A
Abortion: Expulsion or removal of a fetus from the womb prematurely.
Acetylcholine: The most common neurotransmitter, present wherever neurons stimulate muscle contraction and also at a majority of the synapses between neurons.
Acquired: Not present at birth, but developed subsequently, often as a response to some environmental circumstance.
Acquired characteristics: Physiological or other changes developed during the lifetime of an individual but not inherited.
Acquired Immune Deficiency Syndrome (AIDS): Impairment of most of the immune system from infection with the Human Immunodeficiency Virus (HIV), often resulting in death from opportunistic infections or rare cancers.
Acquired immunity: see specific immunity
Actin: A contractile protein found in all eukaryotic cells, especially muscle cells, forming part of the cytoskeleton and responsible (with myosin) for muscle contraction.
Action potential (spike): A large reversal of polarization in a nerve cell membrane, resulting in a nerve impulse.
Active transport: Use of energy to transport a substance from an area where it is in low concentration to an area where it is in higher concentration. In cells, active transport is performed by membrane proteins called transporters.
Acute effect: An effect that ceases soon after its cause is removed.
Adaptation: Any trait that increases fitness or increases the ability of a population to persist in a particular environment. Also, in nervous systems, a physiological change in response to a stimulus that prepares the body to better withstand or react more vigorously to similar stimuli.
Addiction: A strong physiological and psychological dependence.
Additive effect: A physiological response produced by two drugs given together that is the same as the sum of the effects of each drug given separately.
Adenosine TriPhosphate (ATP): A molecule that provides energy for cellular processes.
Adult stem cell: An undifferentiated cell that retains into adulthood the ability to divide and differentiate; bone marrow cells capable of differentiating into blood cells are an example.
Aerobic. Capable of living only in the presence of oxygen (O2).
Age pyramid: A diagram that represents the age distribution of a population by a stack of rectangles, each proportional in size to the percentage of individuals in a particular age group.
Age structure: The distribution of members of a population into different age groups.
Agonist: A drug that stimulates a particular receptor or that has a stated effect.
Alarm: The first phase of the stress response, in which the body quickly secretes hormones such as cortisol and norepinephrine, preparing the body for "fight or flight".
Algae: An informal name for photosynthetic eukaryotes that are not plants.
Alkaptonuria: A genetic condition (inborn error of metabolism) in which urine turns dark upon exposure to air because of the body's inability to break down a compound called homogentisic acid.
Allele: One of the alternative DNA sequences of a gene.
Allele frequency: The frequency of an allele in a population, or the fraction of gametes that carry a particular allele.
Allen’s rule: In any warm-blooded species, populations living in warmer climates tend to have longer and thinner protruding parts (legs, ears, tails, etc.) while the same parts tend to be shorter and thicker in cold climates.
Allergen: Anything provoking an allergic response by the immune system.
Allergy: An inflammatory immune response to a substance that does not usually pose a threat to the body.
Alveolata: A major subgroup of Eukarya that contains such non-photosynthetic unicellular organisms as the ciliates and Apicomplexa, characterized by small, fluid-filled bubbles (alveoli) just inside the plasma membrane.
Alveoli: Small pouches or cavities, especially the air-filled pouches in which gas exchange occurs in the lungs.
Alzheimer's disease: A progressive form of dementia, most often affecting the elderly, characterized by buildup in the brain of amyloid-β clumps ("plaques") and tangles of tau protein, and by memory loss.
Amino acid: A small molecule containing both an amino group (-NH2) and a carboxylic acid group (-COOH) that is capable of being joined with other similar molecules to form long protein chains.
Amniocentesis: A procedure that involves removing a small amount of amniotic fluid from a woman’s uterus to test for possible genetic abnormalities in a developing fetus.
Amyloid-β: A protein that forms abnormal deposits ("plaques") in the brains of Alzheimer's patients.
Anemia: An inability of the blood to efficiently transport oxygen around the body.
Anaerobic: Conditions in which free oxygen (O2) is not present, or organisms that can live under such conditions.
Analogy: Resemblance resulting from similar evolutionary adaptation but not indicative of a shared lineage, as in wings of similar shape made of different materials.
Anaphase: The stage of cell division (mitosis or meiosis) in which sister chromatids or paired homologous chromosomes separate and move to opposite sides of the cell.
Angiogensis: The stimulation of nearby blood vessels to grow into a structure and supply it with blood.
Animal: A multicellular organism that develops from a hollow ball of cells (blastula) and usually has well-developed motility or motile parts.
Anorexia (anorexia nervosa): A psychological eating disorder characterized by self-imposed starvation.
Antagonist: A drug that inhibits another or that inhibits a particular receptor.
Antagonistic interaction: A combined effect in which two drugs together produce less of a physiological response than either drug given separately.
Anthrax: A lung or skin infection caused by Bacillus anthracis and its spores.
Antibodies: Proteins, secreted by B-lymphocytes during an immune response, that bind to and disable the specific type of molecule that induced their secretion, thus helping to protect the body's health.
Anticodon: A three-nucleotide sequence in a transfer RNA molecule that pairs with a messenger RNA codon.
Antigen: Any molecule or part of a cell that is detected by the immune system.
Antigenic drift: Spontaneous single nucleotide mutations that change the shape of antigenic molecules within a species, often reducing the effectiveness of previously acquired immunity within a host.
Antigenic shift: Genetic reassortment of large sequences of nucleotides that result in major changes in the antigen shapes on the cells of organisms, making those organisms 'invisible' to previously immune hosts.
Antioxidant: A substance that prevents oxidation of a molecule by an oxidizing agent.
Anti-parallel: Oriented in opposite directions while remaining a fixed distance apart; in double-stranded DNA, one strand begins with a phosphate group where the opposing strand has a sugar.
Apoptosis: Programmed cell death (or cellular suicide) that is induced by a signal and involves fragmentation of a cell's DNA, cell shrinkage and detachment of a cell from its surroundings without cell lysis (bursting).
Archaeplastida: A major subgroup of Eukarya that contains plants, red algae, and green algae, all of which are photosynthetic.
Artery: A blood vessel that carries blood away from the heart.
Artificial insemination: Insertion of sperm into a female reproductive tract other than directly from a male.
Artificial selection: Consistent differences in the contribution of different genotypes to future generations, brought about by intentional human activity.
Asexual reproduction: Reproduction by one parent, producing a clone of genetically identical offspring without any genetic recombination.
Astrocytes: Star-shaped neuroglial cells that nourish and protect brain cells, also capable of engulfing or destroying cellular debris or abnormal proteins in brain tissue.
Atherosclerosis: Deposits of fat and cellular debris, which may become calcified, on the interior walls of arteries.
ATP, see Adenosine TriPhosphate
Autoantibodies: Antibodies against one's own proteins or other cellular molecules.
Autoimmune disease: Any disease in which the immune system abnormally secretes antibodies that attack the body's own healthy tissues.
Autonomic nervous system: Part of the peripheral nervous system that regulates 'involuntary' physiological processes of the body; consists of the sympathetic and parasympathetic divisions.
Autosome: A chromosome not involved in sex determination.
Autotroph: An organism capable of making its own energy-rich compounds from inorganic compounds.
Axon: An extension of a nerve cell that carries an impulse away from the nerve cell body.
B
B lymphotyte (B cell): A type of white blood cell that makes antibodies.
Balanced diet: A varied diet containing ample (but not excessive) amounts of all nutrients.
Balanced polymorphism: A situation in which different alleles of a gene persist in a population because of the superior fitness of the heterozygous condition.
Barrier method: A birth control method in which a barrier is inserted across the path of sperm to prevent the sperm from reaching the egg.
Basal metabolic rate: The rate at which the body uses energy when awake but lying completely at rest.
Basement membrane: A membrane to which cells are attached at their base, especially in epithelial (sheetlike) tissues.
Benign tumor: An abnormal accumulation of cells that is not cancerous and does not invade neighboring tissues or spread to other parts of the body.
Bergmann’s rule: In any warm-blooded species, populations living in colder climates tend to have larger body sizes, compared to smaller body sizes in warmer climates.
Bilateral symmetry: Body organization in which the left half and right half are mirror images of one another.
Biodegradation. Biological activity that breaks down materials into harmless substances.
Biodiversity: The number and variety of biological species, their, alleles, and their communities.
Biofeedback: Monitoring one's own physiological activity as a means of learning how to modify this activity, e.g., to reduce stress.
Biofilms: Layered aggregates of bacteria or other cells, with specialized cell types expressing different molecules the various layers.
Bioinformatics: The study of large collections of molecular sequences (such as nucleic acid or protein sequences) with the use of computer-based methodologies to catalog, search, and compare these sequences.
Biological determinism: see determinism.
Biomagnification: The increasing concentration of pollutants as one proceeds up the food pyramid from one trophic level to another.
Biomarker: Any molecule or other change that can easily be measured in patients to monitor the course of a disease or its treatment.
Biome: A group of similar ecosystems in various locations around the world.
Biology: The scientific study of living systems.
Bioremediation. The enhancement of biodegradation by adding or encouraging the growth of certain organisms.
Biosphere: The ecosystem that includes the whole Earth and its atmosphere.
Bioterrorism: The use of biological agents to spread harm and also fear in human populations.
Birth control: Any measure intended to prevent unwanted births or to reduce the birth rate.
Birth rate (B): The number of births in a given time period divided by the population size at the beginning of that period.
Blastocyst: An early mammalian embryo at the stage when it is ready to implant into the uterine wall.
Blastula: An early embryonic stage in animal development consisting of a hollow ball of cells.
Blood-brain barrier: A membrane system that separates the bloodstream from the cerebrospinal fluid while allowing certain small molecules to diffuse between the blood and the brain.
Bottleneck effect: A type of genetic drift that occurs when a population is temporarily small.
Branching descent (descent with modification): Relationships among species resulting from a common ancestor giving rise to multiple new species, with repetition creating a treelike pattern overall.
Bulimia: A psychological eating disorder characterized by an overeating binge, followed by self-induced vomiting or laxative abuse.
C
Calorie: The amount of energy required to raise the temperature of a gram of water by one degree Celsius. See also kilocalorie.
Cancer: A group of diseases characterized by DNA mutations in growth control genes in which cells divide without regard to growth control signals and can eventually invade neighboring tissues and spread to other parts of the body.
Carbohydrates: Polar molecules used by organisms as energy sources and consisting of carbon, hydrogen, and oxygen, with hydrogen and oxygen atoms in a 2:1 ratio.
Carbon-neutral. Any process or entity that releases no more carbon dioxide than it consumes.
Carcinogen: A physical, chemical, or viral agent that induces cancer; its action is called carcinogenesis.
Carcinoma: A cancerous growth of epithelial (sheetlike or glandular) tissue.
Cardiovascular disease: Any degenerative (age-related) disease of the heart or blood vessels, including atherosclerosis, arteriosclerosis, heart attack, and stroke.
Carrier: In genetics, an individual carrying a recessive allele in their genotype but not showing its phenotypic effect in their phenotype.
Carrying capacity (K): The maximum population size that can persist in a given environment.
Cas9: An enzyme used in genetic engineering (or by bacteria) to cut DNA sequences at desired locations identified by CRISPR RNA sequences.
Categorical imperative: Kant's ethical criterion, that an act is good or bad depending on whether you wish everyone to copy it.
Census: Any enumeration (counting) of the members of a population.
CD4: see Helper T cells.
CD8: see Cytotoxic T cells.
cDNA (complementary DNA): A strand of DNA produced by transcription from an RNA template molecule using a reverse transcriptase enzyme.
Cell: The smallest unit of living organisms that shows the characteristics of life; can either be free-living or part of a multicellular organism.
Cell cycle: The process by which a cell duplicates its DNA and then divides into two cells.
Cell of origin: The initial cell whose transformation begins a cancerous growth.
Cell theory: The theory that cells are the building blocks and functional units of all organisms and that all cells originate from other cells.
Cellular respiration: The process of energy use in cells by the chemical breakdown of macronutrients.
Central nervous system (CNS): The brain and spinal cord.
Centromere: A constriction point within a chromosome that divides the chromosome into two arms and serves as the place where the spindle becomes attached to the chromosomes during cell division.
Cerebellum: Part of the hindbrain that controls muscular coordination and balance.
Cerebral cortex: The outer part of the cerebrum.
Cerebral hemispheres: The two halves of the cerebrum.
Cerebrospinal fluid: The fluid contained within the cavities of the brain and spinal cord, from which nutrients and oxygen diffuse to the neurons.
Cerebrum: The part of the forebrain controlling conscious activity and thought; it is the major part of the brain in humans.
Channels, see membrane channels
Chargaff's rules: The rules stating that, in a DNA sample, the numbers of adenine (A) and thymine (T) nucleotides are always equal, and the numbers of guanine (G) and cytosine (C) nucleotides are always equal.
Checkpoints: Regulatory mechanisms that do not allow the cell cycle to proceed until certain repairs are made or processes are completed.
Chemical digestion: The use of enzymes and chemical reactions to break down food into simpler molecules that cells can absorb.
Chikungunya: A viral disease transmitted by mosquitoes in tropical regions.
Chlamydia: A sexually transmitted disease caused by the bacteria Chlamydia trachomatis.
Chlorofluorocarbons. Organic chemicals derived from hydrocarbons by replacing some of the hydrogen atoms with chlorine and others with fluorine.
Chlorophyll: A green pigment molecule that traps light in the light reactions of photosynthesis.
Chloroplasts: Cytoplasmic organelles that contain chlorophyll and carry out photosynthesis.
Cholera: A waterborne infection caused by the bacteria Vibrio cholerae.
Cholesterol: A lipid with a multi-ringed structure, found in the membranes of most animal cells.
Chorionic villus sampling: Removal of a small amount of tissue from the placenta to test for possible genetic abnormalities in a developing fetus.
Chromatin: The repeating structure of proteins (histones) associated with DNA in eukaryotic chromosomes, resulting in their compaction.
Chromosomal aberration: A change in a chromosome sequence large enough to contain many genes. Examples include chromosomal inversions, translocations, duplications, and deletions.
Chromosomal rearrangement: Same as a chromosomal aberration.
Chromosomal theory of inheritance: The theory, now undisputed, that chromosomes contain an organism’s hereditary information in the form of genes.
Chromosomes: Long, individual DNA molecules that together make up an organism’s total genetic material. Prokaryotic cells have a single, circular chromosome located in the cytoplasm, while eukaryotic cells have multiple, linear chromosomes located in the nucleus and organized into chromatin by DNA-binding proteins (histones).
Chronic effects: Lasting or life-long effects.
Circadian rhythm: A biological change whose pattern repeats approximately every 24 hours.
Clade: A branch of a family tree or dendrogram, forming a monophyletic group.
Cladistics: The study of family trees (phylogenies) and the basing of classification on the branching patterns in these trees.
Class: A major taxon containing several related orders; examples include insects (Insecta) and mammals (Mammalia).
Classical conditioning: A form of learning in which one stimulus (the conditioned stimulus) that repeatedly precedes or accompanies another (the unconditioned stimulus) becomes capable of evoking the response originally elicited only by the unconditioned stimulus.
Classification: An arrangement of species into a series of taxa, or 'groups within groups'.
Clathrates. see Methane clathrates.
Climate change. Any global change in Earth's temperature, rainfall, or seasonal variations.
Cline: A gradual geographic variation of a trait within a species.
Clone: The genetically identical progeny derived from a single cell or individual by mitotic cell division or asexual reproduction.
Cochlea: The coiled part of the inner ear, responsible for sensing sounds.
Codominant: Alleles capable of producing different phenotypic effects simultaneously.
Codon: A coding unit of three successive nucleotides in a messenger RNA molecule that together determine an amino acid.
Coenzyme: A nonprotein substance needed for an enzyme to function.
Colon: The large intestine, excluding the caecum.
Communicability: The probability that a disease-causing microorganism will be transferred to another individual, either directly or indirectly.
Community: A group of species that interact in such a way that a change in the population of one species has consequences for the other species in the community.
Comparative genomics: Comparison of genomes from different species, used to answer questions about evolution and gene function.
Complement: Blood proteins that, in combination with antibodies, can destroy some bacteria and viruses.
Complementary base pairing: Among nucleotides, a type of chemical bonding that always matches C and G with one another and A and T with one another (except that U substitutes for T in RNA).
Complete protein: A protein that contains all the amino acids considered essential for human nutrition.
Concentration gradient: A situation in which the concentration of a substance is different in different locations or on opposite sides of a membrane.
Concordance: In studies of twins or other matched individuals, the fraction of individuals with a certain trait whose twin (or matched individual) also has the trait.
Condom: A latex or other barrier to the passage of sperm, worn as a covering over the penis.
Consumers: Organisms that consume food energy by eating other organisms.
Contact inhibition: The inability of a normal eukaryotic cell to divide if it is surrounded by other cells.
Continuous variation: Variation in which in-between values are always possible, such as a length of 23.15 cm between the values 23.1 and 23.2.
Contraceptive: Any method that prevents conception (fertilization).
Control group: A group used for comparison in an experiment. For example, if experimental animals are exposed to a drug, then a control group might consist of similar animals not exposed to the drug but treated the same in every other way.
Convergence: Independent evolution of similar adaptations in unrelated lineages, a process that gives rise to analogy.
Corpus callosum: A series of fiber tracts that connect the right and left cerebral hemispheres across the midline of the brain.
Corpus luteum: Progesterone-secreting scar tissue, formed within the ovary by a follicle after the egg has been released.
Correlation by fossils: Judging geological formations to be of the same age if they contain fossils of many of the same or similar species.
COVID-19: An infectious disease caused by the coronavirus SARS-Cov2, responsible for a pandemic in 2019-2022.
Covalent bonds: Chemical bonds formed by the sharing of electrons, either equally (nonpolar covalent bonds) or unequally (polar covalent bonds).
Creutzfeld-Jacob Disease: A brain infection in humans caused by a prion.
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats): Repeated DNA sequences used by certain bacteria to recognize and defend against previously encountered viruses, or similar sequences used to target selected genes in genetic engineering.
CRISPR-Cas9: A method of precise gene editing that relies on CRISPR RNAs designed to target genes of interest and a CRISPR-directed DNA nuclease, Cas9. This method mimics a natural system used by certain bacteria as a protection against previously encountered viruses.
Crossing over: The rearrangement of linked genes when homologous chromosomes break and recombine; one form of genetic recombination.
Cryptosporidiosis: A waterborne infection caused by the parasite Cryptosporidium.
Cuticle: A tough, chemically resistant outer covering, typical of most Ecdysozoa.
Cyclin: A protein whose abundance varies cyclically, increasing at a particular time in the cell cycle to stimulate the transition to the next cell cycle phase.
Cyclin-dependent kinase: An enzyme that controls progress through the cell cycle by adding phosphate groups to other proteins in response to the abundance of cyclins.
Cystic fibrosis: An inherited disease, caused by a defective CFTR chloride-ion transport protein, that results in the build-up of thickened fluids and frequent infections in the lungs, among other symptoms.
Cytokines: Molecular signals that produce effects on cells and functions of the immune system.
Cytokinesis: Division of the cytoplasm into two distinct cells following mitosis, or following either of the divisions of meiosis.
Cytoplasm: The portion of the cell outside the nucleus but within the plasma membrane.
Cytoskeleton: A set of proteins that give shape to eukaryotic cells and in some cases permit movement.
Cytotoxic (CD8) T cells: Lymphocyte cells of the immune system that react specifically to a nonself molecule, becoming activated to kill cells bearing that molecule.
D
Data: Information gathered so as to permit the testing of hypotheses.
Dead-end host: A host in which a pathogen reaches the end of its life cycle and is not transmitted to subsequent hosts.
Death rate (D): The number of deaths in a given time period divided by the population size at the beginning of that period.
Declarative learning: Conscious remembrance of persons, places, things, and concepts, requiring the action of the hippocampus and the temporal regions of the brain.
Deduction: Logically valid reasoning that guarantees a true conclusion whenever the premises are true.
Dementia: A type of brain disease marked by mental deterioration, memory loss, and decreasing ability to carry out such everyday functions as feeding oneself or bathing.
Demographic momentum: A temporary population increase that can be predicted in a population that has more prereproductive members and fewer postreproductive members than a population with a stable age structure would have.
Demographic transition: An orderly series of changes in population structure in which the death rate decreases before a similar change occurs in the birth rate, resulting in a population increase during the transition period.
Demography: The mathematical study of populations.
Dendrites: Nerve cell processes that receive signals and respond by conducting impulses toward the nerve cell body.
Deontological: A type of ethics in which the rightness or wrongness of an action is judged without reference to its consequences.
Deoxyribonucleic acid (DNA): A nucleic acid containing deoxyribose sugar and usually occurring as two complementary strands arranged in a double helix.
Dependence: Inability to carry out normal physiological functions without a particular drug.
Depolarization: The disappearance of a separation of unequal electric charges.
Depression: A mental disorder characterized by low levels of serotonin and other neurotransmitters, by lack of motivation, and, in severe cases, by suicidal thoughts and actions.
Determined: A state of development in which the future identity of a cell's progeny is predictable.
Determinism (genetic determinism): The belief that an individual's characteristics are wholly determined by its heredity (i.e., its genes).
Deuterostomes: Animals in which the blastopore becomes the hind end and the mouth develops at the opposite end.
Discontinuous variation: 'Either/or' variation in which intermediate conditions usually do not exist, as in the presence or absence of a disease.
Diabetes: Either of two metabolic disorders in which blood glucose levels are chronically high. Type 1 diabetes results from a lack of insulin production by the pancreas; type 2 diabetes (often accompanied by obesity) results from the body's inadequate response to insulin.
Diet: An overall pattern of food consumption.
Differentiation: The process of becoming different; a restriction of the set of future possibilities of a cell's progeny.
Diffusion: A process in which molecules move randomly from an area of high concentration to an area of lower concentration until they are equally distributed.
Diploid: Possessing chromosomes and genes in pairs, as in all somatic cells of most multicellular organisms.
Disease: Any condition of an organism in which normal biological function is lessened or impaired.
DNA: See Deoxyribonucleic acid.
DNA ligase: An enzyme that creates a continuous DNA strand by joining separate DNA fragments; used to create a recombinant DNA molecule from restriction enzyme fragments.
DNA marker: Any DNA sequence whose chromosomal location is known, permitting it to be used to help locate genes.
DNA polymerase: An enzyme that builds a new strand of DNA complementary to a preexisting strand used as a template.
DNA probe: See Probe.
DNA replication: The copying of a DNA molecule to make two double-stranded molecules, each with one old and one new DNA strand.
Dominant: Describes a trait that is expressed in the phenotype of heterozygotes or an allele that expresses its phenotype even when only one copy of the allele is present.
Dopamine: A neurotransmitter synthesized from the amino acid tyrosine and terminating in an amino group.
Dose: The amount of a drug given at one time.
Double helix: The shape of a DNA molecule, with two paired, corkscrew-shaped strands running in opposite directions.
Doubling time: The time required for the number of individuals to double in a population.
Down syndrome: A complex syndrome that includes varying degrees of altered brain development, plus eyes with epicanthic folds and in some cases heart malformations, usually arising from trisomy of chromosome number 21, or less often from other chromosomal abnormalities.
Drug: Any chemical substance that alters the function of a living organism other than by supplying energy or needed nutrients.
Drug abuse (substance abuse): Excessive use of a drug, or use that causes harm to the individual or to society.
Duodenum: The initial portion of the small intestine, which receives the secretions of the liver and pancreas.
E
Ebola: A virus, endemic to central Africa, that caused an epidemic in West Africa from 2014-2016.
Ecological succession: An ecological process by which one community replaces another.
Ecosystem: A biological community interacting with its physical environment.
Ectoderm: The outermost germ layer in a developing embryo.
Electrical potential: A form of stored energy resulting from the separation of positive and negative electric charges.
Electrophoresis: A technique used to separate DNA, RNA, or protein fragments by size according to their movement through a gel matrix, driven by an electric field.
Embryo: The earliest stage of development. In plants, this consists of a reproductive cell surrounded and protected by nonreproductive tissue; in humans, it lasts from fertilization to about eight weeks of gestation.
Embryonic stem cell: An undifferentiated cell in an embryo that is able to divide and differentiate into any specialized cell type.
Empirical: Based on observations of the material world that we make with our senses, aided in some cases by scientific instruments.
Endangered species: A species threatened with extinction.
Endemic: Persistently found in a specified location, e.g., a disease that maintains a low to moderate prevalence over a long time.
Endocrine: A form of cell signaling in which a signal molecule produced by one cell type is secreted into the bloodstream to be carried to a target elsewhere in the body.
Endocrine glands: Glands that secrete their products, called hormones, into the blood stream rather than into a duct.
Endocytosis: Bringing a particle into a cell by surrounding it with cell membrane.
Endoderm: The innermost germ layer in a developing embryo.
Endosymbiosis: The engulfing of a smaller cell by a larger cell, followed by the persistence of the smaller cell as a specialized compartment or organelle within the larger cell; also, the theory that eukaryotic cells originated by such a process.
Entropy: A measure of randomness (disorderliness) in physical systems.
Enzyme: A chemical substance (usually a protein, but sometimes an RNA) that speeds up a chemical reaction without getting used up in the reaction; a biological catalyst.
Epidemic: An outbreak of a disease at much greater prevalence than usual.
Epidemiology: The study of the frequency and patterns of diseases in populations.
Epigenetic changes: Changes in gene function that do not involve nucleotide sequence changes in the genes.
Epilepsy: A brain disorder characterized by uncontrollable muscle seizures and other symptoms.
Epiphyte: A plant that lives upon and derives support, but not nutrition, from another plant.
Estrogen: A hormone that stimulates the development of female sex organs prior to reproductive age and the growth of an ovarian follicle each month during the reproductive years.
Ethics: A discipline dealing with the analysis of moral rules and the ways in which moral judgments are made and justified.
Eugenics: An attempt to change allele frequencies through selection or changes in fitness. Raising the fitness of desired genotypes is called 'positive' eugenics; lowering the fitness of undesired genotypes is called 'negative' eugenics.
Eukaryotic: Describes a type of cell possessing a well-formed nucleus and many other internal compartments (organelles) surrounded by membranes.
Euphenics: Measures designed to alter phenotypes (producing phenocopies) without changing genotypes.
Euphoria: A feeling of elation and well-being, especially one unrelated to the true state of affairs.
Eupsychics: Social and educational measures that accommodate people with differences.
Euthenics: Measures designed to assist people to overcome some of the consequences of their phenotypes. Wheelchairs and eyeglasses are examples.
Evidence-based: Decision-making informed by the testing of hypotheses in comparison with empirical data.
Evolution: The process of permanent change in living systems, especially in genes or in the phenotypes that result from them.
Excavata: A subgroup of Eukarya containing single-celled organisms with a feeding groove excavated on one side.
Excretion: The production of waste products, especially by the kidney, and their subsequent removal from the body.
Exhaustion: The third and final phase of the stress response, in which the sympathetic nervous system ceases to respond and the pituitary gland secretes endorphins and enkephalins that lessen the pain.
Exocrine glands: Glands that secrete their product into a duct.
Exon: A segment of a protein-coding gene that is joined together with the gene’s other exons to generate the complete mRNA that is then translated into protein.
Exoskeleton: A supporting skeleton on the outside, as in insects or crabs.
Expressed sequence tag (EST): A short segment of an RNA (or its cDNA) that can be used as a genetic marker to indicate the level of expression of the corresponding gene, which may vary between individuals.
Experiment: An artificially contrived situation in which hypotheses are tested using empirical data in comparison with some known condition called the control condition.
Experimental sciences: Sciences in which hypotheses are tested by conducting experiments and analyzing the results.
Exponential growth: A form of geometric growth without any limit, according to the equation dN/dT = r N.
Extinction: Termination of a lineage without any descendents.
Extracellular matrix: Material produced by cells but located outside any cell. Connective tissues have large amounts of extracellular matrix.
F
Fairness: The ethical principle that all individuals in similar circumstances should receive similar treatment.
False negative: A negative test result in a sample that actually has the condition being tested for; indicates a lack of sensitivity of the test.
False positive: A positive test result in a sample in a sample that does not actually have the condition being tested for; indicates a lack of specificity of the test.
Falsifiable: Capable of being proven false by experience.
Falsified: Proven false by experience (as by comparison with empirical data).
Family: A group of related genera.
Fats: Lipids that are generally solid at room temperature.
Fatty acids: Long-chain, nonpolar organic acids, released by the digestion of fats or phospholipids.
Feedback mechanism or system: Any process in which a later step modifies or regulates an earlier step in the process.
Fertilization: The combining of an egg and a sperm to form a diploid zygote.
Fertilizer: Any substance artificially furnished to promote the growth of crops.
Fetus: A developing embryo after all of its organs have formed; in humans, from about eight weeks post-conception to birth.
Fiber: Food components that pass through the digestive tract without being absorbed.
Fitness: The ability of a particular individual or genotype to contribute genes to future generations, as measured by the relative number of viable offspring of that genotype in the next generation.
Flagellum: A eukaryotic organelle whose tubular fibers create a whiplike motion. Also, an analogous structure that produces a propeller-like motion in some bacteria.
Forebrain: The front portion of the brain, containing the paired olfactory bulbs, olfactory lobes, cerebral hemispheres, and several unpaired portions including the hypothalamus.
Fossils: The remains or other evidence of life forms of past geological ages, as preserved in geological deposits.
Founder effect: A type of genetic drift in which the allele frequencies of a population reflect the restricted variation present among a small number of founders of that population.
Free radicals: Very reactive atoms or molecules with unpaired electrons.
Fruit: A ripened ovary, containing seeds.
Functional genomics: The methods used to assign biological function to the nucleic acid sequences contained in a sequenced genome.
G
G6PD deficiency (favism): A deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD), producing a blood-rupturing (hemolytic) anemia in response to certain drugs or when certain beans, especially fava beans, are eaten.
Gaia hypothesis. The hypothesis that living organisms are responsible for the conditions that make continued life on Earth possible.
Gamete: A reproductive cell (egg or sperm), containing one copy of each chromosome (haploid DNA content).
Ganglion (pl. ganglia): A clump or cluster of nerve cell bodies, usually in the peripheral nervous system.
Gastrula: An early embryonic stage in most animals, containing 2-3 distinct cell layers, formed from a blastula by the invagination (tucking-in) of some cells to form an inner cell layer (endoderm) that is usually separated from an outer cell layer (ectoderm) by an intervening middle layer (mesoderm, absent in Cnidaria).
Gene: A portion of DNA that contains the information for synthesis of a single protein chain (polypeptide) or sometimes a functional RNA that is not translated. In earlier use, a hereditary particle.
Gene editing: Making targeted DNA sequence changes to a gene at its normal position in the genome, most often using the CRISPR-Cas9 method.
Gene expression: Transcription and translation of a gene into its protein product (or, for genes that encode functional RNAs, transcription and possible processing of the RNA).
Gene pool: The sum total of all alleles contained in a population.
Generator potential: The membrane potential generated by sensory cells in response to the sensory information they receive.
Gene therapy: Introduction of a functional copy of a gene to overcome a genetic deficiency, especially when used to treat hereditary diseases.
Genetic code: The correspondence between three-nucleotide codons and the amino acids (or translation stop signals) that they specify. See Table 3-1.
Genetic determinism: see determinism.
Genetic drift: Changes in allele frequencies in populations of small to moderate size as the result of random processes.
Genetic engineering: The intentional altering and manipulation of genotypes.
Genetics: The study of heredity, including genes and how traits are passed from parents to offspring.
Genome: The total genetic makeup of an individual, comprising its entire DNA sequence.
Genome-wide association study: Statistical comparison between the genomes of individuals with and without a certain condition, used as an attempt to understand the genetic basis of the condition.
Genomics: The scientific study of entire genomes.
Genotype: The hereditary makeup of an organism, as revealed by studying its offspring, or, for a given gene, the combination of alleles present in an individual.
Genus (plural, genera): A group of closely related species.
Geographic isolation: Geographic separation of populations by an extrinsic barrier such as a mountain range or an uninhabitable region.
Giardiasis: A severe diarrhea caused by the waterborne intestinal parasite Giardia intestinalis.
Global warming. An increase in the global or average temperature of a planet, such as resulting from a buildup of carbon dioxide.
Gloger’s rule: In any warm-blooded species, populations living in warm, moist climates tend to be darkly colored or black; populations living in warm, arid climates tend to have red, yellow, brown, or tan colors; and populations living in cold, moist climates tend to be pale or white in color.
Glomerulus: A small clump of thin-walled capillaries within the kidneys that filters blood into a surrounding capsule (Bowman's capsule).
Glycogen: A carbohydrate consisting of many glucose units linked together, used as a storage molecule in animals and certain microorganisms.
Glycolysis: The breakdown of carbohydrates into pyruvate.
Gonorrhea: A sexually transmitted disease caused by the spherical bacteria Gonococcus.
Greenhouse effect. The trapping of solar energy in the atmosphere of a planet (or in a greenhouse) in the form of infrared radiation or heat, much of the which is reflected from the atmosphere back to the planetary surface.
Growth factor: A messenger molecule that stimulates a cell to divide.
Growth rate (r): The population increase during a specified time interval (usually a year) divided by the population size at the beginning of that time interval.
H
H1N1 swine flu: see Swine flu.
Habitat: The place and environmental conditions in which an organism or a species lives.
Habituation: A form of learning in which an organism learns not to react to a stimulus that is repeated without consequence.
Half-life: For a drug, the time that it takes for the level of the drug in the body to be reduced by half.
Haploid: Containing only unpaired chromosomes, as in gametes or prokaryotic organisms.
Hardy–Weinberg equilibrium: A genetic equilibrium formed in large, random-mating populations in which selection, migration, and mutation do not occur or are balanced.
Hardy-Weinberg principle: In a large, random-mating population in which selection, migration, and unbalanced mutation do not occur, allele frequencies tend to remain stable from each generation to the next.
Health: The ability of an organism to maintain homeostasis or to return to homeostasis after a disease or injury.
Helper (CD4) T cells: Lymphocyte cells of the immune system that react specifically to a nonself antigen by secreting interleukin-2, a cytokine needed for full activity of B cells and CD8 T cells.
Hemoglobin: The oxygen-carrying protein in red blood cells.
Herd immunity: Resistance of a population to a disease by the presence of many individuals who do not transmit it.
Hereditarianism: see Determinism.
Herpes simplex: A sexually transmitted disease caused by the herpes simplex virus (HSV).
Heterotroph: An organism not capable of manufacturing its own energy-rich organic compounds and therefore dependent on consuming such compounds as food.
Heterozygous: Possessing two different alleles of the same gene in a genotype.
High-Density Lipoproteins (HDL): Proteins that carry lipids (like cholesterol) away from tissues via the bloodstream; often referred to as 'good cholesterol'.
Hindbrain: The rear portion of the brain, containing the cerebellum and medulla.
High-risk behaviors: Behaviors or actions that greatly increase the probability of undesirable outcomes, such as the transmission of a disease.
High-risk group: A subpopulation of people who share some behavioral, geographic, nutritional, or other characteristic and who have a higher frequency of a particular disease than the general population.
HLA antigens: See Transplantation antigens.
Homeostasis: The ability of a complex system (such as a living organism) to maintain conditions within narrow limits. Also, the resulting state of dynamic equilibrium, in which changes in one direction are counteracted by other changes that bring the system back to its original state
Hominin: A human being capable of walking upright.
Homologous pair: A set of two chromosomes that contain the same sets of genes in the same order but possibly different alleles of those genes; these chromosomes line up as a pair on the meiotic spindle and then separate from each other during the first division of meiosis,
Homology: Shared similarity of structure resulting from common ancestry.
Homozygous: Possessing two identical alleles of the same gene in a genotype.
Hormone: A chemical signal transported through the blood, producing physiological responses in target tissues located at a distance from where the signal is produced.
Humus: A dark-colored, nutrient-rich layer of soil in which organic matter is abundant; also called topsoil.
Human Genome Project: A large-scale project to map and sequence the human genome and, subsequently, the genomes of many other species.
Human Immunodeficiency Virus (HIV): The virus that causes AIDS by infecting and inactivating cells of the immune system that bear a molecule called CD4.
Human papilloma virus (HPV): A sexually transmitted virus that causes genital sores and cervical cancer.
Huntington's disease: An inherited condition, usually beginning at midlife, with uncontrolled twitches or spasms, and resulting in a progressive and fatal neurological deterioration.
Hydrogen bond: A weak bond between a hydrogen atom with a partial positive charge and another atom (in water, an oxygen atom) with a partial negative charge.
Hydroponics: The practice of growing plants without soil.
Hypha (plural, hyphae): A threadlike absorptive filament in a fungus.
Hypothesis: A suggested explanation that is both testable and falsifiable.
Hypothalamus: A structure at the base of the forebrain that regulates body temperature and controls the release of various pituitary hormones.
I
Ileum: The final third of the small intestine, in which most absorption of digestive products takes place.
Immortal: A property of transformed cells that relieves them from having a limit on the number of times they can divide.
Immunization (vaccination): Artificial exposure to an antigen that evokes a protective immune response against a potential disease-causing antigen similar in structure to the antigen in the vaccine.
Immunodeficiency: A decreased activity of some part of the immune system as the result of genetic, infectious, or environmental factors.
Immunosenescence: The decline of immune function as a consequence of the aging process.
Immunosuppression: Decreasing the strength of future immune functions in any manner that is not antigen-specific.
Immunosurveillance: Elimination of abnormal cells by the immune system, thus reducing the chance of those cells developing into cancer.
Implantation: The attachment of an early embryo to the wall of the uterus, where it later forms a placenta.
Incidence: The number or frequency of new cases of a disease or other condition. Compare Prevalence.
Independent assortment, law of: Genes on different chromosomes are passed to offspring independently of one another; the separation of alleles for one trait has no influence on the separation of alleles for traits carried on other chromosomes. Also called 'Mendel's second law.'
Induction: Reasoning from specific instances to general principles, which can sometimes be unreliable, as in 'these five animals have hearts, so all animals must have hearts.'
Industrial melanism: The evolution of protective, dark coloration in soot-polluted habitats.
Infanticide: The killing of an infant shortly after birth.
Infection: The colonization and growth of a pathogen within a host organism.
Inflammation: A physiological response to cellular injury that includes capillary dilation, redness, heat, and immunological activity that stimulates healing and repair.
Informed consent: A voluntary agreement to submit to certain risks by a person who knows and understands those risks.
Innate immunity: Host defenses that exist prior to that individual's exposure to an antigen and are not antigen-specific.
Insulin: A hormone, produced by the pancreas, important in the metabolism and cellular uptake of carbohydrates (sugars).
Integrated Pest Management (IPM): An approach to the management of pest populations that emphasizes biological controls and frequent monitoring of pest populations.
Interbreeding: The mating of unrelated individuals or the exchange of genetic information between populations.
Interferon: A cytokine secreted by lymphocytes that prevents viral replication.
Interphase: The long interval between one mitosis and the next, during which cell metabolism is active but chromosomes are not visible under light microscopy.
Interstitial fluid: A water-based fluid filling in spaces between cells, especially in connective tissues, and containing immune system cells such as lymphocytes.
Intrauterine device (IUD): Anything inserted into the uterus to prevent implantation or pregnancy.
Intron: A segment of a protein-coding gene located between exons that is removed during mRNA splicing, and thus does not contribute sequence information for protein synthesis.
Invertebrate: An animal that does not possess a backbone.
In vitro fertilization: Fertilization that takes place outside the body in laboratory glassware.
Ion: An electrically charged atom, one that is either positively charged because it has lost one or more electrons, or negatively charged because it has gained one or more electrons.
J
Jejunum: The second and longest portion of the small intestine, where most digestion is completed.
K
K-selection: Natural selection favoring a stable population size near the carrying capacity, generally found in predictably stable environments.
Karyotype: The chromosomal makeup of an individual.
Kilocalorie (kcal): The amount of energy required to raise the temperature of 1000 grams (1 kilogram, equal to 1 liter) of water by one degree Celsius. Dietary 'calories' are actually kilocalories.
Kingdom: A major group of organisms, such as the animal kingdom or plant kingdom.
Klinefelter syndrome: A condition arising from the chromosomal arrangement XXY, resulting in a sterile male, often thin, with underdeveloped genitalia and varying degrees of lowered intelligence and breast development.
Knockout: A genetically altered cell or organism in which a particular gene is deleted or disabled, usually in an attempt to study the gene's function.
Koch's postulates: A set of test results that must be obtained in order to demonstrate that a particular pathogen is the cause of a particular infectious disease.
Krebs cycle: A series of biochemical reactions that break apart pyruvate and use the chemical bond energy to make ATP and NADH from ADP and NAD.
L
Learning: The modification of behavior or of memory on the basis of experience.
Legionnaire's disease: A waterborne infection caused by Legionella bacteria.
Leishmaniasis: An infection caused by the flagellated parasite Leishmania and transmitted by sandfly vectors.
Life expectancy: The average length of time that individuals with certain characteristics are expected to live, or the average duration of life for the entire population.
Lifespan see Maximum lifespan
Lignin: A material that stiffens the cell walls of woody plants.
Likely-risk behaviors: Behaviors or actions that moderately increase the probability of undesirable outcomes, such as the transmission of a disease.
Limiting amino acid: An amino acid present in small amounts that, when used up, prevents the further synthesis of proteins requiring that amino acid.
Limiting nutrient: Any nutrient whose amounts constrain the growth of an organism or population; supplying greater amounts of this nutrient therefore allows a population of organisms to increase or grow more vigorously.
Lineage: A succession of species in an ancestor-to-descendent sequence.
Linkage: An exception to the law of independent assortment in which genes located on the same pair of chromosomes tend to be inherited together, with the parental combinations of alleles predominating.
Linkage study: A strategy for identifying the gene(s) associated with a phenotype (such as a disease) by following genetic markers that tend to be inherited with the phenotype.
Linked genes: Genes located on the same pair of chromosomes that tend to be inherited together. See Linkage.
Lipids: Nonpolar molecules formed primarily of carbon and hydrogen, occurring in cell membranes and also used as energy sources.
Logistic growth: Growth that begins exponentially but then levels off to a stable population size (K), according to the equation dN/dT = rN (K-N)/K.
Low-Density Lipoproteins (LDL): Proteins that carry lipids (like cholesterol) to tissues via the bloodstream; often referred to as 'bad cholesterol'.
Low-risk behaviors: Behaviors or actions that seldom increase the probability of undesirable outcomes, such as the transmission of a disease.
Lyme disease: A tick-borne infection caused by the bacteria Borrelia burgdorfi.
Lymph: A fluid containing white blood cells but no red blood cells.
Lymphatic circulation: An open circulatory system in vertebrate animals that gathers intracellular fluid and returns it to the blood along with cells of the immune system.
Lymphocytes: White blood cells that have specific receptors for antigens and are therefore capable of forming an antigen-specific immune response.
M
Macronutrients: Carbohydrates, lipids, and proteins, collectively.
Macrophage: A type of phagocytic cell that can engulf and destroy foreign cells or cellular debris in various tissues.
Malaria: A parasitic infection transmitted by mosquitoes, in which a protist of the genus Plasmodium infects blood cells.
Malignant: A tumor that has grown through the basement membrane or extracellular matrix and has the potential to spread to other sites in the body.
Malnutrition: Poor or inadequate nutrition.
Mast cells: Cells whose release of histamine causes inflammation.
Maternal effect gene: A gene that is transcribed in an egg prior to fertilization.
Maximum lifespan: The maximum number of years that an individual can hope to live.
Mean value: The mathematical sum of many values divided by the number of values.
Mechanical digestion: Breaking food into smaller particles by physical means such as chewing and churning, exposing new surfaces for chemical digestion.
Medulla: The innermost part of any organ, such as the medulla oblongata, a portion of the hindbrain that controls breathing and other involuntary activities that continue even during sleep.
Medulla oblongata: See Medulla.
Medusa: A free-swimming body type in many Cnidaria, with the mouth facing down; commonly called 'jellyfish'.
Meiosis: A form of cell division in which the chromosome number is reduced from the diploid to the haploid number; used to generate gametes for sexual reproduction. Compare to Mitosis.
Melatonin: A hormone produced by the pineal body during darkness; changes in its concentration entrains the body to follow circadian rhythms.
Membrane channels: Proteins capable of transporting molecules across membranes.
Memory: The ability to recall past learning.
Memory cells: Cells of the immune system that retain the ability to respond rapidly to an antigen that the body has encountered before.
Memory consolidation: The formation of long-term memory, involving action by the hippocampus.
Menstrual cycle: A female reproductive cycle characterized by periodic loss of blood and uterine tissue approximately two weeks after ovulation.
Mesoderm: The middle layer of cells in three-layered embryos, from which muscles and various other organs (circulatory, excretory, reproductive, etc.) are derived.
Messenger RNA (mRNA): A strand of RNA that leaves the nucleus after transcription and passes into the cytoplasm, where it functions in protein synthesis.
Metaphase: The phase of cell division in which all chromosomes line up in the center of the cell attached to the fibers of the spindle before separating in the next phase (anaphase).
Metastasis: The ability of transformed cells to leave the original tumor, travel through the body, and adhere and form new tumors in other locations.
Methane clathrates. Compounds in which methane (CH4) is trapped within the crystal structure of ice.
Microbiome: Collectively, the microbial flora inside the digestive tract or other body region.
Microglia: A type of small neuroglial cell that creeps through brain tissue with amoeboid motion, engulfing bacteria or cellular debris.
Micronutrients: Collectively, vitamins and minerals, nutrients needed in much smaller quantities than macronutrients.
Microsatellite marker: A type of DNA marker in which a short nucleotide sequence is repeated a different number of times in different individuals.
Midbrain: The middle portion of the brain, containing most of the reticular formation.
Middle East Respiratory Syndrome (MERS): An infection caused by a coronavirus, responsible for a 2012 epidemic.
Mimicry: A situation in which one species of organisms derives benefit from its deceptive resemblance to another species.
Minerals: Inorganic (non-carbon) atoms needed to regulate chemical processes in the body.
Mitosis: The usual form of cell division, in which the number of chromosomes does not change; used for growth, development, tissue renewal, and asexual reproduction. Compare to Meiosis.
Model: A mathematical, pictorial, or physical representation of how something is presumed to work.
Modern synthesis: The modern evolutionary paradigm, since about 1940, based upon Darwinian natural selection and the geographical theory of speciation.
Monoculture: Growth of only one species in a particular place, as in a field planted with only a single crop.
Monophyletic group: A taxonomic group corresponding to a clade, containing an ancestral species and all of its descendents.
Morals: Rules governing human conduct.
Morphological species concept: A now-discarded concept that defined each species according to its morphological (physical) features.
Morphological (typological) race concept: A definition of each race by its physical characteristics, based on the assumption that each characteristic is unvarying and reflects an ideal type or form shared by all members of the group.
Mosaicism: The existence of cells or patches of cells that differ genetically from one another within an organism because of changes that took place during that individual's development.
Motor neuron: A neuron that conducts impulses away from the central nervous system.
Multipotent: Capable of forming a restricted variety of cellular progeny of several different types.
Multiregional model: A model that views the human species as divided into various regional populations that exchange genes with one another frequently enough so that they all evolve together.
Muscular dystrophy: An X-linked genetic disorder in which muscles become weak and nonfunctional, usually resulting in a shortened lifespan.
Mutagen: An agent that causes mutations in DNA.
Mutation: Any sudden, heritable change in a DNA sequence or gene.
Mutualism: An interaction between species in which both species benefit from the interaction.
Mycoplasma genitalium: A bacteria responsible for a sexually transmitted infection.
Mycota: A kingdom of organisms, commonly called 'fungi', characterized by absorptive nutrition.
Myelin sheath: A lipid-rich covering that surrounds and insulates many neurons.
Myosin: A contractile protein found principally in muscle cells.
N
Narcotic: Any drug capable of producing sleep or loss of consciousness.
Natural selection: A naturally occurring process by which different genotypes consistently differ in fitness, i.e., in the number of copies of themselves that they pass on to future generations.
Naturalistic sciences: Sciences in which hypotheses are tested by the observation of and comparison among naturally occurring events under conditions in which nature is manipulated as little as possible.
Necrosis: Death of a damaged or diseased cell by breakage of its outer membrane and release of its contents.
Negative eugenics: Any attempt to change allele frequencies by lowering the fitness of those with undesired traits.
Nephron: One of the many tubules within the kidney that filters blood and produces urine.
Nerve: A bundle of axons outside the central nervous system.
Neuroendocrine system: The nervous system and the endocrine system considered as an interactive whole.
Neuroglia: Cells of the nervous system other than neurons.
Neurons: Specialized cells that conduct nerve impulses along their surface.
Neurotransmitter: Any chemical that transmits a nerve impulse from one cell to another.
Neutrophils: White blood cells that can surround and digest bacteria by phagocytosis.
Next-generation (Next-gen) DNA sequencing: High-throughput, massively parallel DNA sequencing methods developed after the completion of the Human Genome Project that have greatly accelerated genome sequencing due to their speed and their low costs compared to traditional Sanger sequencing.
Niche: The way of life of a species, or its role in the community.
Nitrogen cycle: A cyclical series of chemical reactions occurring in nature in which nitrogen compounds are built up, broken down, and changed from one form to another with the help of living organisms.
Node of Ranvier: The thinning of the myelin sheath where two adjacent myelin cells meet.
Nonpolar: Having electrical charges equally distributed (or nearly so) across chemical bonds; nonpolar molecules do not mix well with polar solvents such as water.
Nonrenewable resource. Anything whose quantity is finite or limited, and whose continued use will eventually deplete the resource.
No-risk behavior: Behaviors or actions that do not increase the probability of undesirable outcomes, such as the transmission of a disease.
Normal distribution: A mathematical description of random variation about a mean value.
Normal science: Science that proceeds step-by-step within a paradigm.
Notochord: A stiff but flexible rod that defines the body axis in the phylum Chordata.
Nonvirulent: Not capable of producing an infectious disease.
Norepinephrine: A neurotransmitter that transmits impulses to postganglionic neurons of the sympathetic nervous system.
Nuclear envelope: A two-layered membrane that surrounds the nucleus in all Eukarya.
Nucleic acids: DNA and RNA; linear chains of nucleotide subunits that can be single-stranded (typical of RNA) or double-stranded (typical of DNA).
Nucleotide: Subunit of a nucleic acid molecule consisting of a phosphate group linked to a five-carbon sugar and then to a nitrogen-containing base.
Nucleus (plural, nuclei): The central compartment of a eukaryotic cell (such as an animal or plant cell) that contains the chromosomes in a double-layered nuclear envelope, separate from the cytoplasm.
O
Obesity: A condition in which ideal body weight is exceeded by at least 20%.
Occam's razor (principle of parsimony): The principle that the simplest explanation (or the one with the fewest assumptions) should always be preferred.
Oligodendrocyte: A type of neuroglial cell with few branches, generally nutritive in function.
Oncogene: A mutated growth control gene that exhibits hyperactive function, causing the transformation of a cell and promoting development of cancer.
Opportunistic infection: Infection in a host with suppressed immunity, resulting from microorganisms that are normally present in the host's environment but that do not cause disease in a host with a normal or healthy immune system.
Order: A group of related families.
Organ: A structure composed of multiple tissues that work together to carry out a function needed by the organism.
Organelles: Membrane-enclosed structures within the cytoplasm of eukaryotic cells that perform specialized cellular functions.
Organic compounds: Carbon compounds held together by covalent bonds.
Organizer: An embryonic tissue whose chemical secretions induce the differentiation of other cells.
Osmosis: Diffusion of water molecules across a semipermeable membrane in response to a concentration gradient of some other molecule or ion.
Ovary: The organ that produces eggs in females.
Ovulation: Release of an egg from the ovary.
Oxidation: Removal of electrons from an atom or molecule.
Oxidize: To remove electrons from an atom or molecule.
Oxidizing conditions: Oxygen-rich conditions or other conditions in which electrons are readily removed from molecules.
Ozone. A reactive molecule (O3) containing three oxygen atoms, highly reactive at Earth's surface, but more prevalent within the stratosphere.
P
Palindrome: A sequence that reads the same forwards or backwards, as in the sentence "Madam, I'm Adam," or a DNA sequence that can base-pair with itself when folded in half, as in AACCCTTTGCAAAGGGTT.
Pandemic: A worldwide epidemic or disease outbreak.
Paracrine: A form of signaling in which the signal molecule is secreted locally, affecting only nearby cells.
Paradigm: A coherent set of theories, beliefs, values, and vocabulary terms used to organize scientific research.
Paradigm shift: The replacement of one paradigm with another.
Parasite: A species that lives in or on another species (the host), to which it causes harm.
Parasympathetic nervous system: A division of the autonomic nervous system that brings about the relaxation response and secretes acetylcholine as the final neurotransmitter.
Parkinsonism: A neurological disorder involving degeneration of certain darkly pigmented brain cells (the substantia nigra), an insufficiency of dopamine, muscle tremors, and difficulty walking or initiating voluntary movement.
Passive immunity: Antigen-specific immunity acquired in an organism by transfer of antibodies or specific immune cells from another organism.
Pathogen: An infectious agent (organism, virus, or protein) that causes a disease.
Pedigree: A chart showing inheritance of a genetic trait within a family.
Peripheral nerves: Nerves outside the brain and spinal cord.
Permafrost. Permanently frozen soil; soil that remains frozen year-round.
Pesticide: A chemical used to kill undesired (pest) organisms.
Phagocytes: Cells that engulf and digest other cells or the remains of dead cells.
Phagocytosis: A process in which one cell surrounds, engulfs, and digests another.
Pharmacology: The study of drugs and their effects.
Phenotype: The visible or biochemical characteristics or traits of an organism
Phenylketonuria: An inherited condition (inborn error of metabolism) in which the amino acid phenylalanine cannot be broken down and instead produces toxic products that impair health.
Phospholipids: Molecules containing long, nonpolar hydrocarbon chains attached at one end to a polar (charged) phosphate group.
Phosphorylation: Addition of a phosphate group to a protein, usually causing activation.
Photosynthesis: A process by which plants and certain other organisms use energy captured from sunlight to build energy-rich organic compounds, especially carbohydrates.
Phylogeny: A family tree or history of a group of organisms, forming a branching pattern of descent in most cases.
Phylum (plural, phyla): A major group of organisms, including several related classes.
Pineal body: A structure on the roof of the diencephalon of vertebrate brains that maintains circadian rhythms.
Placebo: A drug formulation lacking the active ingredient being tested.
Placebo effect: Physiological response to a placebo that does not result from the chemistry of the placebo but that often produces the response expected by the subject.
Placenta: A structure appearing in the development of most mammals, composed of tissue derived from both the embryo and the mother's uterine lining, by means of which the embryo is nourished during its development in the uterus.
Plague: A highly fatal infectious disease caused by the bacteria Yersinia pestis.
Planktonic: Having little or no locomotor ability, and thus drifting at the mercy of water currents.
Plant: A kingdom of photosynthetic organisms that develop from a multicellular embryo in which a fertilized egg (zygote) is surrounded by nonreproductive tissue.
Plasma membrane: The outer membrane enclosing a cell.
Plasmid: A small, circular DNA molecule, separate from the chromosomal DNA, that can be replicated and passed on through cell division and can sometimes integrate into chromosomal DNA. Found mostly in bacteria but also used in genetic engineering as a vector to transfer DNA sequences between different organisms.
Pluripotent: Capable of forming a wide variety of cellular progeny, such as all ectodermal cell types.
Polar: Having chemical bonds with electrons shared unevenly so that one part of the bond has more negative charge than the other. Water is polar and therefore dissolves other polar substances.
Polar covalent bonds: Chemical bonds in which electrons are shared unequally between two elements.
Polarized: Having opposite ends or surfaces differing from one another in electrical charge.
Policy decisions: Decisions that must be made in terms of human preference, especially ethical preference, for one set of consequences over another.
Pollution. Contamination of an environment by substances present in undesirable locations or quantities.
Polymerase chain reaction (PCR): An iterative method for amplifying (making many copies of) a specific DNA sequence using a template containing that sequence, plus a heat-stable DNA polymerase enzyme, and primers that specify the ends of the sequence to be amplified.
Polymorphism: The persistence of several alleles in a population at levels too high to be explained by mutation alone.
Polyp: A body type in some Cnidaria, with the mouth facing upward, usually growing attached to a surface.
Population: A group of organisms capable of interbreeding amongst themselves and usually sharing common descent as well; a group of individuals within a species living a particular time and place.
Population control: All measures that reduce or limit the rate of population growth.
Population ecology: The study of populations and the forces that control them.
Population genetics: The study of genes and allele frequencies in populations.
Positive checks: Involuntary controls that limit or reduce population growth, such as famine, war, and epidemic disease.
Positive eugenics: Any attempt to change allele frequencies by raising the fitness of those with desired traits.
Potentiality: The range of possible futures for a cell's progeny.
Prevalence: The number or frequency of existing cases of a disease or other condition at any particular time. Compare Incidence.
Preventive checks: Voluntary measures that reduce population growth, including voluntary abstinence from sexual activity.
Primary host: The host in which a pathogen or parasite spends the majority of its life cycle, usually including the reproductive stages.
Primer: A short, single-stranded segment of DNA that can base-pair with the complementary DNA sequence that is used for PCR or DNA sequencing.
Prion: A protein capable of producing an infection. Creutzfeld-Jacob disease in humans and BSE (bovine spongiform encephalopathy or 'mad cow disease') are examples of diseases caused by prions.
Probe: A piece of DNA with a sequence complementary to the marker sequence that one wishes to detect, and capable of being revealed by a fluorescent or radioactive tag.
Procedural learning: Learning how to do things, a process that does not require the hippocampus and is not necessarily conscious.
Producers: Plants and other photosynthetic autotrophs that produce food energy from sunlight.
Progeria: Premature aging (senescence) in a young person.
Progesterone: A hormone that maintains the uterine lining in its enlarged, blood-rich condition, ready for implantation of an early embryo.
Progressive disease: A disease that can worsen with time or sometimes not worsen, but can never improve.
Prokaryotic: A simple type of cell, such as a bacterium, without a well-defined nucleus or other internal membranous compartments.
Promoter: A DNA sequence where RNA polymerase binds and where transcription of a gene therefore begins.
Prophase: The first and longest stage of mitosis or meiosis, in which chromosomes condense and attach to the spindle before being fully lined up in the center of the cell at metaphase.
Proteins: Molecules built of amino acids linked together in long chains that fold up to produce complex shapes, functioning often as enzymes or as structural materials in or around cells.
Proteomics: The study of the total set of proteins present in an organism.
Protists: An informal designation of those eukaryotic organisms, typically unicellular, that are neither animals nor plants nor fungi.
Proton gradient: A form of potential (stored) energy created by a separation or unequal distribution of protons (hydrogen ions).
Proto-oncogene: A normal gene from which an oncogene is derived; it encodes a product that promotes cell division in a regulated manner.
Protostomes: Animals developing from an embryo in which the entrance into the archenteron (the principal cavity of the gastrula stage) develops into the mouth.
Prospective experimental design: An experimental design in which subjects are chosen beforehand and data are subsequently gathered on events as they happen.
Pseudoextinction: Extinction of a taxon by its evolution into something else, thus continuing its line of descendents.
Pseudopods: Outgrowths from the surface that are capable of changing shape in many eukaryotic cells to aid in locomotion or in food capture.
Psychoactive drug: Any chemical substance that alters consciousness, mood, or perception.
Psychoneuroimmunology: A theory that postulates that the mind and body are a single entity interconnected through interactions of the nervous, endocrine, and immune systems.
Puberty: A series of hormonal changes and their consequences associated with the onset of sexual maturity.
Punctuated equilibrium: A theory that describes species as remaining the same over long periods of time and then changing suddenly and giving rise to new species.
Punnett square: A box diagram used to predict the proportions of offspring with various genotypes based on the genotypes of the parents.
Pyruvate: A three-carbon molecule, CH3COCOO-, formed mainly from the breakdown of sugars in glycolysis.
Q
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R
r-selection: Natural selection favoring rapid and prolific reproduction, usually in environments where favorable conditions can quickly disappear and mortality can therefore be devastatingly high.
Race: A geographic subdivision of a species distinguished from other subdivisions by the frequencies of a number of alleles; a genetically distinct group of populations possessing less genetic variability than the species as a whole. This concept is called the population genetics race concept, and is distinguished from other, older race concepts by defining race as a characteristic that can only apply to populations and not to individuals. Important older meanings include the socially constructed race concept (a definition of an oppressed group and the individuals in that group by their oppressors, using whatever cultural or biological distinctions the oppressors wish to use) and the morphological (typological) race concept, defined separately above.
Racism: A belief that one race is superior to others.
Rate of concordance: See Concordance.
Rational drug design: Use of structural information about target molecules (such as proteins implicated in diseases) to guide creation of molecules that will interact with those targets for a medicinal purpose.
Reactive oxygen species: Atoms or molecules that owe their reactivity to the presence of oxygen.
Receptor: A protein or other molecule that binds a specific drug or other chemical substance and responds to the binding by initiating some cellular activity.
Recessive: Describes a trait that is not expressed in heterozygotes, or an allele that expresses its phenotype only when no dominant allele of that same gene is present.
Recombination: Production of new combinations of alleles for different genes in the offspring, due either to independent assortment or to crossing over.
'Redox': Electron transfer reactions involving both oxidation and reduction.
Reduce: To add electrons (negative charges) to an atom or molecule.
Reducing conditions: Hydrogen-rich conditions in which no free oxygen is present.
Reduction: Addition of electrons (negative charges) to an atom or molecule.
Regulated: Causing a process to occur either more or less in a given time period, based on some sort of informational input.
REM: A sleep stage characterized by Rapid Eye Movements beneath the closed eyelids and associated with dreaming episodes.
Relaxation response: A stimulation of the parasympathetic nervous system, sometimes voluntary and self-induced, in which the stress response is ended, blood pressure and breathing are reduced, the threshold of excitation of nerve cells becomes higher, and digestive activity is stimulated.
Renewable: Capable of regrowing or repairing any disturbance by natural processes (within a reasonable time span).
Renewable resource. Anything that can replenish itself naturally if not overused.
Replication: See DNA replication.
Reproductive isolating mechanism: Any biological mechanism that hinders the interbreeding of populations belonging to different species.
Reproductive isolation: The existence of biological barriers to interbreeding.
Reserve capacity: The ability of any tissue or organ to function above its normal level.
Resistance: The second phase of the stress response, in which the body's resources are mobilized to regain homeostasis and overcome the stressor by increasing breathing and blood flow and by mobilizing innate immune activity and temporarily causing inflammation.
Resting potential: The difference in electric charge maintained by a nerve cell membrane in the absence of a nerve impulse.
Restriction endonuclease (restriction enzyme): An enzyme that cuts a nucleic acid only at places with a specified sequence of bases.
Restriction enzyme: See Restriction endonuclease.
Restriction fragment: A linear DNA molecule released from a larger molecule by digestion (cutting) of that larger molecule by a particular restriction endonuclease.
Reticular activating system: A system of neurons, radiating upward from the midbrain, maintaining the body alert and responsive to stimuli.
Reticular formation: A center in the midbrain and part of the hindbrain from which the reticular activating system radiates upward.
Retina: The light-sensitive membrane coating the rear of the eye.
Retrovirus: An RNA virus that begins its reproduction by synthesizing DNA from its RNA.
Reuptake: Absorption of a neurotransmitter by the cell that secreted it.
Reverse transcriptase: A viral enzyme that transcribes viral RNA into DNA.
Reverse transcription: Transcription of complementary DNA (cDNA) from an RNA template.
RFLPs (Restriction Fragment Length Polymorphisms): Differences in the lengths of the restriction fragments made by the same endonuclease among different individuals.
Rhizopodia: A major subgroup of Eukarya that includes unicellular organisms with long, needle-like pseudopods.
Rhythm method: A birth-control method of timed abstinence in which the couple avoids having sex during the time when ovulation is most likely.
Ribonucleic acid (RNA): A nucleic acid, usually existing in single-stranded form, containing nucleotides with ribose sugar.
Ribosomal RNA (rRNA): An RNA sequence that combines with protein to make a ribosome.
Ribosome: An intracellular particle containing RNA and protein that serves as the site of protein synthesis during translation.
Rights: Any privilege to which individuals automatically have a just claim or to which they are entitled out of respect for their dignity and autonomy as individuals.
Risk: The probability of occurrence of a specified event or outcome.
Risk behaviors: Behaviors classified according to the probability of disease transmission.
RNA polymerase: An enzyme that transcribes the DNA template strand of a gene into a complementary RNA strand.
RNA world: A theory that RNA-dependent protein synthesis prevailed for a long time before DNA evolved.
RNA sequencing (RNA-seq): A method for studying the transcriptome that involves isolating RNA from cells, generating complementary DNA (cDNA) using a reverse transcriptase enzyme, then sequencing the cDNA using next-generation DNA sequencing methods.
Route of transmission: The means by which an infectious disease spreads from one host individual to another.
S
Sarcoma: A cancerous growth of originating in connective tissue.
Sarcopenia: Loss of physical strength due to loss of muscle tissue.
Saturated fats: Lipids with no double bonds between their carbon atoms.
SCID (Severe Combined Immune Deficiency): A rare genetic condition in which the entire immune system is nonfunctional.
Science: An endeavor in which falsifiable hypotheses are systematically tested.
Scientific literacy: Familiarity with the methods, findings, and limitations of science.
Scientific method: A method of investigation in which hypotheses are tested by comparison with empirical data.
Scientific revolution: The establishment of a new scientific paradigm, including the replacement of earlier paradigms.
Schistosomiasis: A waterborne infection caused by the parasitic flatworm Schistosoma.
Schizophrenia: A disorder that results in an inability to distinguish real from imaginary situations or stimuli, characterized by frequent auditory or other hallucinations.
Secondary sexual characteristics: Features characteristic of one sex but not essential in reproduction; examples include female breasts and male beards in humans, and antlers in male deer.
Second messengers: Molecules within the cytoplasm of a cell that carry information from membrane receptors to other locations in the cell.
Secrete: To produce and release a cellular product to the environment outside of the cell.
Segregation, law of: When a heterozygous individual produces gametes, the individual alleles separate so that some gametes receive one allele and some receive the other, but no gamete receives both. Also called 'Mendel's first law.'
Selective marker: A gene that permits the survival of cells carrying the gene under conditions that will kill cells lacking the gene, such as a gene that confers resistance to an antibiotic.
Senescence: Progressive loss of physical functioning as a result of age. Also, the aging of individual cells as a consequence of the telomeres of chromosomes reaching a threshold shortness and resulting in no further cell division.
Sensitivity: The probability that a test will give a positive result if the condition being tested for is actually present; the lack of false negatives. Also, the smallest amount of a substance that a given test can detect.
Sensitization: A form of learning in which an intense and often aversive stimulus increases subsequent responses to other stimuli.
Sensory neuron: A neuron that conducts impulses toward the central nervous system.
Sensory perception: The interpretation of sensory messages as images, sounds, etc.
Sensory reception: The translation of sensory information into nerve impulses.
Sentinel species. Any species whose numbers or health can be monitored as an indicator of the overall health of an ecosystem.
Serotonin: a neurotransmitter, 5-hydroxytryptamine, synthesized from the amino acid tryptophan, terminating in an amino group.
Severe Acute Respiratory Syndrome (SARS): An infection caused by the coronacirus SARS-Cov1, responsible for a 2003-2004 epidemic.
Sex chromosomes: One of the chromosomes that differ between the sexes, usually distinguished as X and Y.
Sex determination: The genetic or other control of development as a male or female invididual.
Sex-linked: Carried on the X chromosome (more common) or on the Y chromosome (much less common).
Sexual reproduction: Production of offspring by a process that includes genetic recombination and the fusion of two haploid gametes.
Sexual selection: A process by which different genotypes leave unequal numbers of progeny to future generations on the basis of their success in attracting a mate and in reproducing.
Sickle-cell anemia: A genetic disorder in which hemoglobin A is replaced by hemoglobin S, with the resulting deformed (sickle-shaped) red blood cells having a reduced oxygen-carrying capacity but also a resistance to malaria.
Side effect: A drug effect other than the one for which the drug was intended.
Single nucleotide polymorphism (SNP): A nucleotide position within a genome that shows variation across a population and can be used as a DNA marker.
Sister chromatids: Paired duplicate strands within a chromosome, formed during late interphase and separated from one another during anaphase.
Smallpox: A viral disease, once deadly, that has been eradicated by worldwide vaccination.
Social policy: A formal or informal set of rules under which people make decisions in individual cases.
Sodium-potassium pump: A group of membrane proteins that can actively transport sodium ions from the inside to the outside of a cell, such as a nerve cell, while actively transporting potassium ions in the opposite direction.
Somatic cell: Any cell other than an egg or sperm; a diploid body cell.
Speciation: The process by which a new species comes into being, especially by a single species splitting into two new species.
Species: Reproductively isolated groups of interbreeding natural populations.
Specific (acquired) immunity: An acquired, antigen-specific ability to react to a previously encountered antigen.
Specificity: The degree to which a test detects only the molecule it is meant to detect and does not detect other molecules; the lack of false positives.
Spindle: A structure made from protein fibers of the cell’s cytoskeleton that directs the movements of chromosomes during nuclear division by either mitosis or meiosis.
Sporadic: Arising in somatic cells, and therefore not inherited.
Stem cell: An undifferentiated cell that retains the ability to divide and differentiate.
Stomates: Pores on the underside of leaves in most vascular plants, through which gasses are exchanged.
Stramenopiles: A major subgroup of Eukarya that includes brown algae and other photosynthetic organisms possessing chlorophyll c.
Stratigraphy: The study of layered sedimentary rocks.
Stress (stress response): A physiological response or state of heightened activity brought about by the sympathetic nervous system and maintained for a longer time by the endocrine and immune systems.
Stressor: Any stimulus or condition that brings on a stress response.
Subspecies: A geographical subdivision of a species, characterized by less genetic variation within the subspecies than in the species as a whole.
Substance abuse: see Drug abuse.
Succession: See Ecological succession.
Superior colliculi: Midbrain structures that coordinate sensory information to help locate objects spatially.
Susceptibility: The probability that a person who is exposed to a pathogen will become infected with that pathogen and get a disease.
Sustainable: Any practice that could continue indefinitely without depleting any material whose supply is limited.
Swine flu: A viral disease spread from pigs to humans, responsible for a 2009 epidemic.
Symbiosis: Any type of interaction between two species living together; mutualism and parasitism are two types of symbiosis.
Sympathetic nervous system: A division of the autonomic nervous system that brings about the fight-or-flight response and secretes norepinephrine (sometimes epinephrine) as its final neurotransmitter.
Synapse: A meeting of cells in which a nerve cell stimulates another cell by secreting a neurotransmitter; the postsynaptic cell must have receptors to which the neurotransmitter binds.
Syndrome: A condition that includes a combination of multiple symptoms.
Synergistic effect or interaction: A combination of two causes that lead to a greater effect than the sum of the effects of the two causes applied independently; for example, when two drugs together produce a greater physiological response than the sum of the effects of each drug given separately.
Synthetic biology: A new field of biology that aims to extend traditional genetic engineering, creating modular DNA sequences that perform novel biological functions and even engineering entire genomes to create ‘designer’ microbes that may perform useful functions.
Syphilis: A sexually transmitted disease caused by the bacteria Treponema pallidum.
T
T lymphocyte (T cell): A type of white blood cell that helps bring about an antigen-specific immune response without releasing antibodies.
Tau: A protein that normally stabilizes microtubules in cells, but that can form abnormal tangles that impair brain function in Alzheimer's patients.
Taxon: A group of species at any level of classification.
Taxonomy: The study of classifications and how they are made.
Tela choroidea: A thin-walled roof of one of the brain cavities, across which nutrients and wastes are exchanged between the blood and the cerebrospinal fluid.
Telomerase: An enzyme that lengthens the telomeres of embryonic cells or cancer cells to counteract their shortening during cell division.
Telomere: A repetitive DNA sequence at the end of a chromosome whose small, progressive shortening during each cell division limits the number of times a cell can divide.
Telophase: The last phase of mitosis or meiosis, during which the nuclear membrane re-forms and chromosomes begin to return to their interphase state; usually occurs when cytoplasmic division (cytokinesis) is beginning.
Testis (plural, testes): The sperm-producing organs in males.
Testosterone: A steroid hormone that produces male primary and secondary sexual characteristics.
Thalassemia: A form of anemia, common in many Mediterranean countries, resulting from shortened forms of the beta chain of hemoglobin molecules, and protecting the bearers from malaria.
Theory: A coherent set of well-tested hypotheses that guide scientific research.
Threshold: The minimum level of a stimulus that is capable of producing an action potential, or the minimum level of a drug below which no physiological response can be detected.
Tissue: A group of similar cells and their extracellular products, built together (structurally integrated) and working together (functionally integrated).
Tissue culture: A growth of cells and tissues in a laboratory, artificially maintained outside any organism.
Tolerance: An acquired condition in which progressively greater amounts of a drug are required to produce the same physiological effect when the drug is taken repeatedly. Also, in immunology, an acquired lack of response to a specific antigen after repeated contact with that antigen.
Totipotent: Capable of forming cellular progeny of all different types, as in embryonic stem cells.
Transcription: A process in which DNA is used as a template to guide the synthesis of RNA; the first step in gene expression.
Transcriptomics: The study of all RNA sequences expressed in an organism through transcription of DNA to RNA.
Transduction: Introducing new DNA into a cell using a viral vector.
Transfection: In animal or plant cells, a hereditary change caused by incorporation of DNA fragments from outside the cell.
Transfer RNA (tRNA): A small RNA molecule that inserts an amino acid during translation based on matching its anticodon with a messenger RNA codon.
Transformation: In bacteria or yeast, a hereditary change caused by incorporation of DNA fragments from outside the cell. In multicellular organisms, a multistage process in which normal cells acquire the phenotypes of cancer cells, including immortality and lack of contact inhibition or anchorage dependence.
Transgenic: Containing a gene (transgene) from another species.
Translation: A process in which amino acids are assembled into a polypeptide chain (part or all of a protein) in a sequence determined by codons in a messenger RNA molecule; the second step in gene expression.
Transplantation (HLA) antigens: Antigens that cause a host-graft reaction that attacks transplanted tissue from another individual, causing tissue rejection.
Transpiration: Water loss from the leaves of plants by the escape of water vapor through the stomates.
Triglycerides: Lipids formed from glycerol and three fatty acid units at a time.
Tuberculosis: An infectious lung disease caused by the bacteria Mycoplasma tuberculosis.
Tumor: A solid mass of transformed cells that may also contain induced normal cells such as blood vessels.
Tumor initiator: An agent that begins the process of transformation by causing permanent changes in the DNA; mutagens and radiation are tumor initiators.
Tumor promoter: An agent that completes the process of cell transformation after the process is started by a tumor initiator; tumor promoters are not mutagenic by themselves but cause partly transformed cells to go into cell division.
Tumor suppressor genes: Genes whose protein products normally inhibit cell division.
Turgor: Fluid pressure that causes swelling and stiffness in plant cells and other cells that have cell walls.
Turner syndrome: A condition arising from a single unpaired X chromosome (XO), resulting (if untreated) in a sterile female with immature genitals, widely spaced breasts that do not develop fully, webbing of skin at the neck, and varying degrees of lowered intellectual functioning.
U
Unikonta: A major subgroup of Eukarya, including the animal kingdom, the fungi (Mycota), and other groups possessing either broad pseudopods or a posterior (trailing) flagellum or both.
Unsaturated fats: Lipids with one or more double bonds between their carbon atoms.
Utilitarian: A system of ethics in which the rightness or wrongness of an act is judged according to its consequences.
V
Vaccination see Immunization.
Variant CJD: see Creutzfeld-Jacob disease
Vascular plants: Plants containing tissues that efficiently conduct fluids from one part of the plant to another.
Vascular tissues: Tissues (xylem and phloem) that conduct fluids between different parts of plants.
Vector: An insect or other intermediary that transmits a disease organism. Also, in molecular biology, a plasmid or other DNA sequence that can be used to introduce a gene into a target cell.
Vein: A blood vessel carrying blood toward the heart.
Vesicles: Small, membrane-bounded sacs in which materials are transported between organelles of the endomembrane system within eukaryotic cells.
Vestigial structures: Organs reduced in size and nonfunctional, but often showing resemblance (by homology) to functional organs in related species.
Villi: Fingerlike processes such as those lining the inside of the small intestine.
Virulence: The ability of a microorganism to cause a disease.
Virulent: Capable of causing an infectious disease.
Virus: A particle of nucleic acid (DNA or RNA) enclosed in a protein coat that cannot replicate itself but that can cause a cell to replicate it.
Vitamin: Carbon-containing molecules needed in small amounts to facilitate certain chemical reactions in the body.
Vitamin deficiency disease: A disease caused by insufficient amounts of a vitamin and cured, in most cases, by adding the vitamin to the diet.
W
Water cycle. A global cycle in which atmospheric water vapor condenses into liquid and falls as rain or snow, then runs downhill into bodies of water such as lakes and oceans, from whose surface it evaporates into the atmosphere once again, supplemented also by leaf transpiration and other biological processes.
West Nile virus: A virus that can be transmitted to humans by mosquito vectors from a host reservoir in multiple species of birds.
White blood cells (leukocytes): The several types of blood cells that perform various protective (immune) functions but do not possess hemoglobin and do not carry oxygen.
Withdrawal: Physiological changes or unpleasant symptoms associated with the sessation of drug taking.
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Z
Zero population growth: A condition in which a population no longer changes size because its birth rate and death rate are equal.
Zika virus: A brain-damaging virus transmitted to humans by mosquitoes.
Zygote: A diploid fertilized egg, formed by the combining of a haploid sperm with a haploid egg during fertilization.