Life Science Dictionary A


A band: The broad region that correspond to the length of the thick filaments of myofibrils.
A site: One of a ribosome’s three binding sites for tRNA during translation. This site holds the tRNA carrying the amino acid to be added to the growing peptide.
ABC model: A model of flower formation identifying three classes of organ identity genes that direct formation of the the four types of floral organs.
Abdominal cavity: The body cavity in mammals that contains the digestive, excretory, and reproductive systems.
Abiotic: Nonliving
ABO blood groups:  Genetically determined classes of human blood that are based on the presence or absence of carbohydrates A and B on the surface of the red blood cells. The ABO phenotypes are types A, B, AB, and O.
Abortion: The termination of a pregnancy in progress.
Abscisic acid (ABA): A plant hormone that slows down growth (often antagonizing actions of growth hormones), promote dormancy, and facilitates drought tolerance.
Absorption: The uptake of small nutrient molecules by an organism. The third main stage of food processing, following digestion.
Absorption spectrum: The range of a pigment’s ability to absorb various wavelengths of light.
Abyssal: Referring to the very deep benthic zone of the ocean.
Acanthodian: Any of a group of ancient jawed fishes from the Devonian period.
Acclimatization: Physiological adjustment to a change in an environmental factor.
Accommodation: The automatic adjustment of an eye to focus on near objects.
Acetyl CoA: Acetyl coenzyme A; the entry compound for the citric acid cycle in cellular respiration, formed from a fragment of pyruvate attached to a coenzyme.
Acetylcholine: A common neurotransmitter that functions by binding to receptors and altering the permeability of the postsynaptic membrane to specific ion, either depolarizing or hyperpolarizing the membrane.
Acid: A substance that increases the concentration of hydrogen ions of a solution.
Acid chyme: A mixture of recently swallowed food and gastric juice.
Acid precipitation: Rain, snow, or fog that is more acidic than pH 5.6.
Acoelomate: A solid-bodied animal lacking a cavity between the gut and outer body wall.
Acquired immunity: The kind of defense that is mediated by B lymphocytes (B cells) and T lymphocytes (T cells). It exhibits specificity and non-self recognition. Also know as Adaptive immunity.
Acritarch: an organic-walled ancient microfossil, interpreted as the cyst of a eukaryote.
Acrocentric: A chromosome with the centromere close to one end.
Acrosomal reaction: The discharge of a sperm’s acrosome when the sperm approaches an egg.
Acrosome: A vesicle at the tip of a sperm cell that helps the sperm penetrate the egg.
Actin: A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.
Action potential: A rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.
Action spectrum: A graph that depicts the relative effectiveness of different wavelengths of radiation in driving a particular process.
Activator: A protein that binds to DNA and stimulates transcription of a specific gene.
Active immunity: Long-lasting immunity conferred by the action of a person’s B cells and T cells, and the resulting B and T memory cells specific for a pathogen. This immunity can result from natural infection or immunization.
Active site: The specific portion of an enzyme that attaches to the substrate by means of weak chemical bonds.
Active transport: The movements of a substance across a biological membrane against its concentration or electrochemical gradient with the help of energy input and specific transport proteins.
Actual evapotranspiration: The amount of water annually transpired by plants and evaporated from a landscape, usually measured in millimeters.
Adaptation: any alteration in the structure or function of an organism by which the organism becomes better fitted to survive and multiply in its environment.
Adaptive peak: A local maximum in the fitness landscape.
Adaptive radiation: Diversification of a single lineage into species that exploit diverse ecological niches.  The emergence of numerous species from a common ancestor introduced into an environment that presents a diversity of new opportunities and challenges.
Adenohypophysis: see anterior pituitary.
Adenyl cyclase: An enzyme that converts ATP to cyclic AMP in response to a chemical signal.
Additive genetic variance: The total variance contributed by the additive effects of each gene.
Additive tree: A phylogenetic tree in which the branch lengths are proportional to the evolutionary distance between nodes; also known as phylogram.
Adhesion: The attraction between different kind of molecules.
ADH: the enzyme alcohol dehydrogenase.
Adipose tissue: A connective tissue that insulates the body and serves as a fuel reserve; contains fat-storing cells called adipose cells.
Adrenal gland: One of two endocrine glands adjacent to the kidneys in mammals. The outer portion, the cortex, responds to the ACTH hormone and helps maintaining homeostasis during long-term stress; and the central part, the medulla,  secretes epinephrine and norepinephrine in response to short-term stress.
Adrenocorticotrophic hormone (ACTH): A tropic hormone produced and secreted by the anterior pituitary that stimulates the production and secretion of steroid hormones by the adrenal cortex.
Adventitious: A term describing any plant organ that grows in an atypical location such as roots growing from stems.
Aeciospora: Binucleated infective spore produced by a rust fungus.
Aerobic: Containing oxygen; referring to an organism, environment, or cellular process that requires oxygen.
Afferent arteriole: The blood vessel supplying a nephron.
Agamogenesis: Type of sexual reproduction where only female gametes are produce i.e., no male gametes are involved
Age structure: The relative number of individuals of each age in a population.
Aggregate fruit: A fruit derived from a single flower that has more than one carpel.
Agonistic behavior: A type of behavior involving a contest of some kind that determines which competitor gains access to some resource, such as food or mates.
AIDS (acquired immunodeficiency syndrome):  The name of the late stages of HIV infection, defined by a specific reduction of T cells, and the appearance of characteristic secondary infections.
Air spora: The mixture of spores found in the air.
Aging: A decrease in survival or reproduction with age. It is equivalent to senescence.
Alcohol fermentation: The conversion of pyruvate to carbon dioxide and ethyl alcohol.
Aldosterone: An adrenal hormone that acts on the distal tubules of the kidney to stimulate the reabsorption of sodium (Na+) and the passive flow of water from the filtrate.
Alga (plural algae):  A photosynthetic plant-like protist.
Align: to line up different DNA, RNA, or protein sequences.
Alimentary canal: A digestive tract consisting of a tube running between a mouth and an anus.
Allantois: One of four extra-embryonic membranes; serves as a repository for the embryo’s nitrogenous waste.
Allele: any of several forms of a gene, arising through mutation, that are responsible for genetic variability.
Allele frequency: The frequency of a particular allele in a population.
Allelic: two variants are allelic if they are alleles at the same genetic locus.
Allometry: Change in proportions with body size. For example, the bigger the body size of a tag, the bigger the antlers.
Allopatric speciation: The formation of reproductively isolated species due to the divergence of populations that are geographically isolated from each other.
Allopatry: The complete separation of populations by geographical barriers.
Allopolyploid: A polyploid in which the multiple genomes are derived from different populations of species.
Allostery: A change of the shape of a protein or ribozyme due to binding of a molecules at one site, which then change activity at a distant site.
Allozyme: One of several variant forms of an enzyme coded by alternative alleles of a single genetic locus.
α-helix: Common structural motif of proteins in which a linear sequence of amino acids fold into a right-hand helix stabilized by internal hydrogen bonding between backbone atoms.
α-proteobacteria: A major class of bacteria that includes many photosynthetic species, many pathogens, and many mutualistic symbionts, including the ancestors of mitochondria.
Alertness: the state of being watchful and prompt to meet danger or emergency. Quick to perceive and act.
Alternative splicing: The process by which the initial RNA made from a single gene can be spliced into different mature messenger RNAs, which in turn produce different proteins.
Altruistic: A gene, trait, or behavior that reduces the fitness of its bearer but increases the fitness of other individuals.
Alveolate: Member of the Alveolata, one of the major kingdoms of eukaryotes. It includes apicomplexans, dinoflagellates, and cilates.
Alzheimer’s disease: An age-related dementia characterized by confusion, memory loss, and other symptoms.
Amacrine cell: A neuron of the retina that helps integrate information before it is send to the brain.
Amelioration: The process in which DNA that has been acquired by lateral gene transfer changes in composition (G/C content or codon usage) to resemble the genome in which resides.
Amino acid: An organic molecule possessing both a carboxyl and an amino groups and serve as monomers of proteins.
Amino group: A functional group that consist of a nitrogen atom bonded to two hydrogen atoms. It can act as a base in solution and accepting a hydrogen ion and acquiring a charge of +1.
Aminoacyl-tRNA synthetase:  An enzyme that joins each amino acid to the correct tRNA.
Ammonia: A small, very toxic molecule made up on one nitrogen atom bond to four hydrogen atoms. Produced by nitrogen fixation and as metabolic waste product of protein and nuclei acid metabolism.
Ammonite: A shelled cephalopod that was  the dominant invertebrate predator for million of years at the end of the Cretaceous period.
Amniocentesis: removal of amniotic fluid that surrounds the embryo. The fluid contains fetal cells that can be used for prenatal diagnosis.
Amnion: The innermost of four extra-embryonic membranes;  encloses a fluid-filled sac in which the embryo is suspended.
Amniote: Member of a clade of tetrapods that have an amniotic egg containing specialized membranes that protects the embryo. Includes mammals, birds, and some reptiles.
Amoeba: A single cell eukaryote that has no fixed shape, characterized by the presence of pseudopodia. This phenotype is found in many eukaryotic lineages.
Amoebozoa: One of the major kingdoms of eukaryotes. Most species are heterotrophic. Include species that do not have mitochondria.
Amphibian: Member of the tetrapod class Amphibia, including salamanders, frogs, and caecilians.
Amphipathic molecule: A molecule that has both hydrophobic and hydrophilic components like the phospholipids that make up membranes.
Amplification: The strengthening of stimulus energy that is otherwise too weak to be carried into the nervous system.
Amyloid: An insoluble polymer formed by stacking of β-sheets. Amyloids have either physiological or pathological functions.
Anabolic pathway: A metabolic pathway that synthesizes a complex molecule from simpler compounds.
Anaerobic: Laking oxygen referring to an organism, environment, or cellular process that lacks oxygen and may be poison by it.
Anaerobic respiration:  The use of inorganic molecules other than oxygen to accept electrons at the “downhill” end of electron transport chains.
Analogy: Similarity  between two species that is not due to homology (common ancestry) but to convergent evolution.
Anamorph: The form of a fungus produced in its asexual phase.
Anaphase: The fourth stage of mitosis in which the chromatids of each chromosome have separated and the daughter chromosomes are moving to the poles of the cell.
Anaphylactic shock: An acute, whole-body, life-threatening, allergic response.
Anastomosis: fusion between hyphae.
Anatomy: The study of the structure of an organism.
Anchorage dependence: The requirement that to divide, a cell must be attached to the substratum.
Ancestral characteristics: A trait found in both an organism being study and the common ancestor of a group to which the study organism belong.
Androgen: Any steroid hormone, such as testosterone, that stimulates the development and maintenance of the male reproductive system and secondary sex characteristics.
Aneuploid: A cell or organism having an abnormal set of chromosomes.
Aneuploidy: A chromosomal aberration in which one or more chromosomes are present in extra copies or deficient in number.
Angiosperms: A flowering plant which form seeds inside a protective chamber called ovary.
Angiotensin II: A hormone that stimulates constriction of precapillary arterioles and increase reabsorption of NaCl and water by the proximal tubules of the kidney, increasing blood pressure and volume.
Anhydrobiosis: The ability to survive in a dormant state when an organisms’ habitat dries up.
Animal pole: The portion of the egg where the least yolk is concentrated; opposite to vegetal pole.
Animalia: The kingdom that consists of multicellular eukaryotes that ingest their food.
Anion: A negatively charged ion.
Anisogamy: Differentiation of gametes into two or more sizes.
Annelids: Member of Annelida, a phylum within the Lophotrochozoa. This group of segmented worms includes earthworms, leeches, and bristle worms.
Annual: A flowering plant that completes its entire life cycle in a single year or growing season.
Antagonistic pleiotropy: describes alleles that increases on fitness component but decreases another.
Anterior: Referring to the head end of a bilaterally symmetrical animal.
Anterior pituitary: Also called adenohypophysis. Portion of the pituitary that develops from nonneural tissue, consists of endocrine cells that synthesize and secrete several tropic and nontropic  hormones.
Anther: The male reproductive organ where pollen is produced in a flowering plant.
Antheridium: In plants and fungi, the male gametangium; a moist chamber where the gamete develops.
Anthropoid: A member of  a primate group made up of the humans, monkeys, and apes.
Antibody: A protein secreted by differentiated plasma cells (B cells) that binds to a specific antigen and marks it for elimination.
Anticodon: A specialized base triplet at one end of a tRNA molecule that recognizes a particular complimentary  codon on an mRNA molecule.
Antidiuretic hormone (ADH): A hormone produced by the hypothalamus and released from the posterior pituitary. It promotes water retention by the kidneys and helps regulate the osmolarity of the blood.
Antigen: A chemical that triggers an immune response by binding a specific antibody.
Antigen receptor: A general term for a surface protein located on B or T cells that bind antigens and initiates an acquired immune response.
Antigen presenting cell:  A cell that ingest bacteria and viruses and destroy them, generating peptide fragments that are bound by class II MHC molecules and subsequently displayed on the cell surface of helper T cells. Macrophages, dendritic cells, and B cells and the primary antigen-presenting cells.
Antimutator: An allele or genotype that reduces mutation rates.
Antiparallel: The opposite arrangement of the sugar-phosphate backbone in a DNA double helix.
Antisense oligonucleotide: A short synthetic nucleic acid sequence that is complementary to an RNA sequence.
Aphotic zone: The part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
Apical dominance: Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
Apical ectodermal ridge: A limb-bud organizer region consisting of a thickened area of ectoderm at the tip of the limb bud.
Apical meristem: Embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length.
Apicomplexan: Member of the Apicomplexa, a phylum of eukaryotes, in the alveolate kingdom. Many species in this phylum are parasites like Plasmodium falciparum
Apicoplast: An organelle found in apicomplexan species that is involved in metabolic processes. This organelle is derived from photosynthetic chloroplast but have lost the ability photosynthesize.
Apomixis: The asexual reproduction in plants where fertilization is absent i.e., the female gamete develops without fertilization.
Apoplast: In a fungus cell, the space outside the plasma membrane. In plants, the continuum of cell walls plus the extracellular spaces.
Apoptosis: The changes that occur within a cell as it undergoes programmed cell death, which is brought about by signals that trigger the activation of a cascade of suicide proteins in the cell destined to die.
Aposematic coloration: The bright coloration of animals with effective physical or chemical defense that acts as a warning to predators.
Apothecium: Cup-shaped fruit body of some ascomycetes.
Appendix: A small, fingerlike extension of the vertebrate cecum, contains a mass of white blood cells that contribute to immunity.
Appresorium: Adhesive pad formed by a pathogenic fungus on the surface of its host to aid penetration.
Aquaporin: A transport protein in the plasma membrane of a plant or animal cell that specifically facilitates the diffusion of water across the membrane.
Aqueous humor: Plasma-like liquid in the space between the lens and the cornea in the vertebrate eye. It helps maintain the shape of the eye, supplies nutrients and oxygen to its tissues, and dispossed of its waste.
Aqueous solution: A solution in which water is the solvent.
Apex predators: predators that feed on other organisms in their ecosystem, but there are no predators that feed on them.
Arbuscular mycorrhiza: A mycorrhiza produced by species of Glomeromycota, in which highly branched haustorial structures are formed within host cells.
Archaea: One of the three domains of life. These species have no nuclei and thus were group with bacteria as ‘prokaryotes’ before rRNA sequences determine the three domains of life.
Archaic human: A hominid that does not have the modern anatomy of Homo sapiens.
Archegonium: In plants, the female gametangium, a moist chamber where gametes develop.
Archenteron: The endoderm-lined cavity, formed during the gastrulation process, that develops into the digestive tract of an animal.
Archipelago: an expanse of water with many scattered islands. A group of islands.
Archosaur: Member of the reptilian group that includes crocodiles, alligators, dinosaurs, and birds.
Arteriole: A vessel that coveys blood between an artery and a capillary bed.
Artery: A vessel that carries blood away from the heart to organs throughout the body.
Arthropod: Member of Arthropoda, a major phylum within the ecdysozoa. This group is characterized by jointed legs, include the insects, centipedes, millipedes, spiders, and crustaceans.
Arthrospore: Spore formed by breakage of a length of mycelium into segments.
Artificial selection: The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits.
Ascocarp: A structure bearing asci, general term for the fruit body of an ascomycete.
Ascogenous hypha: The dikaryotic hypha emerging from an ascogonium after fertilization, which gives rise to the asci in ascomycetes.
Ascogonium: Cell of ascomycete protoperithecium that takes part in fertilization.
Ascopore: Sexual spore of ascomycetes.
Ascus: The microscopic sac containing ascopores in ascomycetes.
Asexual reproduction: A type of reproduction involving only one parent that produce genetically identical offspring by budding or by the division of a single cell or the entire organism into two or more parts.
Aspartate: An amino acid that functions as a CNS neurotransmitter.
Assisted reproductive technology (ART): Fertilization procedures that involve the surgical removal of eggs (secondary oocytes) from a woman’s ovaries after hormonal stimulation, fertilizing the eggs, and returning them to the woman’s body.
Association studies: A survey of associations between genetic markers and quantitative traits that aim to locate the QTLs that cause trait variation.
Associative learning: The acquired ability to associate one stimulus with another, also called classical conditioning.
Aster: A radial array of short microtubules that extend from each centrosome toward the plasma membrane in a cell undergoing mitosis.
Astrocyte: A glial cell that provides structural and metabolic support for neurons.
Atherosclerosis: A cardiovascular disease in which growth called plaques develops in the inner walls of the arteries, narrowing their inner diameter.
Atom: The smallest unit of matter that retains the properties of an element.
Atomic mass: The total mass of an atom, which is the mass in grams of one mole of the atom.
Atomic nucleus: An atom central core containing protons and neutrons.
Atomic number: The number of protons in the nucleus of an atom, unique for each element and designated by a subscript to the left of the elemental symbol.
ATP (adenosine triphosphate): An adenine-containing nucleoside triphosphate that releases free energy when its phosphate bonds are hydrolyzed.
ATP synthase: A cluster of several membrane proteins found in the mitochondria crista (and bacterial plasma membrane), that function in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion concentration gradient to make ATP. ATP synthases provide a port through which hydrogen ions diffuse into the matrix of a mitochondria.
Atrial natriuretic factor: A peptide hormone that opposes the renin-angiotensin-aldosterone system (RAAS).
Atrioventricular (AV) node: A region of specialized muscle tissue between the right atrium and right ventricle where electrical impulses are delayed for 0.1 second before spreading to the ventricles and cause them to contract.
Atrium (plural atria): A camber that receives blood returning to the vertebrate heart.
Autocatalytic network: A chemical system that outputs a chemical that is a catalyst for the original reaction.
Autoimmune disease: An immunological disorder in which the immune system turns against itself.
Autoinducer: A chemical used in quorum sensing that is secreted by cells and then used to quantify cell density.
Autoinduction: The induction of a regulatory cascade in quorum sensing. It is triggered in response to the crossing of a threshold concentration of autoinducer in the environment.
Automatic selection: The increase in frequency on an allele that increase the rate of selfing.
Autonomic nervous system: Subdivision of the motor nervous system of vertebrates that regulates the internal environment, consist of the sympathetic, parasympathetic, and enteric divisions.
Autophagy: A process in which cells digest their own components or invading microorganisms with the aid of lysosomes. The breakdown products can be recycled for essential cellular processes
Autoplolyploid: A polyploid that carries multiple genomes derived from within the same population.
Autosome: A chromosome that is not directly involved in determining sex, in contrast to sex chromosomes and mDNA.
Autotroph: An organism that obtains organic food molecules without eating other organisms or substances derived from other organisms. Autotrophs use energy from the sun or from the oxidation of inorganic substances to make organic molecules from inorganic ones.
Auxin: Refers to indoleacetic acid (IAA), a natural plant hormone that promotes cell elongation, root formation, secondary growth, and fruit growth.
Average effect: The effect of an allele of a quantitative trait as estimated by regression.
Average heterozygosity: The percentage, on average, of a population loci that are heterozygous in members of the population.
Avirulent: Describes a pathogen that can only mildly harm, but not kill, the host plant.
Axenic: In the absence of contamination, used to described a pure culture.
Axillary bud: A structure located at the angle formed between a leaf and a stem, that has the potential to form a lateral shoot, or branch.
Axon: A typically long extension, or process, from a neuron that carries nerve impulses away from the cell body toward target cells.
Axon hillock: The conical region of a neuron’s axon where it joins the cell body, typically where the nerve signals are generated.