Life Science Dictionary T


T-cell receptor: The antigen receptor on T cells, a membrane-bound molecule consisting of one α chain and one β chain linked by a disulfide bond bridge and containing one antigen binding site.
T lymphocyte (T cell): A type of lymphocyte, including the helper T cell and cytotoxic T cells, that develop to maturity in the thymus. After encountering antigen, T-cells are responsible for cell-mediated immunity.
Tandem (DNA) repeats: A series of repeated sequences, arranged next to each other.
Tandem duplication: A duplication mutation in which the duplicated DNA is found next to the original DNA.
Taproot system: A root system common to eudicots, consisting of one large, vertical root that produces many smaller lateral, or branched, roots.
Taste buds: Collections of modified epithelial cells that are scattered in several areas of the tongue and mouth and are receptors for taste in humans.
TATA box: A promoter DNA sequence crucial in forming the transcription initiation complex.
Tautomerization: The spontaneous isomerization of a nitrogen base from its normal keto (or amino) form to an alternative hydrogen-bonding enol (or imino) form.
Taxis: Movement toward or away from a stimulus.
Taxon (plural: taxa): Taxon is a group of organisms that is recognized as a formal unit. For example, Homo sapiens is a taxon at the species level, while Homo is a taxon at the genus level.
Taxonomy: Classification of living organisms, also known as systematics.
Technology: The application of scientific knowledge for a specific purpose.
Telocentric: A chromosome with the centromere in the middle.
Telomerase: An enzyme complex found in most eukaryotes that maintains the lengths of telomeres through successive divisions. Telomerase catalyzes the lengthening of the telomeres. The enzyme includes a molecule of RNA that serves as a template for new telomere segments.
Telomere: The end of a chromosome. The protective structure at each end of a eukaryotic chromosome.
Telophase: The fifth and final stage of mitosis, in which daughter nuclei are forming and cytokinesis has typically begun.
Temperate broadleaf forest: A biome located throughout midlatitude regions where there is sufficient moisture to support the growth of large, broadleaf deciduous forest.
Temperate grassland: A terrestrial biome dominated by grasses and forbs.
Temperate phage: A phage that is capable of reproducing by either the lytic or lysogenic cycle.
Temperature: A measure of the intensity of heat in degrees, reflecting the average kinetic energy of the molecules.
Template strand: The DNA strand that provides the template for ordering the sequence of nucleotides in an RNA transcript.
Temporal summation: A phenomenon of neural integration in which the membrane potential of the postsynaptic cell in a chemical synapse is determined by the combined effect of ESPS and ISPS produced in rapid succession.
Tendon: A type of fibrous connective tissue that attaches muscle to bone.
Terminal bud: Embryonic tissue at the tip of a shoot, made up of developing leaves and a compact series of nodes and internodes.
Terminator: In prokaryotes, a special sequence of nucleotides in DNA that marks the end of a gene. It signals RNA polymerase to release the newly made RNA molecule, which then departs form the gene.
Territoriality: A behavior in which an animal defends a bounded physical space against encroachment by other individuals, usually of its own species. territory defense may involve direct aggression or indirect mechanisms such as scent marking or singing.
Tertiary consumer: A member of the trophic level of an ecosystem consisting of carnivores that eat mainly other carnivores.
Tertiary structure: Irregular contortions of a protein molecule due to interactions of side chains involved in hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.
Test: The hardened shell of some protists, including forams and radiolarians, or the rigid endoskeleton of sea urchins and dans dollar.
Testcross: Breeding of an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio of phenotypes in the offspring determines the unknown genotype.
Testis (plural testes):  The male reproductive organ, or gonad, in which sperm and reproductive hormones are produced.
Testosterone: The most abundant androgen hormone in the male body.
Tetanus:The maximal, sustain contraction of a skeletal muscle, cause by a very high frequency of action potentials elicited by continual stimulation.
Tetrad: A paired set of homologous chromosomes, each composed of two sister chromatids. Tetrads from during prophase I of meiosis.
Tetraploid: Carrying four copies of each chromosome gene.
Tetrapod: A vertebrate with two pairs of limbs, including mammals, amphibians, and birds, and other reptiles.
Thalamus: One of two integrating centers of the vertebrate forebrain. Neurons with cell bodies in the thalamus relay neural input to specific areas of the cerebral cortex and regulate what information goes to the cerebral cortex.
Thallus: A seaweed body that is plantlike but lacks true roots, stems, and leaves.
Theory: A set of interconnected hypotheses that leads to testable predictions.
Thermophile: An organism with a growth temperature optimum between 50-80 degree centigrade.
Thermocline: A narrow stratum of rapid temperature change in the ocean and in many temperate-zone lakes.
Thermodynamics: (1) The study of energy transformations that occur in a collection of matter. (2) A phenomenon in which external DNA is taken up by a cell and functions there.
Thermoreceptor: An interoreceptor stimulated by either cold or heat.
Thermoregulation: The maintenance of a particular temperature of the living body.
Theropod: A member of an ancient group of dinosaurs that were bipedal carnivores.
Thick filament: A filament composed of staggered arrays of myosin molecules; a component of myofibrils in muscle fibers.
Thigmomorphoganesis: A response in plants to chronic mechanical stimulation, resulting from increased ethylene production. An example is thickening stems in response to strong winds.
Thigmotropism: A directional growth of a plant in response to touch.
Thin filament: The smaller of two myofilaments consisting of two strands of actin and two strands of regulatory protein coiled around one another.
Thoracic cavity: The body cavity in mammals that houses the lungs and heart. It is surrounded in part by ribs and separated from the lower abdominal cavity by the diaphragm.
Threatened species: A species that is considered likely to become endangered in the foreseeable future.
Three-domain system: A system of taxonomic classification based on three super kingdoms: Bacteria, Archaea, and Eukaryotes.
Threshold: The potential an excitable cell membrane must reach for an action potential to be initiated.
Thrombus: A clump of platelets and fibrin that blocks the flow of blood through a blood vessel.
Thylakoid: A flattened membrane sac inside the chloroplast, used to convert light energy into chemical energy.
Thymus: A small organ in the thoracic cavity of vertebrates where maturation of T cells is completed.
Thyroid gland: An endocrine gland, locate din the ventral surface of the trachea, that secretes two iodine-containing hormones; triodothyronine (T3) and thyroxine (T4), and calcitonin.
Thyroid stimulating hormone (TSH): A tropic hormone produced and secreted by the anterior pituitary that regulates the release of thyroid hormones.
Thyroxine (T4): One of two iodine-containing hormones that are secreted by the thyroid gland and help regulate metabolism, development, and maturation in vertebrates.
Ti plasmid: A plasmid of a tumor-inducing bacterium that integrates a segment of its DNA into the host chromosome of a plant: frequently used as a carrier for genetic engineering in plants.
Tidal volume: The volume or air an animal inhales and exhales with each breath.
Tight junction: A type of intercellular junction in animal cells that prevents the leakage of material between cells.
Tip: The terminal node on a phylogenetic tree.
Tissue: An integrated group of cells with a common function, structure, or both.
Tissue system: One or more tissues organized into a functional unit connecting the organs of a plant.
Tonicity: The ability of a solution to cause a cell within it to gain or lose water.
Tonoplast: A membrane that encloses a central vacuole in a plant cell, separating the cytosol from the vacuolar contents, called cell sap. Also known as vacuolar membrane.
Top-down model: A model of community organization in which predation controls community organization because predators control herbivores, which in turn control plants, which in turn control nutrient levels; also called trophic cascade model.
Topoisomerase: A protein that functions in DNA replication, helping to relieve strain in the double helix ahead of the replication fork.
Topsoil: A mixture of particles derived from rock, living organisms, and humus.
Torpor: In animals, a physiological state that conserves energy by slowing down metabolism.
Torsion: A characteristic of gastropods in which the visceral mass rotates during development.
Totipotent: Refers to the ability of a cell to divide and differentiate into any cell type of both embryonic and extra-embryonic origin.
Trace element: An element indispensable for life but required in extremely minute amounts.
Trachea: The windpipe; that portion of the respiratory tube that has C-shaped cartilagenous rings and passes from the larynx to the bronchi.
Tracheal system: A gas exchange system of branched chitin-lined tubes that infiltrates the body and carry oxygen directly to cells in insects.
Tracheid: A long cell with strengthened walls that functions to transport fluid within plants in the xylem of all plants.
Trade-off: A situation where one trait cannot be increased without decreasing another. The term is used to describe constraints in optimization arguments.
Trait: characteristic.
trans: Arrangement of two non-carbon atoms each bound to one of the carbon atoms in a carbon-carbon double bond, where the two non-carbon atoms are on opposite sides relative to the double bond.
Transcription: Replication of an RNA strand complimentary to a DNA sequence.
Transcription factor: A molecule that binds to the promoter and regulates transcription of specific genes.
Transcription initiation complex: The completed assembly of transcription factors and RNA polymerase bound to the promoter.
Transcription unit: A region of a DNA molecule that is transcribe into an RNA molecule.
Transduction: (1) The movement of genes form a donor cell to a recipient cell with a virus as the vector. (2) In cellular communication, the conversion of a signal from outside of the cell to a form that can bring about a specific cellular response.
Transfer cell: A companion cell with numerous ingrowths of its wall, increasing the cell surface area and enhancing the transfer of solutes between apoplast and symplast.
Transfer RNA (tRNA): An RNA molecule that couples a specific amino acid to a specific sequence of three bases. It is responsible for translating the genetic code.
Transformation: The introduction of a fragment of DNA into a genome. Transformation occurs naturally in some bacteria and archaea; in the laboratory, it is the basis of genetic engineering.
Transgenic: Genetically manipulated to carry genes from another individual or species.
Transition: A mutation in which a purine replaces another purine or a pyrimidine replaces another pyrimidine.
Translation: Synthesis of protein with amino acid sequence encoded by an RNA sequence.
Translocation: (1) A rearrangement mutation in which part of one chromosome breaks away and joins another. (2) During protein synthesis, the third stage in the elongation cycle when the RNA carrying the growing peptide moves from the A site to the P site on the ribosome. (3) The transport of organic nutrients in the phloem of vascular plants.
Transmission: The conduction of impulses to the central nervous system.
Transmission electron microscope (TEM): A microscope that passes an electron beam through very thin sections; primarily used to study the internal ultrastructure of the cell.
Transmission disequilibrium test (TDT):  A statistical test that detects association between genetic markers and disease alleles by looking for marker alleles that are transmitted in excess to affected offspring.
Transpiration: The evaporative loss of water from a plant.
Transport epithelium: One or more layers of specialized epithelial cells that regulates solute movement.
Transport protein: A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.
Transport vesicle: A tiny membranous sac in a cell’s cytoplasm carrying molecules produced by the cell.
Transposable element: A genetic element that can move from on location in the genome to another by means of a DNA or RNA intermediate.
Transposon: A transposable element that moves within the genome by means of a DNA intermediate.
Transverse (T) tubules: Infoldings of the plasma membrane of skeletal muscle.
Transversion: A mutation in which a purine replaces a pyrimidine or vice versa.
Tree of life: A phylogenetic tree showing the relationship among all cellular organisms.
Trend: A general course or prevailing tendency. Line indicating upward or downward movement over a period of time.
Triacylglycerol: Three fatty acids linked to one glycerol molecule.
Trigonotarbid: Member of an order of extinct terrestrial spider-like animals (order Trigonotarbida).
Trilobite: An extinct arthropod  with promounced segmentation and appendages that varied little from segment to segment.
Trimerophyte: Member of an early group of vascular plants.
Triple response: A plant growth maneuver in response to mechanical stress, involving slowing of stem elongation, thickening of the stem, and a curvature that causes the stem the start growing horizontally.
Triplet code: A set of three-nucleotides-long words that specify the amino acids for polypeptide chains.
Tripoblastic: Possessing three germ layers; the endoderm, mesoderm, and ectoderm. Most eumetazoans are tripoblastic.
Triploid: Carrying three genomes.
Trisomy: Possession of three copies of one chromosome (in humans is abnormal because there are normally only two copies).
Tristyly: A polymorphism with three different arrangements of anther and stigma. It promotes outcrossing.
Trocophore: Larval type characteristic of many protostomes including annelids and many mollusks.
Trophic: Pertaining to nutrition, concerned to obtaining food.
Trophic efficiency: The percentage of production transferred from one trophic level to the next.
Trophic structure:  The different feeding relationship in an ecosystem which determined the route of energy flow and the pattern of chemical cycling.
Trophoblast: The outer epithelium of the blastocyst, which forms the fetal part of the placenta.
Tropical rain forest:  A terrestrial biome characterized by high levels of precipitation and warm temperatures year-round.
Tropic hormone:  A hormone that has another endocrine gland as a target.
Tropics: Latitudes between 23.5° north and south.
Tropism: A growth response that results in curvature of whole plant organs toward or away from stimulus owing to differential rates of cell elongation.
Tropomyosin: The regulatory protein that blocks the binding of myosin binding sites on the actin molecules.
Troponin complex: The regulatory proteins that control the position of tropomyosin on the thin filament.
True breeding: A population or individual that produces genetically identical offspring.
Truncation selection: Selection that eliminates those with the largest (or smallest) trait values.
Tube foot: One of numerous extensions of an echinoderm’s water vascular system; functions in locomotion, feeding, and gas exchange.
Tumor-suppressor gene: A gene whose protein products inhibits cell division, thereby preventing uncontrolled cell growth (cancer).
Tundra: A biome at the extreme limits of plant growth. At the northernmost limits, it is called arctic tundra, and at high altitudes, where plant forms are limited to low shrubby or matlike vegetation, it is called alpine tundra.
Tunicate: Member of the subphylum Urochordata sessile marine chordates that lack a backbone.
Turgid: Very firm. A walled cell becomes turgid if it has a greater solute concentration than its surroundings, resulting in entry of water.
Turgor pressure: The force directed against a cell wall after the influx of water and the swelling of a walled cell due to osmosis.
Turnover: The mixing of water as a result of changing water temperature profiles in a lake.
Tympanic membrane: Another name for the eardrum.
Tyrosine kinase: An enzyme that catalyzes the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein.