T cell • A type of lymphocyte, involved in the cellular immune response. The final stages of its development occur in the thymus gland. (Contrast with B cell; see also cytotoxic T cell, helper T cell, suppressor T cell.)
T cell receptor • A protein on the surface of a T cell that recognizes the antigenic determinant for which the cell is specific.
T tubules • A system of tubules that runs throughout the cytoplasm of muscle fibers, through which action potentials spread.
Target cell • A cell with the appropriate receptors to bind and respond to a particular hormone or other chemical mediator.
Taste bud • A structure in the epithelium of the tongue that includes a cluster of chemoreceptors innervated by sensory neurons.
TATA box • An eight-base-pair sequence, found about 25 base pairs before the starting point for transcription in many eukaryotic promoters, that binds a transcription factor and thus helps initiate transcription.
Taxis (tak´ sis) [Gr. taxis: arrange, put in order] • The movement of an organism in a particular direction with reference to a stimulus. A taxis usually involves the employment of one sense and a movement directly toward or away from the stimulus, or else the maintenance of a constant angle to it. Thus a positive phototaxis is movement toward a light source, negative geotaxis is movement upward (away from gravity), and so on.
Taxon • A unit in a taxonomic system.
Taxonomy (taks on´ oh me) [Gr. taxis: arrange, classify] • The science of classification of organisms.
Telomeres (tee´ lo merz) [Gr. telos: end] • Repeated DNA sequences at the ends of eukaryotic chromosomes.
Telophase (tee´ lo phase) [Gr. telos: end] • The final phase of mitosis or meiosis during which chromosomes became diffuse, nuclear envelopes reform, and nucleoli begin to reappear in the daughter nuclei.
Template • In biochemistry, a molecule or surface upon which another molecule is synthesized in complementary fashion, as in the replication of DNA. In the brain, a pattern that responds to a normal input but not to incorrect inputs.
Template strand • In a stretch of double-stranded DNA, the strand that is transcribed.
Temporal summation • In the production or inhibition of action potentials in a postsynaptic neuron, the interaction of depolarizations or hyperpolarizations produced by rapidly repeated stimulation of a single point.
Tendon • A collagen-containing band of tissue that connects a muscle with a bone.
Terrestrial (ter res´ tree al) [L. terra: earth] • Pertaining to the land. (Contrast with aquatic, marine.)
Territory • A fixed area from which an animal or group of animals excludes other members of the same species by aggressive behavior or display.
Tertiary structure • In reference to a protein, the relative locations in three-dimensional space of all the atoms in the molecule. The overall shape of a protein. (Contrast with primary, secondary, and quaternary structures.)
Test cross • A cross of a dominant-phenotype individual (which may be either heterozygous or homozygous) with a homozygous-recessive individual.
Testis (tes´ tis) (plural: testes) [L.: witness] • The male gonad; that is, the organ that produces the male sex cells.
Testosterone (tes toss´ tuhr own) • A male sex steroid hormone.
Tetanus [Gr. tetanos: stretched] • (1) In physiology, a state of sustained, maximal muscular contraction caused by rapidly repeated stimulation. (2) In medicine, an often-fatal disease ("lockjaw") caused by the bacterium Clostridium tetani.
Thalamus • A region of the vertebrate forebrain; involved in integration of sensory input.
Thallus (thal´ us) [Gr.: sprout] • Any algal body which is not differentiated into root, stem, and leaf.
Theory • An explanation or hypothesis that is supported by a wide body of evidence. (Contrast with hypothesis, paradigm.)
Thermoneutral zone • The range of temperatures over which an endotherm does not have to expend extra energy to thermoregulate.
Thermoreceptor • A cell or structure that responds to changes in temperature.
Thoracic cavity • The portion of the mammalian body cavity bounded by the ribs, shoulders, and diaphragm. Contains the heart and the lungs.
Thorax • In an insect, the middle region of the body, between the head and abdomen. In mammals, the part of the body between the neck and the diaphragm.
Thrombin • An enzyme that converts fibrinogen to fibrin, thus triggering the formation of blood clots.
Thrombus (throm´ bus) [Gk. thrombos: clot] • A blood clot that forms within a blood vessel and remains attached to the wall of the vessel. (Contrast with embolus.)
Thylakoid • A flattened sac within a chloroplast. The membranes of the numerous thylakoids contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.
Thymine • A nitrogen-containing base found in DNA.
Thymus • A ductless, glandular portion of the lymphoid system, involved in development of the immune system of vertebrates.
Thyroid [Gr. thyreos: door-shaped] • A two-lobed gland in vertebrates. Produces the hormone thyroxin.
Thyrotropic hormone • A hormone that is produced in the pituitary gland of amphibia such as frogs and transported in the bloodstream to the thyroid gland, inducing the thyroid gland to produce the thyroid hormone that regulates metamorphosis from tadpole to adult frog.
Tight junction • A junction between epithelial cells, in which there is no gap whatever between the adjacent cells. Materials may get through a tight junction only by entering the epithelial cells themselves.
Tissue • A group of similar cells organized into a functional unit and usually integrated with other tissues to form part of an organ such as a heart or leaf.
Tonus • A low level of muscular tension that is maintained even when the body is at rest.
Totipotency • In a cell, the condition of possessing all the genetic information and other capacities necessary to form an entire individual.
Toxigenicity [L. toxicum: poison] • The ability of a bacterium to produce chemical substances injurious to the tissues of the host organism.
Trachea (tray´ kee ah) [Gr. trakhoia: a small tube] • A tube that carries air to the bronchi of the lungs of vertebrates, or to the cells of arthropods.
Tracheid (tray´ kee id) • A distinctive conducting and supporting cell found in the xylem of nearly all vascular plants, characterized by tapering ends and walls that are pitted but not perforated.
Tracheophytes [Gr. trakhoia: a small tube + phyton: plant] • Those plants with xylem and phloem, including psilophytes, club mosses, horsetails, ferns, gymnosperms, and angiosperms. (Contrast with nontrachoephytes.)
Trait • One form of a character: Eye color is a character; brown eyes and blue eyes are traits.
Transcription • The synthesis of RNA, using one strand of DNA as the template.
Transcription factors • Proteins that assemble on a eukaryotic chromosome, allowing RNA polymerase II to perform transcription.
Transduction • (1) Transfer of genes from one bacterium to another, with a bacterial virus acting as the carrier of the genes. (2) In sensory cells, the transformation of a stimulus (e.g., light energy, sound pressure waves, chemical or electrical stimulants) into action potentials.
Transfection • Uptake, incorporation, and expression of recombinant DNA.
Transfer cell • A modified parenchyma cell that transports solutes from its cytoplasm into its cell wall, thus moving the solutes from the symplast into the apoplast.
Transfer RNA (tRNA) • A category of relatively small RNA molecules (about 75 nucleotides). Each kind of transfer RNA is able to accept a particular activated amino acid from its specific activating enzyme, after which the amino acid is added to a growing polypeptide chain.
Transformation • Mechanism for transfer of genetic information in bacteria in which pure DNA extracted from bacteria of one genotype is taken in through the cell surface of bacteria of a different genotype and incorporated into the chromosome of the recipient cell.
Transgenic • Containing recombinant DNA incorporated into its genetic material.
Translation • The synthesis of a protein (polypeptide). This occurs on ribosomes, using the information encoded in messenger RNA.
Translocation • (1) In genetics, a rare mutational event that moves a portion of a chromosome to a new location, generally on a nonhomologous chromosome. (2) In vascular plants, movement of solutes in the phloem.
Transpiration [L. spirare: to breathe] • The evaporation of water from plant leaves and stem, driven by heat from the sun, and providing the motive force to raise water (plus ions) from the roots.
Transposable element • A segment of DNA that can move to, or give rise to copies at, another locus on the same or a different chromosome.
Triglyceride • A simple lipid in which three fatty acids are combined with one molecule of glycerol.
Triplet • See codon.
Triplet repeat • Occurrence of repeated triplet of bases in a gene, often leading to genetic disease, as does excessive repetition of CGG in the gene responsible for fragile-X syndrome.
Triploblastic • Having three cell layers. (Contrast with diploblastic.)
Trisomic • Containing three, rather than two members of a chromosome pair.
tRNA • See transfer RNA.
Trochophore (troke´ o fore) [Gr. trochos: wheel + phoreus: bearer] • The free-swimming larva of some annelids and mollusks, distinguished by a wheel-like band of cilia around the middle, and indicating an evolutionary relationship between these two groups.
Trophic level • A group of organisms united by obtaining their energy from the same part of the food web of a biological community.
Tropic hormones • Hormones of the anterior pituitary that control the secretion of hormones by other endocrine glands.
Tropism [Gr. tropos: to turn] • In plants, growth toward or away from a stimulus such as light (phototropism) or gravity (gravitropism).
Tropomyosin (troe poe my´ oh sin) • A protein that, along with actin, constitutes the thin filaments of myofibrils. It controls the interactions of actin and myosin necessary for muscle contraction.
Troposphere • The atmospheric zone reaching upward approximately 17 km in the tropics and subtropics but only to about 10 km at higher latitudes. The zone in which virtually all the water vapor in the atmosphere is located.
Trypsin • A protein-digesting enzyme. Secreted by the pancreas in its inactive form (trypsinogen), it becomes active in the duodenum of the small intestine.
T-tubules • A set of transverse tubes that penetrates skeletal muscle fibers and terminates in the sarcoplasmic reticulum. The T-system transmits impulses to the sacs, which then release Ca2+ to initiate muscle contraction.
Tube nucleus • In a pollen tube, the haploid nucleus that does not participate in double fertilization. (Contrast with generative nucleus.)
Tubulin • A protein that polymerizes to form microtubules.
Tumor • A disorganized mass of cells, often growing out of control. Malignant tumors spread to other parts of the body.
Tumor suppressor genes • Genes which, when homozygous mutant, result in cancer. Such genes code for protein products that inhibit cell proliferation.
Twitch • A single unit of muscle contraction.
Tympanic membrane [Gr. tympanum: drum] • The eardrum.