Biology Dictionary – T

Meanings of biological terminology

EUGENE M. MCCARTHY, PHD GENETICS

Definitions:

t abbr. (1) time; (2) temperature; (3) three times (abbreviation for Latin ter)

T Symbol for (1) thymine; (2) threonine.

tab. abbr. Tablet (abbreviation used in medical prescriptions).

Tabanidae /tə-BAN-ə-dee/ n. An insect family including the horseflies, deer flies, and gadflies. They are significant disease vectors because they feed on the blood of humans and other warm-blooded animals. PICTURE

tabes /TAY-beez/ n. A gradual wasting away (occurring in any disease).

Taenia /TEEN-ee-yə or TEEN-yə/ n. A genus that includes two important human parasites, the pork tapeworm, Taenia solium, and to the beef tapeworm, Taenia saginata. MORE INFORMATION

taiga /TYE-gə/ n. Coniferous forest with thin, acidic soil, a biome characteristic of the northern regions of Eurasia and North America, where summers are short and winters, long and cold. MAP | PICTURE

talofibular /TAL-ō-FIB-yə-ler/ adj. Pertaining to the talus and fibula.

talus (pl tali) /TAY-ləs, pl TAY-lī/ n. The ankle bone or astragalus. It articulates distally with the calcaneus and navicular bone of the tarsus and proximally with the fibula and tibia.

tandem repeats (also tandem duplications) Multiple copies of the same nucleotide base sequence lying in series; often used as markers in physical mapping.

tapeworm See: Taenia.

taphonomy /tah-FAWN-ə-mee/ n. The study of the processes by which living organisms are converted to fossils — taphonomic /TAPH-ə-nōm-ik/

target organ The organ upon which a given hormone has an effect.

tardigrade (also water bear) /TARD-ə-grade/ n. A microorganism of the phylum Tardigrada. MORE INFORMATION

targeted mutagenesis Intentional alteration of a gene, usually to determine the targeted gene's function.

tarsals The bones of the tarsus.

tarsotibial Pertaining to the tarsus and the tibia.

tarsus The portion of the foot between the leg and metatarsus.

Taung Child A famous specimen of Australopithecus africanus discovered by Raymond Dart (MORE INFORMATION).

tautomerism (also dynamic isomerism) A characteristic of certain chemical compounds, where two or more isomers that readily change into each other so that the compound exists only as a mixture of the two.

taxon A taxonomic group, of whatever rank.

taxonomy /TAX-ahn-ə-mee/ (1) the activity of classifying organisms; (2) a classification of living things. — taxonomic adj.

TB Tuberculosis.

tbsp Tablespoon.

TBSV Tomato bushy stunt virus.

TCA (1) trichloroacetic acid; (2) tricarboxylic acid.

T cell (or T lymphocyte) A type of lymphocyte that differentiates in the thymus (the T in T cell stands for thymus); responsible for cellular immunity. T-CELL ACTIVATION

teleology /tee-lee-AWL-ə-jee/ n. The use of design, purpose, or goals in the explanation of the existence of phenomena, particularly natural phenomena thought of as being produced by evolution. Although explanations of this type are unscientific (since they invoke supernatural, planning forces), scientists commonly speak in teleological terms (e.g., "Teeth are for chewing"; "Carnivores evolved acute sensory systems to facilitate capture of prey") — teleological /tee-lee-ə-LAW-jə-kəl/

telocentric chromosome /tell-ə-SEHN-trick/ n. A chromosome having its centromere in a terminal position.

telomerase /tə-LAHM-er-aze/ n. An ribonucleoprotein enzyme that adds individual bases to produce repeating units of one DNA strand at the telomere.

telomere /TELL-ə-mere/ n. A DNA sequence composed of simple repeats at the end of a eukaryotic chromosome. MORE INFORMATION

telophase /TELL-ə-phaze/ n. (1) The fourth stage of mitosis (MORE INFORMATION); (2) The fourth stage of meiosis I (TELOPHASE I) or of meiosis II (TELOPHASE II).

TEM Transmission electron microscope.

template /TEM-plət/ n. A molecule used as a pattern for the synthesis of another molecule.

Temple of Nature A poem written by Erasmus Darwin (1731-1802). It presents, in rhymed verse, the author's vision of Earth's creation, the initial spontaneous generation of microorganisms, and their eventual diversification and elaboration over evolutionary time into complex multicellular organisms. Published in 1803, it had an important influence on the thought of Erasmus' grandson Charles Darwin. TEXT OF POEM

temporal /TEMP-er-əl/ adj. Of or pertaining to a temporal lobe or temporal bone.

temporal bones /TEMP-er-əl/ n. A symmetrical pair of bones, which lie on either side and at the base of the skull. PICTURE 1 | PICTURE 2 | ROTATING VIEW

temporal lobe /TEMP-er-əl/ n. A lobe of the brain underlying either the left or the right temporal bone. ROTATING VIEW OF LEFT TEMPORAL LOBE

tendon /TEN-dən/ adj. A fibrous band of tissue connecting muscle to bone.

teratogenic /TER-ə-tō-JEN-ik/ adj. Causing abnormal development.

teratology /TER-ə-TAWL-ə-jee/ n. The study of developmental abnormalities.

Ternate Paper Name commonly used to refer to an 1858 paper by Alfred Russel Wallace (entitled "On the Tendency of Varieties to Depart Indefinitely from the Original Type") in which he describes in detail a theory of evolution by natural selection identical in all important respects to that later presented by Charles Darwin in the Origin of Species (1859), and worked out independently of Darwin. It was written while Wallace was collecting specimens on the island of Ternate, now in Indonesia.

terpenes /TER-peenz/ n. terpenoids.

terpenoids /TERP-ə-noidz/ (also isoprenes /EYE-sə-preenz/ or isoprenoids /eye-sə-PREE-noidz/) n. A chemical compound composed of isoprene units, usually with the addition of other functional groups. Examples are menthol, citral, pinene, and camphor. Turpentine is composed of plant-derived terpenoids, primarily the monoterpenes alpha-pinene and beta-pinene.

terrestrial /tə-REST-ree-əl/ adj. (1) living on land (as opposed to in the water); (2) living on the ground (as opposed to in the trees); (3) of, relating to, or residing on the planet Earth.

tertian /TAIR-shən/ adj. Occurring every three days (from Latin tertia third).

test n. The shell of a microorganism.

testacea /test-STASH-yə/ n. (1) rhizopods with an external test; (2) formerly, a name for shellfish of any kind.

testicle /TEST-ə-kəl/ n. A testis — testicular /test-TICK-yə-lər/ adj.

testis (pl testes) /TES-tiss, pl TES-teez/ n. A male reproductive gland, of which two are normally present, that produces spermatozoa and androgens. In humans, the testes are contained in the scrotum.

Tethys Seaway in the Oligocene (about 30 mya)

testosterone /tehs-TAWS-tə-rone/ n. In mammals, the primary androgen, an anabolic steroid; secreted primarily in the testes, of males, and the ovaries of females (the adrenals also secrete small amounts).

Tethys Sea (or Tethys Seaway) A sea that opened up between Gondwana and Laurasia as Pangea split in two during the Jurassic. This split was complete by about 170 million years ago. The gap gradually narrowed during the Tertiary. The Mediterranean Sea is a remnant of the Tethys Sea. ANIMATION OF PANGEA'S BREAKUP

tetrahydrofolate (THF or tetrahydrofolic acid) /TEHT-rə-HIDE-rə-FŌ-late/ n. A coenzyme functioning in various metabolic reactions. It is produced from dihydrofolic acid, a folic acid derivative, by dihydrofolate reductase. SYNTHESIS PATHWAY | MOLECULAR STRUCTURE

tetramer /TEHT-rə-mur/ n. A molecule composed of four subunits.

tetraploid /TET-rə-ploid/ adj. (1) exhibiting tetraploidy; (2) an organism, cell, or type exhibiting tetraploidy.

tetraploidy /TET-rə-ploid-ee/ n. A condition in which a cell has, or the individual cells of an organism have, four complete sets of chromosomes. See also: polyploidy.

tetrapod /TET-rə-PAWD/ n. Any amphibian, reptile, bird, or mammal (i.e., a vertebrate having four limbs, or being descended from such an animal).

theca (pl thecae) /THEE-kə/ n. A thick outer covering present in some unicellular protists.

Theileria A genus of protozoan blood parasites transmitted by ticks. Parasites of this type primarily infect cattle. T. annulata causes tropical theileriosis and T. parva is the cause of East Coast fever.

thigmotropism /THIHG-mō-TRŌP-iz-əm/ n. In a plant, directional growth in response to physical contact.

Thelyphonida /thel-ee-FAWN-ə-dee/ n. An order of arachnids composed of the whip scorpions. MORE INFORMATION

thelytoky /thə-LAWD-ə-kee/ n. A mode of asexual reproduction involving the production of females, and only females, from unfertilized eggs.

THF Tetrahydrofolate.

thiol n. An organosulfur compound containing a carbon-bonded sulfhydryl. Thiols are sulfur analogs of alcohols because sulfur replaces oxygen in the hydroxyl group of the equivalent alcohol.

thoracic /thore-ASS-ick/ adj. (1) pertaining to the thorax; (2) of vertebrae: those having ribs.

thorax /THORE-aks/ n. (1) the portion of the body between the neck and diaphragm (syn: chest); (2) in insects, the second of the three body segments, between the head and abdomen. ROTATING TOMOGRAPHIC IMAGE OF THE BONES OF A HUMAN THORAX

thrombasthenia /thrawm-bas-THEE-nee-ə/ n. Deficiency of blood platelets.

thrombocytes (called platelets in mammals) /THRAWM-bə-sīts/ n. Minute cells, found in normal blood, that aid in blood coagulation.

thrombolysis /thrawm-BAWL-lə-sis/ n. Breaking up or dissolution of a thrombus — thrombolytic /THRAWM-bə-lit-ik/

thrombosis /thrawm-BŌ-səs/ n. The formation of a thrombus.

thrombus /THRAWM-bəs/ (pl thrombi /THRAWM-bee/) n. A blood clot that blocks a blood vessel or of a chamber of the heart — thrombotic /thrawm-BAWT-ik/

threonine (T or thr) /THREE-ō-neen/ n. An essential amino acid. CODONS | MOLECULAR STRUCTURE

thylacine /THĪ-lə-seen/ n. A large carnivorous marsupial (Thylacinus cynocephalus) similar in appearance to a dog, but with catlike striping on its tail and hindquarters. MORE INFORMATION.

thylakoid /THĪ-lə-koid/ n. One of the coin-shaped sacs stacked together to form a granum (within a chloroplast). See: chloroplasts.

thymine (T) /THĪ-meen/ n. One of the four nucleotide bases, a pyrimidine, found in DNA, but not RNA; pairs with adenine.

thymus /THĪ-məs/ n. A bodily organ located in the chest behind the sternum. Formerly considered functionless, but now known to play an important role in the immune system. In the thymus lymphocyte precursors become thymocytes, and then mature into T cells. PICTURE

thyroid /THĪ-roid/ n. (1) a secretory gland at the front of the neck. It surrounds the front of the upper trachea (PICTURE); It is made up of two lobes connected by an isthmus and is grossly enlarged in goiter (PICTURE); (2) thyroid extract (derived from the thyroid glands of cattle), used as a therapeutic drug.

tibia /TIB-ee-ə/ n. The "shin bone", the larger of the two bones in the lower leg, it lies at the inner side of the leg while the smaller fibula lies toward the outside. ANTERIOR VIEW | POSTERIOR VIEW

tibiofemoral /TIB-ee-ō-fə-MORE-əl/ adj. Pertaining to the tibia and the femur.

tick An arthropod of the arachnid order Ixodida. MORE INFORMATION

t.i.d. (Latin: ter in die) Three times a day (abbreviation used in medical prescriptions).

t.i.w. Three times a week (abbreviation used in medical prescriptions).

TIM See: triosephosphate isomerase.

tines n. The points on an antler.

tissue /TISH-yoo, Brit: TISS-yoo/ n. A mass or layer of cells that are all of the same type or, if of distinct types, that occur in regular association and perform a joint function. A tissue is generally considered to include any interstitial space between the cells of which it is composed.

tissue culture /TISH-yoo, Brit: TISS-yoo/ n. The growth, by artificial means, of a tissue outside the body.

tissue fluid /TISH-yoo, Brit: TISS-yoo/ n. The fluid within the interstitial spaces of tissues.

titanium (Ti) /tī-TANE-ee-əm/ n. Metallic element; Atomic weight: 47.90; atomic number: 22.

titanoboa /tī-TAN-ə-BŌ-ə/ n. The largest known snake. MORE INFORMATION

TLC Thin layer chromatography.

TMV Tobacco mosaic virus.

tonoplast /TAW-nə-plast, TŌ-/ n. The outer membrane of a vacuole.

tonotaxis /TAW-nō-TAX-əs, TŌ-/ n. Movement along a gradient of osmotic pressure.

top. Topical (abbreviation used in medical prescriptions).

topological isomers /TAWP-ə-LAW-jə-kəl, TOPE-/ n. Two DNA molecules that differ only with respect to linking number.

toxic /TAWK-sick/ adj. Poisonous.

toxicity /tawk-SIS-ə-tee/ n. The characteristic of being, or the degree to which a substance is, toxic.

toxicology /tawks-ə-KAWL-ə-jee/ n. The study of toxins.

toxin /TAWK-sin/ n. A poison, produced by an animal or plant, that elicits the production of an antibody (antitoxin) when introduced into bodily tissue, typically by injection.

TPP Thiamine pyrophosphate.

trace fossil Indirect evidence of past life; for example imprints made by an organism's body (e.g., footprints), burrows, stromatolites, or coprolites.

trachea /TRAYK-ee-ə, British: trə-KEE-ə/ n. (1) in human beings, the single respiratory tube connecting the bronchial tubes with the larynx; also called the windpipe. PICTURE; (2) in a terrestrial invertebrate, one of the many air-admitting tubes permitting the exchange of oxygen and carbon dioxide, which together are called the tracheal system — tracheal /TRAYK-ee-əl/ adj.

tragus /TRAY-gəs/ The small cartilaginous projection in front of the ear opening.

transcribed spacer /tran-SKRĪBD/ The portion of a primary rRNA transcript discarded during the production of functional rRNAs.

transcript /TRAN-skript/ An RNA sequence complementary to, and produced from, a DNA template.

transcription The synthesis of a complementary RNA through the use of a DNA template.

transcription factor /tranz-SKRIP-shən/ A protein that binds to regulatory regions and helps control gene expression.

transcriptome /tranz-SKRIP-tōm/ The full complement of transcripts present in a given tissue at a particular time.

transfection /tranz-FECK-shən/ The introduction of foreign DNA into a eukaryotic cell.

transfer RNA (tRNA) An RNA molecule that adds specific amino acids during translation. Transfer RNAs also serve as primers for reverse transcription. STRUCTURE

transferase /TRANZ-fer-rayz/ An enzyme catalyzing the transfer of a functional group (e.g. a methyl or phosphate group) from a donor molecule to an acceptor molecule.

transformation (1) the introduction of foreign DNA into a prokaryotic cell; (2) the conversion of cultured eukaryotic cells to a state of unrestrained, cancerlike growth.

transformed adj. Having undergone transformation.

transgenic adj. An experimentally produced animals created by the introduction of new DNA into the organism's germ line.

transition /tran-ZISH-shən/ See: base-pair substitution.

translation /tranz-LAY-shən/ n. Synthesis of a protein on an mRNA template.

translocation /tranz-lō-KAY-shən/ n. Transfer of a segment of one chromosome to another chromosome.

transpiration /tranz-sper-RAY-shən/ n. Loss of water from the leaves of a plant; creates an osmotic gradient that draws nutrient-laden water up from the roots. DIAGRAM

transport protein n. See carrier protein.

transposase /trans-PŌZ-aze/ n. An enzyme capable of catalyzing the insertion of a transposon.

transposon /trans-PŌZ-awn/ n. A DNA sequence that can insert itself at a new site in the genome.

transversion /tranz-VERZ-shən/ n. See: base-pair substitution.

trauma (pl tramata or traumas) /TRAW-mə, pl tra-MAW-tə, TRAW-məz/ n. An injury due to the application of an external force (as a sprain, bone fracture or a concussion).

Trematoda /TREM-ə-TŌD-ə/ n. A class of flatworms, commonly known as flukes. PICTURE

Treponema /trep-ə-NEEM-ə/ n. A genus of spirochetes, among which are the causative agents of syphilis (Treponema pallidum) and yaws (Treponema pertenue). PHOTOMICROGRAPH OF TREPONEMA PALLIDUM

triacylglycerols /trī-ASS-əl-GLIS-ə-rawlz/ (also triglycerides) /trī-GLIS-ə-rīidz/ n. Molecules that are the typical form in which fatty acids are stored in cells. A triacylglycerol is formed by esterification of one fatty acid to each of the three hydroxyl groups of a glycerol molecule. Triacylglycerols are insoluble in water and in salt solutions, so they form droplets in cells. For bodily tissues, these droplets are a source of energy. They are stored in adipose cells. GENERIC STRUCTURE OF A TRIACYLGLYCEROL

Triassic Period /trī-ASS-ick/ n. The first period of the Mesozoic Era. Pangaea began to breakup during this time. The ancestors of dinosaurs were present, as were early mammals and synapsids.

trichinosis /TRICK-ə-nō-səs/ n. Disease resulting from ingestion of the nematode Trichina spiralis. The usual source of infection is undercooked pork. PICTURE OF TRICHINELLA SPIRALIS IN MUSCLE CELLS

trichloroacetic acid /trĪ-klore-ō-ə-SEE-tick/ n. An acid commonly used to precipitate proteins in the course of biochemical extractions. MOLECULAR STRUCTURE

trichocysts /TRICK-ə-sists/ n. Threadlike barbs of ciliated protozoans that can be fired for defense or to capture prey.

tricuspid /trī-KƏS-pid/ n. (1) the right atrioventricular valve, which has three cusps; (2) a three-cusped tooth.

trilobites /TRĪ-lə-bites/ n. A group of marine arthropods abundant in the Paleozoic; similar in appearance to the extant horseshoe crab (although received wisdom holds that they became entirely extinct at the end of the Paleozoic Era). PICTURE 1: (Olenoides serratus) | PICTURE 2: (Phacops) | PICTURE 3: (Harpides grimmi) | PICTURE 4: (Andalusiana) | PICTURE 5: (Isotelus brachycephalus)

trilobitic /TRIAL-ə-bid-ik/ adj. Of or pertaining to trilobites.

trioecious /trī-EE-shəs/ adj. Existing in three distinct sexual types: males, females, and hermaphrodites.

triosephosphate isomerase (TIM) A key glycolytic enzyme present in all cells. It interconverts glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

triploblastic /TRIP-lō-BLAST-ik/ adj. Developing from three germ layers (ectoderm, mesoderm and endoderm).

triploblasty /TRIP-lō-BLAST-ee/ n. Condition of being triploblastic.

triploid /TRIP-ploid/ adj. (1) adj. exhibiting triploidy; (2) n. an organism, cell, or type exhibiting triploidy.

triploidy /TRIP-ploid-ee/ n. A condition in which a cell has, or the individual cells of an organism have, three complete sets of chromosomes.

trisomy /TRĪ-sōm-ee/ The condition of having three copies of a particular chromosome instead of the normal two. Compare monosomy.

triturated /TRICH-ə-rate-ed/ adj. Crushed by grinding or rubbing (as seashells in the surf).

trivalent /TRĪ-vale-ənt/ n. An association of three homologous chromosomes during synapsis in meiotic prophase I.

tRNA See: transfer RNA.

trophoblast /TRAW-fə-blast/ The outer layer of cells of a blastocyst. PICTURE

tropism /TRŌP-izm/ A tendency to grow or move toward or away from a stimulus. The change can be rotational or translatonal.

trp Tryptophan.

Trypanosoma /trəp-an-nō-SŌ-mə, trip-/ A genus of protozoans including the causative agents of sleeping sickness and Chagas' disease. PICTURE

trypanosomiasis /trəp-an-nō-sō-MĪ-ə-səs, trip-/ Any of the diseases of humans and animals caused by the various protozoans of the genus Trypanosoma.

tryptophan (W or trp) /TRIP-tə-fan/ An essential amino acid. CODONS | MOLECULAR STRUCTURE

tsp. Teaspoon.

TTP Thymidine triphosphate.

tuber /TOO-ber/ A fleshy, swollen underground stem that serves to store nutrients and as a means of vegetative reproduction. Examples are potatoes, yams, and Jeruselem artichokes,

tuberculosis (TB) /tə-BIRK-yə-LŌ-səs, too-/ n. A common, often fatal disease caused in humans primarily by the bacillus Mycobacterium tuberculosis. Most often, TB attacks the lungs, but it can affect almost any part of the body. It is extremely contagious. The infection spreads through the air, and from surfaces contaminated with infected mucous and saliva. Frequently, too, infected cattle and milk spread the disease to humans. The usual symptoms of pulmonary tuberculosis are a chronic cough productive of bloody sputum, night sweats, fever and weight loss. A wide range of other symptoms can occur depending on the region of the body affected. However, the vast majority of infected individuals are asymptomatic (latent TB). Transmission occurs only from people with active, not latent, TB.

tubulin /TOOB-yə-lin/ The protein that forms the primary component of microtubules. Each unit of tubulin is a dimer with an alpha and beta subunit.

tuff /təf/ Extrusive igneous rock formed by the agglomeration of volcanic ash.

tumor /TOO-mer/ n. New, autonomous growth forming an abnormal mass that performs no physiologic function.

Turner syndrome (also Ullrich-Turner syndrome) n. A condition in human beings, which occurs in individuals who both lack a Y chromosome and either possess a single X chromosome or one normal X chromosome and one abnormal one. Individuals affected by Turner syndrome are anatomically female, but sterile; the ovaries are usually absent or small.

tympanic membrane (also membrana tympani) /tim-PAN-ik/ n. The eardrum. PICTURE

tympanitic /tim-pə-NID-ik/ adj. Pertaining to tympanites.

tympanites /tim-pə-NĪD-eez/ n. Abdominal distention resulting from accumulation of gas in the intestines or peritoneal cavity.

tympanum cavity /TIM-pə-nəm/ n. The cavity of the middle ear. PICTURE

type specimen The specimen designated as the representative type of a particular taxonomic group.

tyrosine (tyr or Y) /TĪ-rə-seen/ One of the nonessential amino acid. CODONS | MOLECULAR STRUCTURE.

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