t abbr. (1) time; (2) temperature; (3) three times (abbreviation for Latin ter)
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).
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
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.
targeted mutagenesis Intentional alteration of a gene, usually to determine the targeted gene's function.
tarsals The bones of the tarsus.
tarsus The portion of the foot between the leg and metatarsus.
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 (1) the activity of classifying organisms; (2) a classification of living things.
TBSV Tomato bushy stunt virus.
TCA (1) trichloroacetic acid; (2) tricarboxylic acid.
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.
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 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.
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.
thelytoky /thə-LAWD-ə-kee/ n. A mode of asexual reproduction involving the production of females, and only females, from unfertilized eggs.
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/
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
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.
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.
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.
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.
TPP Thiamine pyrophosphate.
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.
transcriptome /tranz-SKRIP-tōm/ The full complement of transcripts present in a given tissue at a particular time.
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.
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.
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).
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
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.
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.
triturated /TRICH-ə-rate-ed/ adj. Crushed by grinding or rubbing (as seashells in the surf).
tRNA See: transfer RNA.
tropism /TRŌP-izm/ A tendency to grow or move toward or away from a stimulus. The change can be rotational or translatonal.
TTP Thymidine triphosphate.
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.
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.
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.
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