Online Biology Dictionary - T

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

T (1) thymine; (2) threonine.

Tabanidae 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 /TAH-beez/ A gradual wasting away, occurring in any disease.

Tapeworm heads ("scolices") with         attached segments ("proglottids").         Left: pork tapeworm, Taenia         solium (note hooks allowing gut wall         attachment); Right: beef tapeworm,         Taenia saginata, which lacks hooks         ("unarmed scolex").

Taenia /TEEN-ee-yuh or TEEN-yuh/ A genus that includes two important human parasites, the pork tapeworm, Taenia solium, and to the beef tapeworm, Taenia saginata. A tape worm is composed of (1) a head, or scolex, by which it attaches itself to the wall of the small intestine its host; and (2) a long chain of segments, or proglottids -- typically a thousand or more -- each of which may contain up to 100,000 eggs. Eggs and gravid proglottids are passed in the feces of the host. When ingested, they hatch in the gut of the new host, invade the blood stream, and then encyst, primarily in muscle tissue. When a carnivore eats infected tissue, the encysted larvae complete the life cycle, by developing into mature tapeworms in the small intestine.

taiga /TYE-guh/ Coniferous forest with thin, acidic soil and characteristic of the northern regions of Eurasia and North America, where summers are short and winters, long and cold. MAP | PICTURE 1 | PICTURE 2

talofibular Pertaining to the talus and fibula.

talus The ankle bone or astragalus. It articulates distally with the calcaneus and navicular bone of the tarsus and proximally with the fibula and tibia. PICTURES

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.

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

Water bear (tardigrade), Hypsibius dujardini Scanning electron micrograph by Bob Goldstein

tardigrade A microorganism of the phylum Tardigrada (example shown at right). Also known as water bears, these tiny animals have four pairs of legs and live in water, most often being found in mosses. They are famed for their ability to survive in harsh environments by entering an anabiotic state known as the "tun state." When experimentally dessicated for a decade or more they have been rehydrated and show no ill effects. They have also survived boiling, deep freezing to temperatures as low as 1° Kelvin and, after being sent up in a rocket, exposure to the vacuum of outer space.

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.

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

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.

TB Tuberculosis.

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

telology The use of design, purpose, or goals in the explanation of the existence of phenomena, particularly natural phenomena 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.").

telocentric chromosome A chromosome having its centromere in a terminal position.

telomerase An ribonucleoprotein enzyme that adds individual bases to produce repeating units of one DNA strand at the telomere.

telomere A DNA sequence composed of simple repeats at the end of a eukaryotic chromosome (see figure right). Telomeres stabilize linear chromosomes by protecting their ends from loss during replication. During interphase, the enzymes that duplicate the chromosomes cannot continue replication all the way to the chromosomes' ends. The ends are rebuilt during repication by the enzyme telomerase reverse transcriptase. PICTURE

telophase /TELL-oh-phaze/ (1) The fourth stage of mitosis (MORE DETAILS); (2) The fourth stage of meiosis I (telophase I) or of meiosis II (telophase II).

TEM Transmission electron microscope.

template /TEM-pluht/ 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 bones /TEMP-er-uhl/ A symmetrical pair of bones, one lying on either side of the skull. PICTURE 1 | PICTURE 2 | Rotating View

Ternate Paper Name commonly used to refer 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. TEXT OF TERNATE PAPER

teratogenic /TER-uh-toh-JEN-ik/ Causing abnormal development.

terrestrial (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.

test The shell of a microorganism.

testis (pl testes) /TEST-is, TEST-eez/ A reproductive gland, of which two are normally present in males, that produces spermatozoa and androgens.

testosterone /tehs-TAWS-tuh-rone/ 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 Seaway in the Oligocene (about 30 mya)

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. SEE ANIMATION OF PANGEA'S BREAKUP

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

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

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

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

theca 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-moh-TROPE-iz-uhm/ In a plant, directional growth in response to physical contact.

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

THF Tetrahydrofolate.

thorax /THORE-aks/ (1) the portion of the body between the neck and diaphragm; (2) in insects, the second of the three body segments, between the head and abdomen.

thrombus /THRAWM-buhs/ Blockage of a blood vessel by a clot.

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

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

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

thymus 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 (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 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. PICTURE 1 | PICTURE 2

t.i.d (Latin: ter in die) Three times a day.

TIM See: triosephosphate isomerase.

titanium (Ti) Metallic element; Atomic weight: 47.90; atomic number: 22.

TLC Thin layer chromatography.

TMV Tobacco mosaic virus.

tonoplast The outer membrane of a vacuole.

toxic /TAWK-sick/ Poisonous.

toxicity /tawk-SIS-uh-tee/ The degree of being toxic.

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-uh/ The windpipe; in human beings the portion of single respiratory tube connecting the bronchial tubes with the larynx. PICTURE

tragus The small cartilaginous projection in front of the ear opening.

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

transcript 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 A protein that binds to regulatory regions and helps control gene expression.

transcriptome The full complement of transcripts present in a given tissue at a particular time.

transfection 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. PICTURE

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.

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

transition See: base-pair substitution.

translation Synthesis of protein on an mRNA template.

translocation Transfer of a segment of one chromosome to another chromosome.

transpiration 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 See carrier protein.

transposase An enzyme capable of catalyzing the insertion of a transposon.

transposon A DNA sequence that can insert itself at a new site in the genome.

transversion See: base-pair substitution.

Trematoda A class of flatworms, commonly known as flukes. PICTURE

Treponema A genus of spirochetes, among which is the causative agent of syphilis (Treponema pallidum). PICTURE

triacylglycerols (also triglycerides) 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 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 Disease resulting from ingestion of the nematode Trichina spiralis. The usual source of infection is undercooked pork. PICTURE

trichloroacetic acid An acid commonly used to precipitate proteins in the course of biochemical extractions. MOLECULAR STRUCTURE

trichocysts Threadlike barbs of ciliated protozoans that can be fired for defense or to capture prey.

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

trilobites /TRI-luh-bites/ 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-uh-bid-ik/ Of or pertaining to trilobites.

trioecious /try-EE-shuhs/ 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-low-BLAST-ik/ Developing from three germ layers (ectoderm, mesoderm and endoderm).

triploblasty /TRIP-low-BLAST-ee/ Condition of being triploblastic.

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

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

trisomy The condition of having three copies of a particular chromosome instead of the normal two. Compare monosomy.

trivalent An association of three homologous chromosomes during meiosis.

tRNA See: transfer RNA.

trophoblast The outer layer of cells of a blastocyst. PICTURE

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

trp Tryptophan.

trypanosomiasis Any of the diseases of humans and animals caused by the various protozoans of the genus Trypanosoma.

Trypanosoma A genus of protozoans including the causative agents of sleeping sickness and Chagas' disease. PICTURE

tryptophan (W or trp) An essential amino acid. CODONS | MOLECULAR STRUCTURE

TTP Thymidine triphosphate.

tuber 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) 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 loss of weight. 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 The protein that forms the primary component of microtubules. Each unit of tubulin is a dimer with an alpha and beta subunit.

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

tumor New, autonomous growth forming an abnormal mass that performs no physiologic function.

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

tympanum cavity /TIM-puh-num/ The cavity of the middle ear. SEE PICTURE

Turner syndrome A condition in human beings, which occurs in individuals who both lack a Y chromosome and possess only a single X chromosome (X monosomy). Such individuals are anatomically female, but sterile; the ovaries are usually absent or small.

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

tyrosine (tyr or Y) A nonessential amino acid. CODONS | MOLECULAR STRUCTURE.