Park, Wyoming. This bacterium became the source of the Taq DNA polymerase used in the polymerase chain reaction (q.v.) and the Taq DNA ligase used in the ligase chain reaction (q.v.). T. aquaticus is also the source of the ribosomes that were used in the crystallographic studies which showed the 3D struc- ture of 70S ribosomes complexed with tRNA and mRNA molecules. See Appendix A, Bacteria, Deino- cocci; Appendix C, 2001, Yusupov et al. theta replication a bidirectional mode of replica- tion of circular DNA molecules from a replication origin in which, midway through the replication cy- cle, the structure produced resembles the Greek let- ter θ. Also called a Cairns molecule. See Appendix C, 1963, Cairns; D loop, sigma replication. thiamine vitamin B1, the anti-beriberi factor.
thiamine pyrophosphate a coenzyme of the car- boxylases and aldehyde transferases. thin layer chromatography See chromatography. thioglycolic acid treatment a procedure used to rupture disulphide bridges linking adjacent protein chains. Thio-tepa trade name for triethylenethiophos- phoramide, a mutagenic, alkylating agent.
third cousin See cousin. thirty-seven percent survival dose the radiation dose at which the number of hits equals the number of targets. The dose at which there is an average of one hit per target. See target theory. thr threonine. See amino acid. three-point cross a series of crosses designed to determine the order of three, nonallelic, linked genes upon a single chromosome on the basis of their crossover behavior. three-strand double exchange See inversion. threonine See amino acid. threshold dose the dose of radiation below which the radiation produces no detectable effect.
threshold effect hypothesis the notion that cer- tain traits with a polygenic basis develop only if the additive effects of contributory alleles exceed a criti- cal value. This hypothesis is often used to explain many all-or-none phenomena with a polygenic mode of inheritance (e.g., susceptibility vs. resistance to certain diseases). thrifty gene hypothesis the proposal that there exist in human populations genes that facilitate the efficient utilization of food and its conversion into stored fat. The bearers of such genes gain weight during periods of plenty, and as a result are more likely to survive periods of famine.
Since popula- tions in early stages of human evolution were alter- nately exposed to unpredictable periods of feast and starvation, thrifty genes were selected and retained in the gene pool. However, in contemporary socie- ties food is usually available in unlimited amounts and life is sedentary; now thrifty genes become dele- terious, since they increase susceptibility to the dia- betes type 2 family of diseases. The observation that the symptoms of diabetes disappear under condi- tions of starvation supports this hypothesis. See Ap- pendix C, 1962, Neel; diabetes mellitus, obese. thrombin See blood clotting. thrombocyte blood platelet. See platelets. thrombocytopenia the condition in humans where the circulation has an abnormally low concen- tration of platelets (q.v.).
thrum the type of flower characterized by short styles and high anthers found among distylic species such as seen in the genus Primula. See pin. Thy-1 antigen an antigen on the plasma mem- brane of thymocytes (q.v.) that can be used to distin- guish them from other lymphocyte groups. thylakoid See chloroplast. thymidine the deoxyriboside of thymine. See nu- cleoside. thymidine kinase an enzyme catalyzing the phos- phorylation of thymidine to thymidine monophos- phate. thymidylate kinase an enzyme catalyzing the phosphorylation of thymidine monophosphate and thymidine diphosphate to thymidine diphosphate and thymidine triphosphate, respectively.
thymidylic acid See nucleotide. thymine See bases of nucleic acids. thymine dimer two thymine molecules joined by bonds between their number 5 and 6 carbons, as shown in the accompanying illustration. The reac- tion forming such dimers occurs when ultraviolet ra- diations interact with DNA. These dimers, which block future DNA replication, are removed during cut-and-patch repair (q.v.). See Appendix C, 1961, Wacker, Dellweg, and Lodemann; error-prone re- pair, xeroderma pigmentosum.
thymocyte a thymus-derived lymphocyte or T lymphocyte. thymonucleic acid See nucleic acid. thymus an organ lying in the chest of mammals, formed embryologically from gill pouches. This or- gan functions to populate the body with lymphoid cells. It reaches a maximum size about the time of sexual maturity and then atrophies. thyroglobulin See thyroid hormones. thyroid hormones thyroxine and triiodothyronine are synthesized by the thyroid gland in response to thyrotropin, a hormone from the anterior pituitary gland.
The synthesis of thyroid hormones begins with the selective accumulation of inorganic iodide by the epithelial cells of the gland. The trapped io- dide is then oxidized to iodine, and tyrosine residues of the glycoprotein thyroglobulin are then iodinated, converting them to monoiodotyrosine residues (a). The iodotyrosyl residues are then coupled with the elimination of the alanine side chain to form triiodo- thyronine (b) and thyroxine (c). The structures of these compounds are shown in the accompanying drawings. See cretinism. thyroid-stimulating hormone (TSH) a glycopro- tein hormone stimulating secretion by the thyroid. TSH is produced by the adenohypophysis. thyrotropin See thyroid hormones.
thyroxine See thyroid hormones. TIGR the abbreviation for The Institute for Geno- mic Research, a not-for-profit research institute whose primary concern is the comparative analysis of the genomes of prokaryotes and eukaryotes of ge- netic interest. TIGR was founded in 1992 by J. Craig Venter, and it became the world’s largest DNA se- quencing institute of that time. The first two bacte- rial genomes were worked out at TIGR. However, in 1998 Venter left TIGR to set up an even larger facility at Celera (q.v.), and his wife, Claire M. Fra- ser, took the reins at TIGR. See Appendix C, 1995, Fleischmann, Venter, and Fraser; Appendix E, Master Web Sites; Haemophilis influenzae, Mycoplasma genetalium. tiller a grass side shoot produced at the base of a stem. TIM See mitochondrial translocase. timber line the line in high latitudes and in high elevations in all latitudes beyond which trees do not grow. time-lapse photomicrography a technique in which living cells are photographed using phase con- trast, Nomarski, or fluorescence microscopy with a camera that records images at selected intervals (e.g., one frame per minute), and the sequence of images subsequently displayed at a more rapid speed (e.g., 24 frames per second). Time is thus speeded up for a clearer understanding of cellular dynamics.
Ti plasmid a tumor-inducing (hence the acronym) plasmid found in the bacterium Agrobacterium tumefaciens (q.v.) that is responsible for crown gall disease of dicotyledonous plants. When Ti DNA is integrated into the DNA of the host plant, cytoki- nins and auxins (both of which see) are produced. It is these hormones that cause gall formation. Only a small part of the plasmid actually enters the plant; the rest stays in the bacterium, where it has other functions. The wild-type plasmid produces tumor cells, but it can be modified so that it can carry for- eign genes into cells without making the recipient cells tumorous. During tumor induction, a specific segment of the Ti plasmid, called the T-DNA (trans- ferred DNA), integrates into the host plant nuclear DNA. Ti-mediated tumorigenesis is the first case of a horizontal mobile element (q.v.) that transfers DNA between cells that belong to separate king- doms.
See Appendix C, 1974, Zaenen et al.; 1981, Kemp and Hall; genetic engineering, promiscuous DNA, selfish DNA. Tiselius apparatus electrophoresis apparatus. tissue a population consisting of cells of the same kind performing the same function. tissue culture the maintenance or growth of tissue cells in vitro in a way that may allow further differ- entiation and preservation of cell architecture or function, or both. Primary cells are those taken di- rectly from an organism. Treating a tissue with the proteolytic enzyme trypsin dissociates it into indi- vidual primary cells that grow well when seeded onto culture plates at high densities. Cell cultures arising from multiplication of primary cells in tissue culture are called secondary cell cultures.
Most sec- ondary cells divide a finite number of times and then die.
A few secondary cells may pass through this “crisis period” and be able to multiply indefinitely to form a continuous cell line. Cell lines have extra chromosomes and are usually abnormal in other re- spects as well. The immortality of these cells is a fea- ture shared in common with cancer cells. See Appen- dix C, 1907, Harrison; 1965, Hayflick; telomerase. tissue typing identification of the major histo- compatibility antigens of transplant donors and po- tential recipients, usually by serological tests. Donor and recipient pairs should be of identical ABO blood group, and in addition should be matched as closely as possible for H antigens in or- der to minimize the likelihood of allograft rejection. See histocompatibility molecules. titer the amount of a standard reagent necessary to produce a certain result in a titration (q.v.). titin the largest protein known, consisting of a con- tinuous chain of 27,000 amino acids. Each molecule spans a distance greater than one micrometer (from the Z discs to the M discs in the striated muscles of vertebrates). Titins act as springs, pulling the muscle fiber back into shape after it is stretched. Titins come in a variety of isoforms generated by alterna- tive splicing (q.v.).
The human titin gene (TTN) is located on the long arm of chromosome 2 and con- tains 80,780 base pairs. It is subdivided into 178 ex- ons, the largest of which contains 17,106 bases. The Drosophila titin gene is located at the distal end of 3L. Drosophila titin is a component of muscle sar- comeres, but it also is localized in chromosomes. Here it presumably organizes higher-order chromo- some structure and provides elasticity. See Appendix C, 1995, Labeit and Kolmer. TLC thin-layer chromatography. See chromatog- raphy. T lymphocyte the lymphocyte responsible for cell- mediated immunological reactions, such as graft re- jection, and characterized by the possession of T cell receptors (q.v.). T lymphocytes differentiate within the microenvironment of the thymus gland.
Mature T cells can be divided into two groups (CD4 and CD8) on the basis of their ability to recognize cer- tain classes of histocompatibility molecules (HCMs). CD4+ cells, which recognize class II HCMs, function as helper T lymphocytes (q.v.). CD8+ cells, which recognize class I HCMs, function as cytotoxic T lym- phocytes (q.v.). See histocompatibility molecules, V(D)J recombination. Compare B lymphocyte. Tm the temperature at which a population of dou- ble-stranded nucleic acid molecules becomes half- dissociated into single strands; referred to as the melting temperature for that system. TMV tobacco mosaic virus. TNFs tumor necrosis factors (q.v.). Tn 5 a bacterial transposon that is favorite for stud- ies of the three-dimensional interactions of its trans- posase proteins with the DNA target sites at the ends of the element during its excision from one host and transfer to another. Tn 5 transposition intermediate a hairpin-shaped complex between the Tn 5 transposon and two transposases, each bound to recognition sequences at the ends of the Tn 5 element, after its release from the host chromosome. The intermediate can now at- tach to target sequences in a new host chromosome where it can catalyze the reintegration of the se- quence. See Appendix C, 2000, Davies et al.
NA or DNA that has been transcribed respectively from a DNA or RNA tem- plate.