Telomeric repeat-binding factor 2 (TRF2)
value for chimpanzees, gorillas, and orangutans. See Appendix C, 1991, Ijdo et al.. telomeric repeat-binding factor 2 (TRF2) a pro- tein that binds to TTAGG repeats and controls telo- mere length by inhibiting telomerases (q.v.). In hu- mans the gene that encodes it is located at 16q22.1. TRF2 protects human telomeres from end-to-end fusions. telomeric silencing the repression by telomeres of transcription by genes in adjacent DNA domains.
Telomeres also appear to reduce the accessibility of subtelomeric chromatin to modification by DNA methylases. See DNA methylation. telophase See mitosis. telotrophic meroistic ovary See insect ovary types. telson the most posterior arthropod somite in which the posterior opening of the alimentary canal is located. See maternal polarity mutants. TEM transmission electron microscope. See elec- tron microscope. temperate phage a nonvirulent bacterial virus that infects but rarely causes lysis. It can become a pro- phage and thereby lysogenize the host cell. temperature-sensitive mutation a mutation that is manifest in only a limited temperature range. The product of such a gene generally functions normally, but is unstable above a certain temperature. Thus, the mutant when reared at the lower (permissive) temperature is normal, but when placed at the higher (restrictive) temperature shows the mutant phenotype. See Appendix C, 1951, Horowitz and Leupold; 1971, Suzuki et al.; albinism, Himalayan mutant. template the macromolecular mold for the syn- thesis of a negative antitemplate macromolecule. The antitemplate then serves as a mold for the tem- plate. Thus the duplication of the template requires two steps.
A single strand of DNA serves as a tem- plate for the complementary strand of DNA or mRNA. template strand the strand of a DNA segment that is transcribed into mRNA. See strand terminolo- gies. template switching in E. coli, a bizarre in vitro re- action often accompanying strand displacement in which DNA polymerase I shifts from the original template strand to the displaced strand. temporal isolation See seasonal isolation. teosinte various Mexican wild grasses that are in- terfertile with corn. The wild ancestor of corn has been identified as Zea mays ssp. parviglumis, an an- nual teosinte growing in the Balsas river valley of southern Mexico. The initial domestication occurred about 9,000 years ago. See Appendix C, 1939, Bea- dle; 2002, Matsuoka et al.; consilience, Zea mays spp mays. teratocarcinoma embryonal tumors originating in the yolk sac or gonads of amniotes and capable of differentiating into a variety of cell types.
These tu- mors are used to study the regulatory mechanisms involved in embryological development. See Appen- dix C, 1975, Mintz and Illmensee. teratogen any agent that raises the incidence of congenital malformations. teratoma a tumor composed of an unorganized ag- gregation of different tissue types. terminal chiasmata the end-to-end association of homologous chromosome arms resulting from ter- minalization (q.v.). terminal deletion See deletion. terminal inverted repeats (TIR) sequences of nu- cleotides found at each end of a transposon (q.v.), but in reverse order. Each transposon family is de- fined by the fact that its members share the same TIRs. In maize, Ac and Ds have the same 11 bp TIR; whereas the Suppressor-mutator (Spm) transposable element has a 13 bp TIR.
A Spm transposase does not recognize an Ac TIR and vice versa. terminalization in cytology, the progressive shift of chiasmata from their original to more distal posi- tions as meiosis proceeds through diplonema and diakinesis. See Appendix C, 1931, Darlington. terminal redundancy referring to the repetition of the same sequence of nucleotides at both ends of a DNA molecule. terminal taxa the groups that occur at the ends of branches in a cladogram. terminal transferase a deoxyribonucleotidyltrans- ferase that is used by molecular biologists to add a homopolymer tail, e.g., polydeoxyadenylate, to each end of a vehicle DNA. The enzyme is then used to add poly T tails to a passenger DNA. The passenger and vehicle are then annealed via their complemen- tary termini, ligated, and cloned. See Appendix C, 1972, Lobban and Kaiser.
termination codon a codon that signals the termi- nation of a growing polypeptide chain. See Appendix C, 1965, Brenner et al.; amber mutation, ochre muta- tion, opal codon, stop codon, universal code theory. termination factors See release factors. termination hairpin, termination sequence See terminators. terminators nucleotide sequences in DNA that function to stop transcription; not to be confused with terminator codons that serve as stop signals for
translation. In the illustration here, the lower DNA strand is being transcribed from left to right. The RNA segment transcribed from the underlined DNA forms a hairpin-shaped loop because the two blocks of nucleotides have complementing base se- quences.
This tends to force the adjacent region of the DNA/RNA hybrid to open up. Since it consists of polyribo-U and polydeoxy-A regions that bind weakly, the mRNA molecule will detach at this point. See attenuator, exon. territoriality the defense by an animal or group of an area against members of the same species. territory an area of the habitat occupied by an in- dividual or group. If members belonging to the same species enter the territory, they are attacked as tres- passers. Tertiary the older of the two geologic periods making up the Cenozoic era. See geologic time divi- sions. tertiary base pairs the specific base pairs of a tRNA molecule responsible for its three-dimensional folding. Most of these base pairs are evolutionarily conserved in all tRNA molecules. tertiary nucleic acid structure the three-dimen- sional conformation of a nucleic acid strand (chain) formed by folding of the strand and formation of in- trastrand complementary base pairing (e.g., transfer RNA, q.v.). tertiary protein structure See protein structure. tesserae functionally different patches of endo- plasmic reticulum, each bearing a characteristic set of enzymes. test cross a mating between an individual of un- known genotype, but showing the dominant pheno- type for one or more genes, with a tester individual known to carry only the recessive alleles of the genes in question. The test cross reveals the genotype of the tested parent.
For example, an individual show- ing the A and B phenotypes is crossed to an aabb tester. If the F1 contains individuals of AB, Ab, aB, and ab phenotypes in a 1 : 1 : 1 : 1 ratio, this reveals that (1) the tested parent produced gametes with genotypes identical to the F1 phenotypes and in the same proportions, and (2) the tested parent was AaBb. The first test crosses were made in 1862 by Gregor Mendel. tester strain a multiply recessive strain that pro- vides the genotypically known mate used in a test cross. testicular feminization See androgen insensitivity syndrome. testis (plural testes) the gamete-producing organ of a male animal. testosterone a masculinizing, steroid hormone se- creted by interstitial cells of the testis.
test-tube baby the production of a child by in vitro fertilization, followed by embryo transplanta- tion to complete gestation in a normal uterus. This may be provided by the biological or surrogate mother. tetra-allelic referring to a polyploid in which four different alleles exist at a given locus. In a tetraploid A1A2A3A4 would be an example. tetracyclines a family of antibiotics obtained from various species of Streptomyces. Tetracyclines bind to the 30S subunit of prokaryotic ribosomes and pre-
vent the normal binding of aminoacyl-tRNA at the A site. The structure of a typical tetracycline appears below. See cyclohexamide, ribosome, ribosomes of organelles, translation.
tetrad 1. four homologous chromatids (two in each chromosome of a bivalent) synapsed during first meiotic prophase and metaphase. See meiosis. 2. four haploid products of a single meiotic cycle. tetrad analysis the analysis of crossing over by the study of all the tetrads arising from the meiotic divi- sions of a single primary gametocyte. To perform such an analysis, one must use an organism in which the meiotic products are held together, as for exam- ple, in the case of meiospores confined in an ascus sac. Genera suitable for such analyses include Asco- bolus, Aspergillus, Bombardia, Neurospora, Podo- spora, Saccharomyces, Schizosaccharomyces, Sordaria, and Sphaerocarpus. tetrad segregation types For a bivalent contain- ing the genes A and B on one homolog and a and b on the other, three patterns of chromatid segrega- tion are possible: AB, AB, ab, ab (referred to as the parental ditype); AB, Ab, aB, ab, where two chro- matids are recombinant (the tetratype); and Ab, Ab, aB, aB, where all chromatids are recombinant (the nonparental ditype). tetrahydrofolate See folic acid.
Tetrahymena a genus containing T. pyriformis, the species for which the most genetic information is available, and T. thermophila, the species in which UAA and UAG were shown to encode glutamine rather than serving as stop codons. The nuclear reor- ganization that takes place following conjugation (q.v.) in these ciliates makes them a rich source of telomeres and the enzymes that work on them. This is because during the regeneration of a new macro- nucleus, the DNA of the micronucleus is split at specific sites into hundreds of thousands of pieces. New telomeres are synthesized at each new end, and each chromosome fragment undergoes many cycles of replication.
The Tetrahymena macronucleus con- tains 20,000 to 40,000 telomeres! In T. thermophila, each macronucleus carries about 45 copies of each expressed gene, and it is responsible for the pheno- type of the cell. The micronucleus, which is tran- scriptionally inactive, contains five pairs of metacen- tric chromosomes. See Appendix A, Protoctista, Ciliophora; Appendix E; genetic code, nuclear dimor- phism, telomerase, telomere. tetramer a structure resulting from an association of four subunits. If the subunits are all identical, they form a homotetramer; if the subunits are not all identical, they form a heterotetramer. tetranucleotide hypothesis the proposal that DNA is a linear, single-stranded polynucleotide con- sisting of four repeating bases (adenine, thymine, guanine, and cytosine) linked to each other by a de- oxyribose phosphate ester backbone. See Appendix C, 1929, Levene and London; Chargaff rule.
Tetraodon nigroviridis the green spotted puffer- fish. A species, which like Takifugu rubripes (q.v.), possesses a very small genome. However, it has the advantage of being a popular aquarium fish that is easily reared in tap water. tetraparental mouse a mouse developed by arti- ficial fusion of embryonic cells from two genetically different blastulas. tetraploid having four haploid sets of chromo- somes in the nucleus. See allotetraploid, autotetra- ploid, polyploidy. tetrasomic having one chromosome in the com- plement represented four times in each nucleus. tetratype See tetrad segregation types. tetravalent See quadrivalent. thalassemias a group of human anemias due to imbalance in the ratio of alpha and/or beta hemoglo- bin subunits. Since there are four alpha genes per genome, deletions (commonly produced by unequal crossing over) can result in an individual having any number of alpha genes from zero to four. The com- plete absence of alpha genes produces hydrops fetalis (q.v.). With only one alpha gene, excess beta chains form a tetramer (β4), resulting in hemoglobin H dis- ease. Individuals with two or three alpha genes are almost indistinguishable from normal. Epidemiologi- cal studies have shown that individuals with alpha thalassemia trait (–/aa or -a/-a) are more resistant to malaria than aa/aa individuals. Incomplete beta chains can be produced by nonsense codons. Dele- tions in beta genes are commonly produced by un-
equal crossing over, as are the hybrid chains contain- ing δ and β segments (Hb Lepore) or Aγ and β segments (Hb Kenya). Beta thalassemia (also called Cooley anemia) is a hemoglobinopathy in which few functional beta globin chains are made. A point mu- tation, within an intron that alters the cutting and splicing signal, causes an extra piece of intron RNA to be present in processed mRNA; the extra piece shifts the reading frame and causes translation to stop prematurely, yielding a truncated and nonfunc- tional beta globin molecule. See Appendix C, 1976, Kan et al.; 1986, Costantini et al.; Desferal, hemoglo- bin fusion genes, hemoglobin homotetramers. http:// www.thalassemia.org thelytoky a type of parthenogenesis in which dip- loid females are produced from unfertilized eggs and males are absent or rare. There are two types of the- lytoky, meiotic (automictic) and ameiotic (apomic- tic). In automictic thelytoky, meiosis takes place, but the reduction in chromosome number is com- pensated for later in the life cycle. The most wide- spread method of doing this is to have a haploid po- lar body nucleus fuse with a haploid egg nucleus (autofertilization). In apomictic thelytoky, the mat- uration division in the egg is equational and there- fore the egg nucleus remains diploid. Theobroma cacao the cacao tree, source of choco- late. theobromine a mutagenically active purine ana- log. It is the main alkaloid stimulant in chocolate. The enzyme caffeine synthase catalyzes the conver- sion of theobromine into caffeine (q.v.). See alkaloid, bases of nucleic acids.
therapeutic cloning the proposal (sometimes also called somatic cell nuclear transfer) to generate em- bryonic stem cells (q.v.) that are genetically matched to a donor organism. The purpose is to later induce them to differentiate into a specific tissue to provide grafts. The procedure would involve transplanting the nucleus from a somatic cell of an adult individ- ual to an enucleated egg cell. The diploid egg would be stimulated to undergo early embryonic develop-
ment in vitro. The cultured cells would be geneti- cally identical to the individual that provided the transplanted nucleus. If by appropriate chemical treatment the cells could then be stimulated to dif- ferentiate into a specific tissue, it would be a perfect match for the nuclear donor. Therefore diseased or damaged tissues could be replaced by the cloned cells without risk of graft rejection (q.v.). See nuclear transfer. thermal denaturization profile See melting profile. thermal neutron a fast neutron from uranium fis- sion that has been slowed down by elastic collision with a moderator such as graphite to energies equiv- alent to those of gas molecules at room temperature (approximately 0.025 electron volts). The biological effect of thermal neutrons is attributable to the sum- mation of capture and decay radiations. In biological material, the reactions 1H (n, γ) 2H and 14N (n, p) 14C are the most important sources of tissue ioniza- tion.
The relative importance of these reactions de- pends on the size of the organism. Protons from ni- trogen capture are the major cause of the biological effects of thermal neutrons in an organism the size of Drosophila. thermoacidophiles bacteria that live in extremely acidic hot springs. Species belonging to the genus Thermoplasma are examples. They are placed in the archaebacteria (q.v.) on the basis of the nucleotide sequences of their 16S rRNAs. thermophilic heat loving. Said of bacteria that grow at temperatures between 45°C and 65°C (found in fermenting manure and hot springs). See hyperthermophile. Thermotoga maritima a thermophilic bacterium that lives in geothermal marine sediments. It has an optimum growth temperature of 80°C. Its genome is a circular DNA molecule that contains 1,860,725 base pairs. There are 1,877 ORFs, each with an aver- age size of 947 bps. These coding sequences cover 95% of the chromosome.
The largest gene family en- codes ABC transporters (q.v.). The organization of the majority of the genes of T. maritima places it in the bacteria. However, a quarter of the genome is archaeal in nature. The mosaic nature of the T. mari- tima genome suggests that extensive lateral gene transfer as occurred during the evolution of this spe- cies. See Appendix A, Bacteria, Thermotogae; Ap- pendix C, 1999, Nelson et al.; Appendix E; horizontal transmission, hyperthermophile. Thermus aquaticus an aerobic, Gram-negative, heterotrophic, thermophilic bacterium discovered in the natural hot springs of Yellowstone National