a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z

T (1) Thymine or thymidine. (2) See tetratype. T-cell receptor Surface protein of T lymphocyte cells that allow the T cells to recognize infected host cells. t-dna A portion of the Ti plasmid that is inserted into the genome of the host plant cell. TACTAAC box A consensus sequence surrounding the lariat branch point of eukaryotic pre-mRNA introns and used during splicing. tandem duplication adjacent identical chromosome segments. target theory A theory that predicts response curves based on the number of events required to cause the phenomenon. Used to determine whether point mutations are single events. targeted gene knockout The introduction of a null mutation in a gene by a designed alteration in a cloned DNA sequence that is then introduced into the genome by homologous recombination and replacement of the normal allele. TATA box A DNA sequence (concensus TATAAA) at about minus 25 (ie. 25 nucleotides upstream of the transcription start site) in the promoter region of many eukaryotic genes involved in binding RNA polymerase via a TATA binding protein. Analogous to the Pribnow box in prokaryotes. tautomeric shift The spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding form, possibly resulting in a mutation. Reversible shifts of proton position in a molecule. Bases in nucleic acids shift between keto and enol forms or between amino and imino forms. telocentric chromosome A chromosome whose centromere lies at one end. telomerase An enzyme that adds telomeric sequences to the ends of eukaryotic chromosomes. No template is necessary. telomere The ends of linear chromosomes that are required for replication and stability. The tip (or end) of a chromosome. telophase The terminal stage of mitosis or meiosis in which chromosomes uncoil; the spindle breaks down; and cytokinesis usually occurs. The late stage of nuclear division when daughter nuclei re-form. temperate phage A phage that can enter into lysogeny with its host. A phage that can become a prophage. temperature-sensitive mutant An organism that has a wild-type phenotype at a permissive temperature but a mutant phenotype at a restrictive (non-permissive) temperature. temperature-sensitive mutation A conditional mutation that produces the mutant phenotype in one (restrictive or non-permissive) temperature range and the wild-type phenotype in another (permissive) temperature range. template (1) A pattern serving as a mechanical guide. (2) In DNA replication each strand of the duplex acts as a template for the synthesis of a new double helix. (3) A molecular mold that shapes the structure or sequence of another molecule. For example, the nucleotide sequence of DNA acts as a template to control the nucleotide sequence of RNA during transcription. teratogen An agent that interferes with normal development. terminator sequence A sequence in DNA that signals termination of transcription to RNA polymerase. tertiary structure The further folding of a protein bringing alpha-helices and beta-sheets into three-dimensional arrangements. The folding or coiling of the secondary structure to form a three-dimensional molecule. test-cross The crossing of an organism, with an unkown genotype, to a homozygous recessive organism (tester). A cross between an individual of unknown genotype or a heterozygote (or a multiple heterozygote) to a homozygous recessive individual. tester An individual homozygous for one or more recessive alleles and used in a test-cross. testicular feminization syndrome A human condition, inherited as an X-linked recessive, caused by a mutation in a gene coding for the androgen (testosterone) receptor, in which genetical (XY) males develop female secondary sexual characters. testing of hypotheses The determination of whether to accept or reject a proposed hypothesis based on the likelihood of the experimental results. testis-determining factor (TDF) General term for the gene determining maleness in human beings (Tdf in mice). tetrad (1) Four homologous chromatids in a bundle in the first meiotic prophase and metaphase. The meiotic configuration of four chromatids first seen in pachytene. There is one tetrad per pair (bivalent) of homologous chromosomes. (2) The four haploid product cells from a single meiosis. tetrad analysis The use of tetrads (definition 2) to study the segregation of chromosomes and genes during meiosis. tetraparental mouse A mouse that develops from an embryo created by the experimental fusion of two separate embryos (blastulas) from matings of different parents. tetraploid (1) A cell having four chromosome sets. (2) an organism composed of such cells. tetratype (T) A tetrad containing four different genotypes, two parental and two recombinant. A spore arrangement in Ascomycetes that consists of two parental and two recombinant spores indicating a single crossover between two linked loci. See parental ditype (PD) and non-parental ditype (NPD). theta structure An intermediate structure formed during the replication of a circular DNA molecule. Thr Threonine (an amino acid). three-point cross A cross involving three loci. three-point testcross A testcross involving one parent with three heterozygous gene pairs and another (tester) with three homozygous recessive gene pairs. thymidine The nucleoside having thymine as its base. thymine A pyrimidine base that pairs with adenine. thymine dimer A pair of abnormally chemically bonded adjacent thymine bases in DNA, resulting from damage by ultra-violet irradiation. The cellular processes that repair this lesion often make errors that create mutations. Ti plasmid A circular plasmid of Agrobacterium tumefaciens that enables the bacterium to infect plant cells and produce a tumour (crown gall tumour). topoisomerase Enzymes of two types that can remove (or create) supercoiling in duplex DNA by creating transitory breaks in one (type I) or both (type II) strands of the sugar-phosphate backbone. See topoisomer topoisomer Topological form of DNA with the same sequence as another but differing in linking number (supercoiling). totipotency The ability of a cell to proceed through all the stages of development and thus produce a normal adult. totipotent The state of a cell that can give rise to any and all adult cell types as compared with a differentiated cell whose fate is determined. trans Meaning across and referring usually to the geometric configuration of two mutant alleles (ab) across from each other on a pair of homologous chromosomes (Ab/aB). See cis. trans conformation In a heterozygote involving two mutant sites (ab) within a gene or gene cluster, the arrangement, Ab/aB. See cis conformation trans-acting Referring to mutations of, for example a repressor gene, that act through a diffusable protein product and can therefore act at a distance not simply on the DNA molecule in which they occur. transcription The synthesis of RNA using a DNA template. The process whereby RNA is synthesized from a DNA template. transcription factor A protein that binds to a cis-regulatory element (eg. an enhancer, a TATA box) and thereby, directly or indirectly, affects the initiation of transcription. Eukaryotic proteins that aid RNA polymerase to recognize promoters. Analogous to prokaryotic sigma factors. transducing particle A defective phage carrying part of the host genome in place of part of the phage genome. transduction The movement of genes from a bacterial donor to a bacterial recipient using a phage as the vector. A process whereby a cell can gain access to and incorporate foreign DNA brought in by a viral particle. transfection The process by which exogenous DNA in solution is introduced into cells. The introduction of foreign DNA into eukaryotic or prokaryotic cells. transfer operon (tra) Operon containing genes that produce the male (F-pili producing) bacterial phenotype. The F+(plus) cell (male cell) can transfer the fertility factor (F plasmid) to an F-(minus) (female) cell. transform (1) In microbiology and genetics the modification of one genotype by the external application of DNA from a cell of another genotype. (2)In oncology (the study of cancer formation) the conversion of a normal cell into a cancerous one by mutation or transfection. transformation (1) The modification of a genome by the external application of DNA from a cell of different genotype. (2) Conversion of normal eukaryotic cells to a cancer-like state of uncontrolled division. transformer An allele in fruit flies (Drosophila) that converts chromosomal females into sterile males. transgenic Organisms that have had foreign DNA stably integrated into their genome. transgenic organism An organism whose genome has been modified by introduction of novel DNA. transient diploid The stage of the life cycle of predominantly haploid fungi (and algae) during which meiosis occurs . transition A type of nucleotide-pair substitution involving the replacement of a purine with another purine, or of a pyrimidine with another pyrimidine for example GC with AT. See transversion. transition mutation A mutation in which a purine/pyrimidine base pair is replaced with a base pair in the same purine/pyrimidine relationship eg. GC with AT. translation The process of protein synthesis whereby the primary structure of the protein is determined by the nucleotide sequence in mRNA. The ribosome-mediated production of a polypeptide whose amino acid sequence is derived from the codon sequence of an mRNA molecule. translocase (EF-G) Elongation factor in prokaryotes necessary for proper translocation at the ribosome during the translation process. Replaced by eEF2 in eukaryotes. translocation 1. The relocation of a chromosomal segment in a different position in the genome. A chromosomal configuration in which part of a chromosome becomes attached to a different chromosome. 2. Also a part of the translation process in which the mRNA is shifted one codon in relation to the ribosome. transmembrane receptor A protein that spans the plasma membrane of a cell, with the extracellular domain of the protein having the ability to bind to a ligand and the intracellular domain having an activity (such as a protein kinase) that can be altered (either increased or decreased) upon ligand binding. transmission genetics The study of the mechanisms involved in the passage of genes from one generation to the next. transposable genetic element A general term for any genetic unit that can insert into a chromosome, exit, and relocate; includes insertion sequences, transposons, some bacteriophages, and controlling element. A region of the genome, flanked by inverted repeats, a copy of which can be inserted at another place; also called a transposon or a jumping gene. transposon A mobile piece of DNA that is flanked by terminal repeat sequences and typically bears genes coding for transposition functions. See transposable genetic element. transversion A type of nucleotide-pair substitution involving the replacement of a purine with a pyrimidine, or vice versa for example GC with TA. See transition transversion mutation A mutation in which a purine/pyrimidine replaces a pyrimidine/purine base pair or vice versa eg. GC with TA. trihybrid An organism heterozygous at three loci. triplet The three nucleotide pairs that compose a codon. triploid A cell having three chromosome sets, or an organism composed of such cells. trisomic A diploid cell with an extra chromosome. Basically a diploid with an extra chromosome of one type, producing a chromosome number of the form 2n + l. tritium A radioactive isotope of hydrogen. trna (transfer RNA) Transfer RNA. Small RNA molecules that carry amino acids to the ribosome for polymerization into a polypeptide. During translation the amino acid is inserted into the growing polypeptide chain when the anticodon of the tRNA pairs with a codon on the mRNA being translated. Trp Tryptophan (an amino acid). truncation selection A breeding technique in which individuals in whom quantitative expression of a phenotype is above or below a certain value (the truncation point) are selected as parents for the next generation. tumour Abnormal growth of tissue, literally means a swelling. tumour suppressor gene A gene encoding a protein that suppresses tumour formation. A gene that normally prevents unlimited cell division. When both copies of the gene are lost or mutated the cell is transformed to a cancer phenotype. Examples are the P53, retinoblastoma and Wilm's tumour. tumour virus A virus that is capable of inducing a cancer. Turner syndrome An abnormal human female phenotype produced by the presence of only one X chromosome (XO). twin spot A pair of mutant sectors within wild-type tissue, produced by a mitotic crossover in an individual of appropriate heterozygous genotype. two-point cross A cross involving two loci. two-strand double crossover A double crossover that involves only two of the four chromatids of a tetrad. type I error In statistics the rejection of a true hypothesis. type II error In statistics the accepting of a false hypothesis. typological thinking The concept that organisms of a species conform to a specific norm. In this view variation is considered abnormal. Tyr Tyrosine (an amino acid).