Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Glossary
T
T (1) Thymine or thymidine. (2) See tetratype.
tagging The use of a piece of foreign DNA or a transposon to tag a gene so that a clone of that gene can be identified readily in a library.
tandem duplication Adjacent identical chromosome segments.
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 through homologous recombination and replacement of the normal allele.
tautomeric shift The spontaneous isomerization of a nitrogen base to an alternative hydrogen-bonding condition, possibly resulting in a mutation.
T-DNA A part of the Ti plasmid that is inserted into the genome of the host plant cell.
telocentric chromosome A chromosome having the centromere at one end.
telomerase An enzyme that adds repetitive units to the ends of linear chromosomes to prevent shortening after replication, by using a special small RNA as a template.
telomere The tip (or end) of a chromosome.
telophase The late stage of nuclear division when daughter nuclei re-form.
temperate phage A phage that can become a prophage.
temperature-sensitive mutation A conditional mutation that produces the mutant phenotype in one temperature range and the wild-type phenotype in another temperature range.
template 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 nucleo-tide sequence of RNA during transcription.
teratogen An agent that interferes with normal development.
terminal redundancy In phage, a linear DNA molecule with single-stranded ends that are longer than is necessary to close the DNA circle.
tertiary structure of a protein The folding or coiling of the secondary structure to form a globular molecule.
testcross A cross of an individual of unknown genotype or a heterozygote (or a multiple heterozygote) with a tester individual.
tester An individual homozygous for one or more recessive alleles; used in a testcross.
testicular feminization syndrome A human condition, caused by a mutation in a gene encoding androgen receptors, in which XY males develop into phenotypic females.
tetrad (1) Four homologous chromatids in a bundle in the first meiotic prophase and metaphase. (2) The four haploid product cells from a single meiosis.
tetrade analysis The use of tetrads (definition 2) to study the behavior of chromosomes and genes in meiosis.
tetramer A protein consisting of four polypeptide subunits.
tetraparental mouse A mouse that develops from an embryo created by the experimental fusion of two separate blastulas.
tetraploid A cell having four chromosome sets; an organism composed of such cells.
tetratype (T) A tetrad type containing four different genotypes, two parental and two recombinant.
Thr Threonine (an amino acid).
three-point testcross A testcross in which one parent has three heterozygous gene pairs.
thymidine The nucleoside having thymine as its base.
thymine A pyrimidine base that pairs with adenine.
thymine dimer A pair of chemically bonded adjacent thymine bases in DNA; the cellular processes that repair this lesion often make errors that create mutations.
Ti plasmid A circular plasmid of Agrobacterium tumifaciens that enables the bacterium to infect plant cells and produce a tumor (crown gall tumor).
tissue-specific gene expression The expression of a gene in a higher eukaryote in a specific and reproducible subset of tissues and cells during development.
topoisomerase An enzyme that can cut and re-form polynucleotide backbones in DNA to allow it to assume a more relaxed configuration.
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 lineage that is able to give rise to all possible cell fates found within a given organism.
trailer An untranslated segment at the 3 end of mRNA.
trans conformation In a heterozygote with two mutant sites within a gene or gene cluster, the arrangement a11 / 1a2.
trans-acting factor A diffusible regulatory molecule (almost always a protein) that binds to a specific cis-acting element.
transcription The synthesis of RNA with the use of a DNA template.
transcription factor A protein that binds to a cis-regulatory element (for example, an enhancer) and thereby, directly or indirectly, affects the initiation of transcription.
transduction The movement of genes from a bacterial donor to a bacterial recipient with the use of a phage as the vector.
transfection The process by which exogenous DNA in solution is introduced into cultured cells.
transfer RNA See tRNA.
transformation (1) The directed modification of a genome by the external application of DNA from a cell of different genotype. (2) Conversion of normal higher eukaryotic cells in tissue culture into a cancerlike state of uncontrolled division.
transgene A gene that has been modified by externally applied recombinant DNA techniques and reintroduced into the genome by germ-line transformation.
transgenic organism One whose genome has been modified by externally applied new 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 pyrimidinefor example, GC AT.
translation The ribosome-mediated production of a polypeptide whose amino acid sequence is derived from the codon sequence of an mRNA molecule.
translocation The relocation of a chromosomal segment in a different position in the genome.
transmembrane receptor A protein that spans the plasma membrane of a cell, with the extracellular part of the protein having the ability to bind to a ligand and the intracellular part having an activity (such as protein kinase) that can be induced on ligand binding.
transmission genetics The study of the mechanisms involved in the passage of a gene 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 phages, and controlling elements.
transposition See translocation.
transposon A mobile piece of DNA that is flanked by terminal repeat sequences and typically bears genes coding for transposition functions.
transversion A type of nucleotide-pair substitution in which a pyrimidine replaces a purine or vice versafor example, G·C T·A.
triplet The three nucleotide pairs that compose a codon.
triplet expansion The expansion of a 3-bp repeat from a relatively low number of copies to a high number of copies that is responsible for a number of genetic diseases, such as fragile X syndrome.
triploid A cell having three chromosome sets or an organism composed of such cells.
trisomic Basically a diploid with an extra chromosome of one type, producing a chromosome number of the form 2n + 1.
tritium A radioactive isotope of hydrogen.
tRNA (transfer RNA) A class of small RNA molecules that bear specific amino acids to the ribosome during translation; the amino acid is inserted into the growing polypeptide chain when the anticodon of the tRNA pairs with a colon on the mRNA being translated.
Trp Tryptophan (an amino acid).
true-breeding line or strain See pure-breeding line or strain.
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.
tumor-suppressor gene A gene encoding a protein that suppresses tumor formation. The wild-type alleles of tumor-suppressor genes are thought to function as negative regulators of cell proliferation.
tumor 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-hybrid system A pair of bakers' yeast vectors used for detecting proteinprotein interaction. Each vector carries the gene for a different foreign protein under test; if these vectors unite physically, a reporter gene is transcribed.
2- m m (2-micrometer) plasmid A naturally occurring extragenomic circular DNA molecule found in some yeast cells, with a circumference of 2 m m. Engineered to form the basis for several types of gene vectors in yeast.
Tyr Tyrosine (an amino acid).

© 1999 by W. H. Freeman and Company. All rights reserved.