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- T -

T  The thymine residue in DNA.

tag  See label.

tailing  The in vitro addition of the same nucleotide by the enzyme terminal transferase, to the 3´-hydroxyl ends of a duplex DNA molecule. a.k.a. homopolymeric trailing.

tandem array  The existence of two or more identical DNA sequences in series, i.e., end -to-end.

tank bioreactor  Vessel in which fermentation takes place. A tank bioreactor is a vessel in which a micro-organism is grown in a large volume of liquid. This contrasts with fibre or membrane bioreactors and immobilized cell reactors. The large majority of bioreactors used in biotechnology are tank bioreactors, and most tank bioreactors are stirred-tank bioreactors, because stirring helps to distribute effectively gas and nutrients to the growing organism.

tap root  Root system in which the primary root has a much larger diameter than the lateral roots. Opposite: fibrous root.

Taq polymerase  A heat-stable DNA polymerase isolated from the thermophilic bacterium Thermus aquaticus, and used in PCR. See polymerase.

target  For diagnostic tests, the molecule or nucleic acid sequence that is being sought in a sample.

target site duplication  A sequence of DNA that is duplicated when a transposable element inserts; usually found at each end the insertion.

targeted drug delivery  A method of delivering a drug to the site in the body where it is needed, rather than allowing it to diffuse into many sites.

targeting vector  A cloning vector carrying a DNA sequence capable of participating in a crossing-over event at a specified chromosomal location in the host cell.

TATA box  A conserved adenine- and thymine-rich promoter sequence located 25-30 bp upstream of a gene, which is the binding site of RNA polymerase. See Pribnow box.

tautomeric shift  The transfer of a hydrogen atom from one position in an organic molecule to another position.

tautomerism  A type of isomerism in which the two isomes are in equilibrium.

T cells; T lymphocyte  Lymphocyte that pass through the thymus gland during maturation. Different kinds of T cells play important roles in the immune response, being primarily responsible for the T cell-mediated response or cellular immune response.

T cell receptor  An antigen-binding protein that is located on the surfaces of killer T cells and mediates the cellular immune response of mammals. The genes that encode T cell antigens are assembled from gene segments by somatic recombination processes that occur during T lymphocyte differentiation.

T-cell-mediated (cellular) immune response  The synthesis of antigen-specific T cell receptors and the development of killer T cells in response to an encounter of immune system cells with a foreign immunogen.

T-DNA  The segment of DNA in the Ti plasmid of Agrobacterium tumefaciens that is transferred to plant cells and inserted into the chromosomes of the plant.

T4 DNA ligase  An enzyme from bacteriophage-T4-infected cells, and that catalyses the joining of duplex DNA molecules and repairs nicks in DNA molecules. The enzyme requires that one of the DNA molecules has a 5´-phosphate group and that the other has a free 3´-hydroxyl group.

tRNA; transfer RNA  The class of small RNA molecules that transfer amino acids to the ribosome during protein synthesis. Transfer RNA molecules are folded into a 'clover-leaf' secondary structure by intrastrand base pairing. The anticodon loop contains a nucleotide-triplet complementary to a specific codon within the mRNA molecule. Each tRNA is 'charged' with the correct amino acid molecule, via its 3´ adenosine moiety, by an enzyme called aminoacyl-tRNA synthetase.

telomerase  An enzyme that adds telomeric sequences to the ends of eukaryotic chromosomes.

telemeter  The unique structure found at the end of eukaryotic chromosomes containing specialized sequences of DNA that assures the completion of a cycle of DNA replication.

telophase (Gr. telos, end + phase)  The last stage in each mitotic or meiotic division, in which the chromosomes are assembled at the poles of the division spindle.

TEM  See transmission electron microscope.

temperate phage  A phage (virus) that invades but may not destroy (lyse) the host (bacterial cell). However, it may subsequently enter the lytic cycle.

temperature-sensitive mutant  An organism that can grow at one temperature but not at another.

temperature-sensitive protein  A protein that is functional at one temperature but loses function at another (usually higher) temperature.

template  An RNA or single-stranded DNA molecule upon which a complementary nucleotide strand is synthesized. A pattern or mould. DNA stores coded information and acts as a model or template from which information is copied into complementary strands of DNA or transcribed into mRNA.

template strand  The polynucleotide strand that a polymerase uses for determining the sequence of nucleotides during the synthesis of a new nucleic acid strand.

term finalization  Repelling movement of the centromeres of bivalents in the diplotene stage of the meiotic prophase, that tends to move the visible chiasmata toward the ends of the bivalents.

terminal bud  A branch tip, an undeveloped shoot containing rudimentary floral buds or leaves, enclosed within protective bud scales.

terminal transferase  An enzyme that adds nucleotides to the 3´ terminus of DNA molecules.

termination codon  See stop codon.

termination signal  In transcription, a nucleotide sequence that specifies RNA chain termination.

terminator (of transcription)  

    1. A DNA sequence just downstream of the coding segment of a gene, which is recognized by RNA polymerase as a signal to stop synthesizing mRNA. In prokaryotes, terminators usually have an inverted repeat followed by a short stretch of Us at the very end of the transcribed portion. There may also be sequences beyond the transcribed part of the gene which influence the termination of transcription.

    2. A name given to antisense DNA inserted in plants to make impossible the use of a second generation of seed by a farmer.

terminator codon  See stop codon.

terminator region  A DNA sequence that signals the end of transcription.

test-tube fertilization  In vitro fertilization.

test tube  Tube in which cells, tissues, etc., can be cultured.

testcross  Backcross to the recessive parental type, or a cross between genetically unknown individuals and a fully recessive tester to determine whether an individual in question is heterozygous or homozygous for a certain allele. It is also used as a test for linkage, i.e., to estimate recombination fraction.

tetracycline  An antibiotic that interferes with protein synthesis in prokaryotes.

tetrad  1. The four cells arising from the second meiotic division in plants (pollen tetrads) or fungi (ascospores).

    2. The quadruple group of chromatids that is formed by the association of duplicated homologous chromosomes during synapsis in meiosis I. a.k.a. quadrivalent.

tetraploid  An organism whose cells contain four haploid (4x) sets of chromosomes.

tetrasomic (noun: tetrasome)  Pertaining to a nucleus or an organism with four members of one of its chromosomes, whereas the remainder of its chromosome complement is diploid. Chromosome formula: 2n + 2.

tetratype  In fungi, a tetrad of spores that contains four different types; e.g., AB, aB, Ab and ab.

thallus (Gr. thallos, a sprout)  Plant body without true roots, stems, or leaves.

therapeutic agent  A compound that is used for the treatment of a disease or for improving the well-being of an organism. a.k.a. pharmaceutical agent; drug; protein drug.

thermic shock Exposure to reduced or increased temperature for several days.

thermolabile  Not heatproof, e.g., a substance which disintegrates or is unstable upon heating.

thermophile  An organism which grows at a higher temperature than most other organisms. In general, a wide range of bacteria, fungi and simple plants and animals can grow at temperature up to 50°C; thermophiles are considered to be organisms which can grow at above 50°C. They can be classified according to their optimal growth temperature, into simple thermophiles (50-65°C), thermophiles (65-85°C), and extreme thermophiles (>85°C). Thermophiles and extreme thermophiles are usually found growing in very hot places, such as hot springs and geysers, smoker vents on the sea floor, and domestic hot water pipes.

thermosensitivity  Loss of activity of a protein at high temperature.

thermostability  Retention of activity at high temperature.

thermotherapy  Technique mainly used for virus or mycoplasma elimination. Plants are exposed to elevated temperatures as a treatment. Thermotherapy is used alone or in combination with meristem culture or meristem tip culture.

thinning  1. Removal of older stems to promote new growth.

    2. Removal of excess fruits to improve the size and quality of the remaining fruits.

    3. Removal of seedlings spaced too closely for optimum growth.

thymidine  A nucleoside present in DNA but absent in RNA.

thymidine kinase (tk)  An enzyme that allows a cell to utilize an alternate metabolic pathway for incorporating thymidine into DNA. Used as a selectable marker to identify transfected eukaryotic cells.

thymine  A pyrimidine base found in DNA. The other three organic bases - adenine, cytosine, and guanine - are found in both RNA and DNA; in RNA, thymine is replaced by uracil.

Ti plasmid  Tumour-inducing plasmid. A giant plasmid of Agrobacterium tumefaciens that is responsible for the induction of tumours in infected plants. Ti plasmids are used as vectors to introduce foreign DNA into plant cells. See vector.

tissue  A group of cells of similar structure which sometimes performs a special function.

tissue culture  A general term used to describe the culture of cells, tissues or organs in a nutrient medium under sterile conditions.

T lymphocyte  See T cell.

tk  See thymidine kinase.

tolerance  A form of genetic resistance in which an organism attacked or affected by a disease pathogen (or pest) exhibits less reduction in yield or performance in comparison with members of other affected cultivars or breeds.

tonoplast (Gr. tonos, stretching tension + plastos, moulded, formed) The cytoplasmic membrane bordering the vacuole, with a role in regulating the pressure exerted by the cell sap.

topo-isomerase  An enzyme that introduces or removes supercoils from DNA.

Torr  An obsolete unit of pressure equal to that exerted by a column of mercury 1mm high at 0°C and standard gravity (1mm Hg); named after Evangelista Torricelli (1608-1647), the inventor of the mercury barometer. 1 Torr = 1/760 atm = 133.322 Pa.

totipotency  Having the potentiality of forming all the types of cells in the body. The property of somatic cells to be induced to undergo regeneration. The diploid zygote formed at fertilization is a single cell which is capable of division and differentiation to give rise to the total range of cell types found in the adult organism.

totipotent cell; totipotent nucleus  An undifferentiated cell (or nucleus), such as a blastomere, that, when isolated or suitably transplanted, can develop into a complete embryo.

toxicity  Negative effect of a compound, as shown by altered morphology or physiology. It is meaningful only when the effect itself is also described, such as changes in the rate of cell growth, cell death, etc.

toxin (L. toxicum, poison)  A compound produced by an organism and poisonous to plants or animals.

trace element  See microelement.

tracer  An added or injected substance that can be followed within a reaction or an organism, such as radioactive isotopes and certain dyes.

tracheid (Gr. tracheia, windpipe)  An elongated, tapering xylem cell, with lignified pitted walls, and adapted for conduction and support. Found in conifers, ferns and related plants.

trait  See phenotype.

trans-acting  A term describing substances that are diffusable and that can affect spatially separated entities within cells.

transcapsidation  The partial or full coating of the nucleic acid of one virus with a coat protein of a differing virus. See coat protein.

trans configuration  See repulsion.

transcript  An RNA molecule that has been synthesized from a specific DNA template. In eukaryotes, the primary transcript produced by RNA polymerase must often be processed or modified in order to form the mature, functional mRNA, rRNA or tRNA.

transcription  Process through which RNA is formed along a DNA template. The enzyme RNA polymerase catalyses the formation of RNA from ribonucleoside triphosphates.

transcription factor  A protein that regulates the transcription of genes.

transcription unit  A segment of DNA that contains signals for the initiation and termination of transcription, and is transcribed into one RNA molecule.

transcription vector  A cloning vector that allows the foreign gene or DNA sequence to be transcribed in vitro.

transcriptional anti-terminator  A protein that prevents RNA polymerase from terminating transcription at specific transcription termination sequences.

transducing phage  See transduction.

transduction (t)  The transfer of DNA sequences from one bacterium to another via lysogenic infection by a bacteriophage (transducing phage). Genetic recombination in bacteria mediated by bacteriophage. Abortive t: Bacterial DNA is injected by a phage into a bacterium, but unable to replicate.

transfection  The transfer of DNA to an eukaryotic cell.

transfer  The process of moving cultured tissue or cells to a fresh medium.

transferred DNA  See T-DNA.

transferase  Enzyme that catalyses the transfer of a group of atoms from one molecule to another.

transfer RNA (tRNA)  RNA that transports amino acids to the ribosomes, where the amino acids are assembled into proteins.

transformant  In prokaryotes, a cell that has been genetically altered through the uptake of foreign DNA. In higher eukaryotes, a cultured cell that has acquired a malignant phenotype. See transformation.

transformation  1. The uptake and establishment of DNA in a bacterium or yeast cell, in which the introduced DNA often changes the phenotype of the recipient organism.

    2. Conversion by various means of animal cells in tissue culture from controlled to uncontrolled cell growth. Typically through infection by a tumour virus or transfection with an oncogene. See transformant; transformation efficiency.

transformation efficiency  The number of cells that take up foreign DNA as a function of the amount of added DNA; expressed as transformants per microgram of added DNA. See transformation.

transformation frequency  The fraction of a cell population that takes up foreign DNA; expressed as the number of transformed cells divided by the total number of cells in a population.

transforming oncogene  A gene that upon transfection converts a previously immortalized cell to the malignant phenotype. See oncogene.

transgene  A gene from one genome that has been incorporated into the genome of another organism. Often refers to a gene that has been introduced into a multicellular organism.

transgenesis  The introduction of a gene or genes into animal or plant cells, which leads to the transmission of the input gene (transgene) to successive generations. cf transfection.

transgenic  An organism in which a foreign gene (a transgene) is incorporated into its genome. The transgene is present in both somatic and germ cells, is expressed in one or more tissues, and is inherited by offspring in a Mendelian fashion. See transgenic animal; transgenic plant.

transgressive variation  The appearance in the F2 (or later) generation of individuals showing more extreme development of a trait than either of the original parents.

trans heterozygote  A double heterozygote that contains two mutations arranged in the trans configuration.

transient  Of short duration.

transition  The substitution in DNA or RNA of one purine by another purine, or of one pyrimidine by another pyrimidine.

transition stage  The integration period of juvenile and reproductive stages of growth.

transition-state intermediate  In a chemical reaction, an unstable and high-energy configuration assumed by reactants on the way to making products. Enzymes are thought to bind and stabilize the transition state, thus lowering the energy of activation needed to drive the reaction to completion.

translation  The process of polypeptide synthesis in which the amino acid sequence is determined by mRNA, mediated by tRNA molecules, and carried out on ribosomes.

translational initiation signal  See initiation codon.

translational start codon  See initiation codon.

translational stop signal  See termination codon.

translocation (L. trans, across + locare, to place)  1. The movement of nutrients or products of metabolism from one location to another.

    2. Change in position of a segment of a chromosome to another, non-homologous chromosome.

transmission electron microscope (TEM)  A microscope which uses an electron beam to obtain images of objects, with a much greater resolving power than a light microscope. See electron microscope.

transplant  1. noun: A plant grown in a cold frame, greenhouse, tissue culture or indoors for later planting outdoors.

    2. To dig up and move a plant to another location.

transposable genetic element  A DNA element that can move from one location in the genome to another. See transposon.

transposase  An enzyme encoded by a transposon gene and that catalyses the movement of a DNA sequence to a different site in a DNA molecule, by catalysing the excision of the transposon from one site and its insertion into a new chromosomal site.

transposition  The process whereby a transposon or insertion sequence inserts itself into a new site on the same or another DNA molecule. The exact mechanism is not fully understood and different transposons may transpose by different mechanisms. Transposition in bacteria does not require extensive DNA homology between the transposon and the target DNA. The phenomenon is therefore described as illegitimate recombination.

transposon  A transposable or movable genetic element. A relatively small DNA segment that has the ability to move (mobile genetic element) from one chromosomal position to another, e.g., Tn 5 is a bacterial transposon that carries the genes for resistance to the antibiotics neomycin and kanamycin and the genetic information for insertion and excision of the transposon.

transposon tagging  The insertion of a transposable element into or nearby a gene, thereby marking that gene with a known DNA sequence.

trans test  See complementation test.

transversion  The substitution of a purine for a pyrimidine, or a pyrimidine for a purine.

tribrid protein  A fusion protein that has three segments, each encoded by parts of different genes.

trichome (Gr. trichoma, a growth of hair)  A short filament of cells. See hair.

tri-hybrid  The offspring from homozygous parents differing in three pairs of genes.

tri-nucleotide repeats  Tandem repeats of three nucleotides that are present in many genes. In several cases, these trinucleotide repeats have undergone expansions in copy number, and that has resulted in inherited diseases.

tripartite mating  A process in which conjugation is used to transfer a plasmid vector to a target cell when the plasmid vector is not self-mobilizable. When (1) cells that have a plasmid with conjugative and mobilizing functions are mixed with (2) cells that carry the plasmid vector and (3) target cells, mobilizing plasmids enter the cells with the plasmid vector and mobilize the plasmid vector to enter into the target cells. Following tripartite mating, the target cells with the plasmid vector are separated from the other cell types in the mixture by various selection procedures.

triplet  A sequence of three nucleotides of DNA which specifies an amino acid. The elucidation of the genetic code involved the binding of charged tRNA species to chemically synthesized ribonucleotide triplets.

triploid  A cell or organism containing three times the haploid number of chromosomes.

trisomy (adj: trisomic) (Gr. treis, three + soma, body)  An otherwise diploid cell or organism that has an extra chromosome of one pair (chromosome formula: 2n + 1). cf disomy; monosomy.

triticale  The species formed by crossing wheat and rye.

tropism (Gr. trope, a turning)  An involuntary plant response to a stimulus, in which a bending, turning or growth occurs, such as phototropism, geotropism or hydrotropism. The response may be positive (towards) or negative (away from) to the stimulus.

true-to-type  Applied to a plant or propagation source, this term denotes correct cultivar identification and lack of variation in productivity or performance. Verification is determined visually by an expert or through biochemical, serological or other means.

trypsin  A proteolytic enzyme that hydrolyzes peptide bonds on the carboxyl side of the amino acids arginine and lysine.

trypsin inhibitors  Substances inactivating the enzyme trypsin, which is needed for digestion of peptides.

tuber  Food-storing modified roots in plants like potato.

tubulin  The major protein component of the microtubules of eukaryotic cells.

tumble tube  A glass tube mainly used in vitro to agitate and consequently aerate suspension cultures. The tube, which is commonly attached to a slowly revolving platform, is closed at both ends, with a side-neck opening.

tumour virus  A virus capable of transforming a cell to a malignant phenotype. See virus.

tumour-inducing plasmid  See Ti plasmid.

tunica  An outer one- to four-cell layered region of the apical meristem, where cell division is anticlinal, i.e., perpendicular to the surface. See apical meristem.

turbidostat  An open continuous culture in which a pre-selected biomass density is uniformly maintained by automatic removal of excess cells. The fresh medium flows in response to an increase in the turbidity (usually cell density) of the culture.

turgid (L. turgidus, swollen, inflated)  Swollen, distended; referring to a cell that is firm due to water uptake.

turgor potential  See pressure potential.

turgor pressure (L. turgor, a swelling)  The pressure within the cell resulting from the absorption of water into the vacuole and the imbibition of water by the protoplasm.

turion  An underground bud or shoot from which an aerial stem arises. cf sucker.

twins  Two individuals originating from the same zygote.

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