T

T7 RNA Polymerase:

A bacteriophage RNA polymerase which is commonly used to transcribe plasmid DNA into RNA. The plasmid must contain a T7 promoter upstream of the relevant sequence.

Tandem Repeat Sequences:

Multiple copies of the same base sequence on a chromosome; used as a marker in physical mapping.

Taq Polymerase:

A DNA polymerase isolated from the bacterium Thermophilis aquaticus and which is very stable to high temperatures. It is used in PCR procedures and high temperature sequencing.

TATA Box:

A sequence found in the promoter (part of the 5' flanking region) of many genes. Deletion of this site (the binding site of transcription factor TFIID) causes a marked reduction in transcription, and gives rise to heterogeneous transcription initiation sites.

Technology Transfer:

The process of converting scientific findings from research laboratories into useful products by the commercial sector.

Tel:

A helix-loop-helix transcription factor fused to the PDGFß-receptor in CMML t(5:12) and to other genes in AML or MDS.

Telomerase:

A ribonucleoprotein complex that maintains the repeat sequence structures at the telomeric ends of chromosomes.

Telomere:

The natural distal end of a chromosome. Contain some form of simple repeating sequence, usually with a single stranded distal end that may form a hairpin. These specialized structures are involved in the replication and stability of linear DNA molecules. See DNA replication.

Terminator:

A sequence downstream from the 3' end of an open reading frame that serves to halt transcription by the RNA polymerase. In bacteria these are commonly sequences that are palindromic and thus capable of forming hairpins. Sometimes termination requires the action of a protein, such as Rho factor in E. Coli.

Terminal Deoxynucleotidyl:

This lymphocyte-specific enzyme normally transfers available (random) nucleotides to the 3' end of a growing nucleic acid chain. In recombinant DNA technology, these enzymes can be used to add a homogeneous tail to a piece of DNA, thereby allowing its specific recognition in PCR reactions or in cloning efforts.

Tertiary Structure:

(also see Primary and Secondary Structure) Refers to higher ordered structures conferred on proteins or nucleic acids by interactions between amino acid residues or nucleotides which are not closely positioned within the sequence (primary structure) of the molecule.

Thermostabile Polymerases:

The prototype polymerase, Taq, and newer versions such as Vent and Tth polymerase are derived from microorganisms that normally reside at high temperature. Consequently, their DNA polymerase enzymes are quite stable to heat denaturation, making them ideal enzymes for use in the polymerase chain reaction.

Tetracycline (Tet):

An antibiotics.

Thymine (T):

One of the pyrimidine bases found in DNA one member of the base pair A- T (adenine- thymine). (2,4-dihydroxy-5-methylpyrimidine).

Thymine Dimer:

It comprises a chemically cross linked pair of adjacent thymine residues in DNA, a result of damage induced by ultraviolet irradiation.

Tissue-Specific Expression:

Gene function which is restricted to a particular tissue or cell type. For example, the glycoprotein hormone alpha subunit is produced only in certain cell types of the anterior pituitary and placenta, not in lungs or skin; thus expression of the glycoprotein hormone alpha-chain gene is said to be tissue-specific. Tissue specific expression is usually the result of an enhancer which is activated only in the proper cell type.

Tm:

The midpoint of the temperature range over which DNA is melted or denatured by heat; the temperature at which a duplex nucleic acid molecule is 50% melted into single strands, it is dependent upon the number and proportion of G-C base pairs as well as the ionic conditions. Often referred to as a measure of the thermal stability of a nucleic acid probe:target sequence hybrid.

Topoisomerase:

An enzymatic activity responsible for relieving excessive supercoiling in DNA.

Trans:

Located on two physically dis-contiguous DNA molecules.

Trans-Acting Factors:

Proteins that are involved in the transcriptional regulation of a gene of interest.

Transcription:

The process of copying a gene into RNA transcript. This is the first step in the expression of any gene. The resulting RNA, if it codes for a protein, will be spliced, polyadenylated, transported to the cytoplasm, and by the process of translation will produce the desired protein molecule, although not all transcripts lead to proteins. Compare translation.

Transcription Factor:

A protein which controls the transcription of genes. These usually bind to DNA as part of their function (but not necessarily). A transcription factor may be general (i.e. acting on many or all genes in all tissues), or tissue-specific (i.e. present only in a particular cell type, and activating the genes restricted to that cell type). Its activity may be constitutive, or may depend on the presence of some stimulus; for example, the glucocorticoid receptor is a transcription factor which is active only when glucocorticoids are present.

Transcriptional Regulation:

Gene regulation is determined by the rate of transcriptional initiation. This usually results from alteration in the level of activity of transacting proteins, which, in turn, are regulated either by the amount of the transcriptionally active protein or by their state of activation.

Transcription Start Site:

The point in a DNA sequence at which transcription of a gene into RNA begins.

Transcriptome:

The complete set of RNAs transcribed from a genome.

Transduction:

The incorporation of a cellular gene into a viral genome, that can then be introduced into other cells.

Transfection:

A method by which foreign DNA may be put into a cultured mammalian cell. Such experiments are usually performed using cloned DNA containing coding sequences and control regions (promoters, etc) in order to test whether the DNA will be expressed. Since the cloned DNA may have been extensively modified (for example, protein binding sites on the promoter may have been altered or removed), this procedure is often used to test whether a particular modification affects the function of a gene.

Transfer RNA (tRNA):

A class of RNA having structures with triplet nucleotide sequences that are complementary to the triplet nucleotide coding sequences of mRNA. The role of tRNAs in protein synthesis is to bond with amino acids and transfer them to the ribosomes, where proteins are assembled according to the genetic code carried by mRNA.

Transformation:

A process by which the genetic material carried by an individual cell is altered by incorporation of exogenous DNA into its genome. The cancerous alteration of mammalian cells.

Transformation (With Respect to Cultured Cells):

A change in cell morphology and behavior which is generally related to carcinogenesis. Transformed cells tend to exhibit characteristics known collectively as the "transformed phenotype" (rounded cell bodies, reduced attachment dependence, increased growth rate, loss of contact inhibition, etc).

Transformed Phenotype:

Acquisition of a phenotype characteristic of oncogenic transformation.

Transgenic:

Creation of an individual in which genetic modification has occured in the germline tissues and may be transmitted to subsequent generations.

Transgenic Mouse:

A mouse which carries experimentally introduced DNA. The procedure by which one makes a transgenic mouse involves the injection of DNA into a fertilized embryo at the pro-nuclear stage. The DNA is generally cloned, and may be experimentally altered. It will become incorporated into the genome of the embryo. That embryo is implanted into a foster mother, who gives birth to an animal carrying the new gene. Various experiments are then carried out to test the functionality of the inserted DNA.

Transient Transfection:

When DNA is transfected into cultured cells, it is able to stay in those cells for about 2-3 days, but then will be lost (unless steps are taken to ensure that it is retained - see Stable transfection). During those 2-3 days, the DNA is functional, and any functional genes it contains will be expressed. Investigators take advantage of this transient expression period to test gene function.

Transition:

A nucleotide substitution in which one pyrimidine is replaced by the other pyrimidine, or one purine replaced by the other purine (e.g., A is changed to G, or C is changed to T).

Translation:

The process of converting the genetic code into polypeptides, catalyzed by the ribosome and a host of soluble factors. mRNA codons are recognized by tRNA anti-codons. Each tRNA codes for a single amino acid, resulting in synthesis of polypeptide wherein the amino acid sequence is dictated by and matches the order of the codons in the mRNA.

Translational Frame Shifting:

A mechanism used by certain viruses and even higher organisms to change the reading frame used during translation of an mRNA molecule, in a controlled manner, so that the polypeptide product is the result of translation in more than one reading frame.

Translocation:

The process by which a newly synthesized protein is directed toward a specific cellular compartment (i.e, the nucleus, the endoplasmic reticulum).

Transmembrane Protein:

It is a component of a membrane, a hydrophobic region or regions of the protein residues in the membrane, and hydrophilic regions are exposed on one or both sides of the membrane.

Transposible Elements:

Genetic elements characterized by their abilities to insert into and withdraw from a given location within the genome, resulting in movement from site to site within the genome over a period of time. Transposable elements may cause epigenetic changes in phenotype.

Transposon:

Naturally occurring genetic elements that are naturally easily removed and inserted into the genome, allowing for the recombination of genetic segments, giving rise to genetic diversity. These same elements can be utilized for gene therapy.

Transposition:

The movement of DNA from one location to another location on the same molecule, or a different molecule within a cell.

Transversion:

A nucleotide substitution in which a purine replaces a pyrimidine, or vice versa (e.g., A is changed to T, or T is changed to G).

Transvection:

It describes the ability of a locus to influence activity of an allele on the other homolog only when two chromosomes are synapsed.

tRNA (transfer RNA):

Special RNAs that are charged with amino acids and which carry anticodons for recognition of the codons in mRNA. tRNAs are responsible for translation from the language of nucleic acids into the language of polypeptides. Each tRNA can be charged with only one type of amino acid, although multiple tRNAs exist for many of the amino acids.

Tumor Suppressor:

A gene that prevents tumor formation until deleted or mutated. The best-known examples of tumor suppressors are the proteins p53 and Rb.

Turnover:

The balance between synthesis and degradation of a product.

Twisting Number:

Twisting number of a DNA is the number of base pairs divided by the number of base pairs per turn of the double helix.