Tandem duplication
A DNA sequence that is repeated in direct orientation.

3'-terminus
The end of a polynucleotide which carries the hydroxyl group attached to the 3' position of the sugar.

5'-terminus
The end of a polynucleotide which carries the phosphate group attached to the 5' position of the sugar.

T-DNA
The portion of the Ti plasmid transferred from Agrobacterium to the plant DNA.

Tandem duplication
Two adjacent copies of a DNA sequence.

Tandem repeats
Multiple adjacent copies of the same sequence.

Tautomeric shift
A reversible change in the position of a hydrogen atom in a molecule which results in the conversion of the molecule between different isomers. A shift between the keto group and a enoyl group in nucleotides can result in altered base-pairing.

Taxonomy
The classification of organisms.

Telomere
The terminal part of a linear chromosome. Replication of the ends of linear DNA molecules requires specialized enzymes or structures. Often the telomers have a DNA sequence with a single-stranded end that can fold into a hairpin structure.

Temperate phage
A phage that is capable of becoming a prophage in the bacterial host (i.e. maintain itself in a relatively quiescent state). Improperly but frequently called a lysogenic phage.

Temperature-sensitive mutation
A mutation that results in a gene product that is functional within a certain temperature range (e.g. at less than 30°C), but nonfunctional at different temperatures (e.g. at 42°C).

Template
A single-stranded polynucleotide (or region of a polynuceotide) that can be copied to produce a complementary polynucleotide.

Terminal redundancy
The presence of identical DNA sequences repeated at the two ends of DNA molecule (e.g. phage particles).

Terminator
A DNA sequence that results in termination of transcription. See Transcription terminator.

Terminus
The region of DNA sequences where DNA replication terminates.

Tetracycline (Tet)
An antibiotic that inhibits protein synthesis by preventing aminoacyl tRNA from binding to ribosomes. There are several possible mechanisms of Tetracycline resistance. TetR encoded by Tn10 and pBR plasmids is due to a membrane protein that actively transports tetracycline out of the cell.

Tetrad analysis
A method for establishing linkage relationships in fungi by analysing the four products from individual meiotic divisions.

Tetrad
The four products of a single meiosis.

Theta-replication
Replication of a circular molecule of double-stranded DNA by initiation at a unique origin and proceeding in one or both directions around the molecule. The resulting intermediate looks like the greek letter theta.

Three-factor cross
A method for determining the genetic map position of three linked loci based upon relative frequency of coinheritance of each locus during a cross between two strains with different allelic forms in each gene.

Thymine dimer
See pyrimidine dimer.

Ti plasmid
The large plasmid found in those Agrobacterium tumefaciens cells able to direct crown gall formation on certain species of plants.

Topoisomerase
An enzyme which introduces or removes overwinding or underwinding of the DNA circular duplex by causing a nick, rotating the strands, and then ligating them.

trans
Genes located on different DNA molecules present in the same cell (the opposite of cis). For example, one copy of a particular gene may reside on the chromosome and the second copy may reside on a plasmid.

Trans acting factor
A molecule that can diffuse through the cell to act at a distance from where it is made. Often used to distinguish gene products (protein or RNA) that regulate gene expression in trans vs the DNA site where the gene products binds.

Transcript
A strand of RNA copied from a DNA template.

Transcription bubble
A region where the double-stranded DNA is separated while RNA polymerase is actively transcribing RNA. A short region of RNA-DNA duplex is formed between the newly synthesized RNA and the template DNA in this region.

Trans-dimer synthesis
A process which permits nucleotides to be inserted opposite a pyrimidine dimer. Because this process is not based upon complementary base pairing, the wrong base pairs may be inserted, resulting in a mutation.

Transcription
The synthesis of RNA from a DNA template. Catalyzed by RNA polymerase.

Transcription terminator
A nucleotide sequence that acts as a signal for termination of transcription. There are two common types of transcription terminators: Rho-independant terminators (usually a stem-loop structure in the transcribed RNA followed by a run of U residues) are typically located at the end of operons, and Rho-dependant terminators (typically an unstructured region of RNA that, when untranslated, is recognized by Rho factor) are typically responsible for translational polarity.

Transcription unit
A region of DNA (a gene or an operon) transcribed as a single RNA.

Transcriptome
The complete set of RNA transcripts made by a cell under a particular condition. Typically determined by microarray analysis.

Transductant
A genetic recombinant formed by transduction.

Transduction
A method of gene transfer between bacteria in which the bacterial donor DNA is carried by a phage. There are two types of transduction: generalized transduction and specialized transduction. Generalized transduction can transfer of any region of the chromosome from the bacterial host into a recipient cell. Specialized transduction can only transfer regions of DNA adjacent to an integrated phage (prophage).

Transfection
The introduction of purified phage DNA into a bacterial cell by transformation or electroporation.

Transfer RNA (tRNA)
Adaptor molecules which translate the triplet code from the mRNA sequence into the corresponding chain of amino acids. tRNAs are short (about 74-95 bases), single-stranded RNA molecules that contain a high proportion of modified nucleosides. When drawn in two-dimensions, tRNAs can be folded into a characteristic cloverleaf structure with three stem-loop structures. The anticodon at the base of the second loop region. A specific amino acid is are added to the 3' end of each tRNA by a specific aminoacyl tRNA synthetase. The aminoacylated tRNA binds to the ribosome-mRNA complex via interactions with the ribosome and, if the anticodon in the tRNA is properly paired with the complementary bases in the codon at that position, the amino acyl group is transferred to the growing polypeptide chain.

Transformation
Transfer of naked donor DNA into a cell.

Transformation frequency
The relative proportion of cells in a population that are transformed in a single experiment.

Transition mutation
A base substitution mutation where a purine is replaced by a different purine, or a pyrimidine is replaced by a different pyrimidine.

Translation
The assembly of amino acids into polypeptides using the genetic information encoded in the molecules of mRNA.

Translesion synthesis
A mechanism that resumes stalled replication due to a damage on the template strand. The stalled replicative polymerase is replaced by translesion polymerase(s) that synthesises a short stretch of DNA across the lesion. Once this occurs, the replicative polymerase resumes DNA synthesis.

Transposable element
A transposon or insertion sequence. An element that can insert in a variety of DNA sequences.

Transposase
An enzyme (or enzyme complex) required for the transposition of a particular transposable element. A transposase must recognize specific sites on the ends of a transposon, cut the transposon out of the original site, and insert the transposon into a new site.

Transposition
The movement of a discrete segment of DNA from one location in the genome to another.

Transposon
A genetic element which, in addition to encoding the proteins required for its own transposition, confers one or more new observable phenotypes (often resistance to one or more specific drugs) on the host cell.

Transposon tag
Use of a transposon insertion in a gene to follow the inheritance of the gene. Because transposons often express phenotypes that are simple to select and screen for (e.g. antibiotic resistance), it is often much easier to follow inheritance of the transposon insertion than the unmarked gene.

Transversion
A base substitution mutation where a pyrimidine replaces a purine, or a purine replaces a pyrimidine.

Triplet
A sequence of three nucleotides. Typically refers to the codons and the corresponding genetic code.

Truncation
To shorten. For example, a truncated protein results if a premature stop codon interrupts the gene.

Two-componant system
A regulatory mechanism that includes at least two functional activities defined as a sensor and a response regulator. Phosphorylation of the sensor domain is modulated in response to specific physiological stimulus. Phosophotransfer between a histidine residue on the sensor domain and an aspartate residue inĘthe response regulator domain determines the phosphorylation state of the response regulator. The phosphorylation state of the response regulator determines its physiological role (DNA binding, repression, activation, protein-protein interactions, or enzymatic activity). In many cases the two componants are present on separate proteins, but in some cases both componants are present in a single protein, and in other cases there are more than two proteins involved in the phosphotransfer reactions.

Two µm circle
A plasmid found in the yeast Saccharomyces cerevisiae and used as the basis for a series of cloning vectors.