Transfer RNA See ribonucleic acid

transformation An alteration in cell morphology and/or behavior, involving loss of contact inhibition and usually the acquisition of neoplastic potential. Transformation may occur spontaneously, or after exposure to certain chemical carcinogens. But it is most usually observed after infection with oncogenic viruses, retroviruses and DNA tumor viruses. Retroviruses which transform cells in vitro usually carry a transduced oncogene, such as v-src, which transforms the cells with very high efficiency. Alternatively, they may activate cellular proto-oncogenes following integration into the DNA genome. Both RNA and DNA tumor viruses may also induce proteins in infected cells (e.g. T antigens) which combine with tumor suppressor genes and inactivate them, leading to cell transformation. Transformed cells can be maintained indefinitely in culture, unlike non-transformed cells. See immortalization.

Wisdom R and Verma IM (1999) In Encyclopedia of Virology, Second edition, edited by A Granoff and RG Webster. London: Academic Press, p. 1817

transformation assay When viruses have a sufficiently high transforming activity (e.g. Rous sarcoma virus), it is possible to assay the frequency with which cells are transformed by observing the effect of the virus on a monolayer culture. Transformed cells grow in a manner different from that of normal cells, forming small, heaped-up colonies (foci) of morphologically altered cells. A focus assay for transformation is analogous to a plaque assay for infectivity.

transgenic mice Strains of mice that have a deliberately altered genome. Used for experimental purposes. Genes may be added (e.g. adding human genes specifying poliovirus receptors) or deleted (e.g. deleting the prion gene), in which case they are called 'knock-out' mice.

transition temperature Temperature at which double-stranded nucleic acid dissociates into single strands.

translation The process of making a protein chain from the information in the mRNA. The four-letter language of the nucleic acid (sequence of bases) is translated into translation

a 20-letter protein (sequence of amino acids).

translocation A chromosomal aberration which results in a change in position of a gene within the genome.

transmethylase Enzyme catalyzing the addition of methyl groups, e.g. to RNA or DNA. Present in virions of Reoviridae or Rhabdoviridae and involved in formation of the 5' cap structure on mRNA.

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