| transition mutation | A point mutation involving substitution of one base-pair for another, i.e., replacement of one purine for another and of one pyrimidine for another pyrimidine without change in the purine-pyrimidine orientation. (05 Mar 2000) |
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| transition probability model | A model to account for the apparently random variation in cell cycle time between individual animal tissue cells in culture that postulates that transition from G1 to s phase is probabilistic. Contrasts with hypotheses that require the accumulation of critical levels of particular proteins. (18 Nov 1997) |
| transition protein | <molecular biology> In spermatogenesis, group of proteins that displace histones from nuclear DNA and that are in turn displaced by protamines to produce the transcriptionally inactive nuclear DNA characteristic of the sperm nucleus. (18 Nov 1997) |
| transition state | The activated state of a molecule that has partly undergone a chemicalreaction. (09 Oct 1997) |
| 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 bindand stabilise the transition state, thus lowering the energy of activation needed to drive the reaction to completion. (09 Oct 1997) |
| transition temperature | <chemistry> The temperature at which there is a transition in the organisation of, for example: the phospholipids of a membrane where the transition temperature marks the shift from fluid to more crystalline. Usually determined by using an Arrhenius plot of activity against the reciprocal of absolute temperature, the transition temperature being that temperature at which there is an abrupt change in the slope of the plot. In membranes such phase transitions tend to be inhibited by the presence of cholesterol. (18 Nov 1997) |