Volume 3

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

    78/952 (page 58)
    ing the total number of viable bacteria and the other to discover the number of mutants present. The mutation rate can then be represented by the expression M f - Mj . _ m = — er— 'oge 2 Nf - N¡ where M f and are the number of mutants detected on the final and initial plates. These data are readily obtained from the control (undis turbed) plates of the Newcombe test for variation. A drawback of this direct technique is that it can be applied only to mutant characteristics which allow selection to be imposed on the plates without disturbing the colonies on them. Another approach to calculate mutation rates derives from the Luria-Delbrück fluctuation test. The Poisson distribution can be used to calculate the average number of mutations per culture (usually from the proportion of cultures that shows no mutations). The mutation rate is then given by m = Ma :7 age • loge2 N f Other, more sophisticated methods of analyzing the data have been de veloped from the concept that in an exponentially growing culture the proportion of mutants increases with time (although these approaches usually assume that the growth rates of the mutant and wild type bacteria are the same, often not a valid condition). Since old mutants divide in harmony with the population and new ones are continually added, the mutant proportion increases (up to a plateau when the mutant number is so great that reversion and other factors establish an equilibrium). These methods of analysis have been reviewed by Hayes (1968). A complicating factor in analyzing bacterial mutations is that there is often a delay in the appearance of the mutant phenotype. One cause for such delay is the segregation lag. Under conditions of rapid growth, each bacterial cell may possess more than one haploid nucleus (see Volume 1 of Gene Expression). When a mutation is dominant, it may be expressed immediately by the cell, but as Figure 2.1 shows, the number of mutant bacteria will not increase until sufficient divisions have taken place to yield a bacterium all of whose nuclei are mutant. If a mutation is reces sive, the segregation lag remains the same, but the mutant phenotype is not expressed until after segregation has yielded wholly mutant cells; this delay in expression is th ephenotypic lag. In calculating rates of mutation, it is therefore necessary to take into account the existence of multinu-