. Cell heredity. Cytogenetics. 54 CELL HEREDITY F(0) as the fraction of cultures which had no mutational events, and from this calculate m. The assumption is that the mutational events are distributed at random, and this has been amply demonstrated to be the case. Experimentally, ^(O) niay be measured by plating cultures of bacteria on agar media where mutants may be detected. The frequency of mutants is usually so small (ca. 10~^) that the medium preferably should be selective, allowing no growth of the parents. Thus, among the milnv millions of the latter on the plate, only the mutants prese
. Cell heredity. Cytogenetics. 54 CELL HEREDITY F(0) as the fraction of cultures which had no mutational events, and from this calculate m. The assumption is that the mutational events are distributed at random, and this has been amply demonstrated to be the case. Experimentally, ^(O) niay be measured by plating cultures of bacteria on agar media where mutants may be detected. The frequency of mutants is usually so small (ca. 10~^) that the medium preferably should be selective, allowing no growth of the parents. Thus, among the milnv millions of the latter on the plate, only the mutants present will form colonies. This method of measuring mutations has been adapted for use with liquid cultures and also is conveniently employed in cases where mutants form papillae on the surface of colonies. In this case some colonies may have no papillae at all. Letting this fraction equal F(0), the average number of papillae, m, can be calculated from equation Since each papilla represents one mutational event, the calculated m can be com- pared with the m obtained by counting the number of papillae directly. The two estimates are found to agree because mutation is random. Once m is known, the chance of mutation per cell per division, a, can be estimated from the equation defining mutation rate: a = mid () where d is the total number of divisions that have taken place. The term d cannot be measured directly but it is in some cases approximately equal to N, the total number of cells present per culture. This can be seen by considering the properties of a clone produced by repeated divi- sion into two:. Here one division gave rise to two, three divisions to four, and seven divisions to eight cells. The number of divisions counted from the first cell is one less than the number of cells produced. As a , when a large culture is started from a few cells, d^N, and N may be substituted for (/ in equation In this way one can measure the rate of mutation of manv mi
Size: 2144px × 1165px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiv, bookpublishernewyorkwiley
