Point Mutations 



191 



methods of analysis, the gene appears to be 

 a very stable entity. 



Consider the following method for obtain- 

 ing information with regard to gene muta- 

 tion. All mutants involving the one or more 

 genes being investigated are collected and 

 then analyzed. Some mutants involving a 

 given locus prove to be based upon changes 

 in the number of whole chromosomes; others 

 prove to be associated with gross or small 

 chromosomal rearrangements. All these 

 mutants are eliminated from further consid- 

 eration even though gene mutation may also 

 have occurred. Insofar as feasible, all ge- 

 netic and cytological tests known are applied 

 to eliminate mutants involving the minutest 

 chromosomal rearrangements including, for 

 example, tiny duplications or deficiencies. 

 All, or a considerable number, of the mu- 

 tants remaining can then be assumed — for 

 lack of evidence to the contrary — to have 

 resulted from mutations involving either a 

 single gene (gene mutations) or at most 

 only a few genes (intergenic mutations). 

 Each of the remaining mutants behaves as 

 though it resulted from a change at a single 

 point in the crossover and cytological maps 

 and is, therefore, called a point mutant. 

 Since at this point we have no criterion for 

 differentiating between a mutant involving 

 only one gene (including its complete loss) 

 and one involving a few genes, the entire 

 heterogeneous class of point mutants will 

 have to be studied in the hope of revealing 

 some of the characteristics of gene mutation. 

 Consider some of the characteristics of 

 spontaneous and induced point mutations. 

 Since point mutation of a vast number of 

 different genes occurs, this process is not 

 restricted to a very limited type of gene. 

 Although the conditions causing point muta- 

 tion might be of such a nature that, in the 

 diploid cell, both members of a pair of alleles 

 tend to mutate at the same time, actually the 

 evidence is that only one gene of the pair 

 is affected. Because only one member of 



a pair of genes in a nucleus mutates, point 

 mutation must be a very localized, submicro- 

 scopic event. 



If point mutation usually involved either 

 a series of stable gene changes or an in- 

 stability of the gene extending over more 

 than one cell generation, the resultant mu- 

 tants would usually occur in clusters and 

 within a cluster the same gene might not 

 always mutate to the same allele. But many 

 point mutants occur singly. Moreover, 

 those which do appear in a cluster often 

 seem to be identical. Such a cluster can 

 usually be explained by assuming a single 

 cell has undergone mutation, having divided 

 a number of times before the tests to detect 

 the mutants were performed. Although 

 such data do not prove that point mutation 

 is instantaneous, they indicate that it is usu- 

 ally completed within one cell generation 

 and the change in this respect is quick more 

 than it is gradual. The number of point 

 mutations obtained from X-ray or ultra- 

 violet ray treatments is reduced, however, 

 if posttreatment with certain types of visible 

 light or chemical substances is given im- 

 mediately (but not if such treatment is post- 

 poned for some hours). Such immediate 

 posttreatments produce photo- or chemo- 

 recovery from point mutation and prove that 

 the point mutation process often does not 

 occur or is not completed for some minutes. 

 Certain chemical changes, which themselves 

 may or may not be mutational, can lead to 

 other, genetic changes such as breakage. If 

 the first changes are repaired before they can 

 induce the second, an apparent recovery 

 from mutation is observed. Only after the 

 point mutation process is completed is the 

 new genetic alternative just about as stable 

 as the old. 



Because point mutants are just about as 

 stable as their parent genes or other genes 

 in the genotype, it does not necessarily mean 

 that all allelic and nonallelic genes have 

 the same spontaneous mutation frequency. 



