[Development of spontaneous mutations in somatic mamalian cells and DNA replication. II. Expression of gene mutations in cultured cells].

The dependence of spontaneous rate and phenotypic expression of 6-mercaptopurine resistance mutations of DNA replication synthesis was studied in cultured Chinese hamster cells. Spontaneous mutations arising with a constant rate per cell per time unit independently on DNA replication rate were shown to be expressed only in the course of subsequent cell divisions. The frequency of N-nitrosomethylurea induced mutations in cells with reduced and normal DNA replication rate is approximately the same. However, DNA replication synthesis is necessary for the phenotypic expression of both induced and spontaneous mutations. The causes of the phenotypic lag are discussed.