Study of a haploid yeast strain with an unusually high rDNA content. III. Unequal meiotic segregation of the gamma-DNA fraction.

DNA from different strains of Saccharomyces cerevisiae has been fractionated in preparative Ag+/Cs2-SO4 density gradients. The results show that there are real differences in amount of the nuclear satellite component, the gamma-DNA, from one strain to the other. The gamma-DNA forms a homogeneous dense band that contains all the rDNA, and the amount of gamma-DNA estimated from the gradients can be correlated to amount of rDNA derived from rRNA-DNA hybridizations. By various crossings and sporulations we have obtained diploid and haploid strains with gamma-DNA contents ranging from 7 to 20% of the nuclear DNA. During meiosis, the amount of gamma-DNA appears to segregate in a pattern that indicates unequal crossing over as a possible mechanism for differences in gamma-DNA contents.