[Induction of saccharomyces cerevisiae extrachromosomal-DNA].

In Saccharomyces yeast the significant portion of rRNA genes may be located as extrachromosomal copies. Extrachromosomal rDNA is represented by covalently-closed circular molecules with a major class contour length of 3.3 micrometer (the same as the length of one rDNA repeating unit). Covalently-closed circular rDNA can only be isolated from the stationary phase yeast culture (more than 60 hours of growth). Electron microscopy revealed several discrete length classes of circular molecules. Three of them are 3.3, 6.6 and 9.9 micrometers. Circular molecules were found with contour length less than 3.3 micrometers (1.1, 2.0 and 2.6 micrometers). The proportion of these molecules is 8% from the total number.