Circular DNA and Rolling Circles in Nucleolar rDNA from Mitotic Nuclei of Physarum polycephalum

Abstract

About 15% of nucleolar DNA (1.712 g/cm3) from Physarum polycephalum displaying maximum hybridization to ribosomal RNA, is composed of circular DNA of 3.9 ± 0.2 μm contour length or multiples thereof. A portion of these circular molecules (25%) contained linear DNA pieces longer than circumference length. In a small fraction of circular DNA linear pieces, shorter than the unit length, were observed. Most nucleolar DNA, [3H]thymidine‐labeled or hybridizable to ribosomal RNA was separable from chromosomal DNA during G2 phase, mitosis and S phase of the cell cycle. Ribosomal DNA content was not amplified during the cell cycle, was unchanged during exponential or stationary growth phase and amounted to about 0.11–0.21% of nuclear DNA in diploid and hexaploid strains of Physarum or 100–200 ribosomal genes per diploid genome. Copyright © 1975, Wiley Blackwell. All rights reserved