Dependence of the nucleolar structure on DNA and RNA synthesis

The dependence of nucleolar structure on DNA and RNA synthesis in synchronous cultures of the slime mold Physarum polycephalum was traced through the mitotic cycle. The blockage of RNA synthesis produces a characteristic abnormality of the nucleolar structure when imposed at any time during interphase. But differences in the function of the early and late replicating DNA molecules were observed. The blockage of DNA synthesis causes abnormality of nucleolar structure only when imposed during the early part of the S-period.