The nucleolus and RNA metabolism in Urechis eggs

Abstract RNA synthesis in unfertilized and fertilized eggs of Urechis caupo has been studied by autoradiography using H 3 -uridine. The nucleolar apparatus as well as the chromosomes synthesize RNA in unfertilized eggs. RNA synthesis stops in disappearing nucleoli and in chromosomes after nuclear membrane breakdown following sperm entry and does not resume during 2- to 4-cell stages. The persisting nucleolar remnant at prometaphase-metaphase I retains most of the labeled RNA synthesized immediately before sperm entry. This suggests that the large fraction of the nucleolar substance which disappears by prometaphase-metaphase I consists of ribonucleoproteins which had been produced at an earlier time, prior to labeling of unfertilized eggs.