ULTRASTRUCTURE OF THE NUCLEOLUS AND REPLICATION SITES OF RIBOSOMAL DNA IN ALLIUM-CEPA

Our conventional EM observations indicate that the nucleolus of Allium cepa is composed of the fibrillar centre (FC), fibrillar component (F) and granular region (G). FC is an electron-lucid zone which contains condensed or loosened chromatin. F is a circular, highly electron-dense component which surrounds FC and consists of compact fibrils. EM autoradiographic (EM ARG) studies with H-3-UdR reveal that the synthesis of ribosomal RNA (rRNA) takes place in F. G is situated around F and composed of granules about 25 to 30 nm in diameter. EM ARG studies with H-3-TdR demonstrate that silver grains are predominantly located in G of the nucleolar periphery and the region of the nucleolus-associated chromatin. In addition, H-3-TdR labels are also present, but with a much lower frequency, in FC, F and G of the nucleolar interior. According to the distribution and sphere of silver grains in different components of the nucleolus, we suggest that the replication sites of ribosomal DNA (rDNA) are mainly located in G of the nucleolar periphery and the region of the nucleolus-associated chromatin. Our observations also suggest that some segments of rDNA initiate their replication in FC and then move to the nucleolar periphery.