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