Abstract
The cyclic change of nucleolar number per nucleus in synchronized L-929 cells was observed during the traverse of cell cycles. The reduction of the nucleolar number, however, is not due to the disappearance of nucleoli during the replication of their chromatins for the following reasons : 1) The average number of nucleoli of synchronized L-cells was 1.84 for S phase and 1.46 for G2 and/or G1 phase. 2) Incorporation of [3H]thymidine into nucleolar DNA of synchronized L-cells was observed mainly in the S phase. Therefore, the lowest average number of nucleoli per nucleus was not observed in the S phase, in which the bulk of nucleolar DNA was under replication. 3) In randomly growing mouse ascites tumor cells, the specific activities of DNAs of nucleolar and extranucleolar chromatins which incorporated [3H]thymidine in vivo were similar. Therefore, the newly synthesized extranucleolar chromatin was not the precursor of nucleolar chromatin. 4) Three known DNA polymerases α, β and γ were found to be present in the extracts of isolated nucleoli from Ehrlich ascites tumor cells. So nucleoli themselves appear to have the machinery for DNA replication. These results suggest that the reproduction of nucleolar chromatins in mouse cells is not associated with disorganization of the whole nucleolar structure.
