Sequence of DNA replication in Allium fistulosum chromosomes during S-phase

Bromodeoxyuridine pulse labelling and immunodetection were applied to synchronized Allium fistulosum cells to study sequential changes in the chromosome replication pattern during S-phase. The replication patterns were classified into five main types depending on the location of the replication signals: (1) over the whole chromosomes; (2) at proximal and interstitial regions; (3) at proximal, interstitial and distal regions; (4) at interstitial and distal regions; and (5) at distal regions. The frequencies of each pattern changed sequentially according to the timing of BrdU incorporation, demonstrating the temporal order of chromosome replication during S-phase. The distal regions that correspond to the major C-bands replicated throughout S-phase except for the earliest stage, but most intensely in late S-phase. The replication time of different chromosome sites overlapped, which is quite different from the biphasic mode of replication that occurs in mammalian chromosomes.