Chromosome condensation from prophase to late metaphase: relationship to chromosome bands and their replication time

As chromosomes condense during early mitosis, their subbands fuse in a highly coordinated fashion. Subband fusion occurs when two large subbands flanking one minor subband come together to form one band, which takes on the cytological characteristics of the original flanking subbands. Using four different banding techniques--GTG (G-bands obtained with trypsin and Giemsa), GBG (G-bands obtained with BrdU and Giemsa), RHG (R-bands obtained by heating and Giemsa), and RBG (R-bands obtained with BrdU and Giemsa)--we studied subband fusion from prophase (1,250 bands per haploid set) to late metaphase (300 bands). To quantify the condensation process, a fusion index was established. We found that chromosomes contain preferential zones of condensation. From prophase to late metaphase, the early replicating subbands (R-subbands) fuse more readily with each other than do the late-replicating subbands (G-subbands). R-bands usually replicate early and condense late independently of the adjacent G-bands, which replicate late but condense early. Therefore, chromosome bands can undergo DNA replication and chromatin condensation relatively autonomously. Our data suggest that (1) chromosome replication and condensation are closely connected in time, (2) the metaphase bands represent independent units of chromatin condensation, and (3) the condensation process is an important feature of chromosome organization.