Competence for assembly of sister chromatid cores is progressively acquired during S phase in mammalian cells

Condensed sister chromatids possess a protein scaffold or axial core to whi ch loops of chromatin are attached. The sister cores are believed to be dyn amic frameworks that function in the organization and condensation of chrom atids. Chromosome structural proteins are implicated in the establishment o f sister chromatid cohesion and in the maintenance of epigenetic phenomena. Both processes of templating are tightly linked to DNA replication itself. It is a question whether the structural basis of sister chromatid cores is templated during S phase. As cells proceed through the cell cycle, chromat id cores undergo changes in their protein composition. Cytologically, cores are first visualized at the start of prometaphase, Still, core assembly ca n be induced in G(1) and G(2) when interphase cells are fused with mitotic cells. In this study, we asked if chromatid cores are similarly able to ass emble in S-phase cells. We find that the ability to assemble cores is trans iently lost during local replication, then regained in chromosome regions s hortly after they have been replicated. We propose that core templating occ urs coincident with DNA replication and that the competence for the assembl y of the sister chromatid cores is acquired shortly after passage of replic ation forks.