Organization of DNA into foci during replication.

In metazoan cells, DNA replication during the S phase of the cell cycle takes place at discrete locations within the nucleus. These sites, or foci, appear to participate in clustering many replicons together and synchronously regulating the activation of these replicon units. Consistent with this role. many replication proteins have been observed to attach to foci during the S phase of the cell cycle. Recently, cell-free replication extracts have been used both to characterize the events that are involved in either the formation of these sites or in the regulation of foci assembly, and to purify candidate proteins which may be integral core structural proteins responsible for forming foci. These advances provide a foundation for investigating foci structure and function at a biochemical level.