Figure 3. Cell-cycle regulation of CENP-A assembly.
a | Pre-existing nucleosomes are distributed equally to sister-chromatids at the replication fork. The chromatin assembly factors CAF and ASF1 coordinate the deposition of new histone H3 and H4, and assemble new nucleosomes behind the replication fork. CENP-A density at centromeres drops by one half following replication, most probably owing to CENP-A-nucleosome segregation at the fork. The resulting nucleosome ‘gaps’ could be ‘filled’ with new H3 nucleosomes during S phase, or might be maintained as gaps until new CENP-A nucleosomes are assembled. CENP-A nucleosomes could also be split during replication, forming half-nucleosomes (not shown). b | Pulse–chase experiments in human cells show that new CENP-A is assembled at centromeres starting in telophase and continuing through G1. Thus, the amount of CENP-A at each centromere is highest at the end of G1, and is halved during S phase owing to the distribution of CENP-A to both sister chromatids. Cellular CENP-A protein levels are highest in G2, but new CENP-A assembly does not initiate until telophase. The amount of CENP-A in centromeres does not return to maximal levels until the end of G1, suggesting that kinetochore assembly occurs at centromeres containing half the maximal amount of CENP-A. MCM, minichromosome maintenance helicase; PCNA, proliferating cell nuclear antigen; DNA Pol, DNA polymerase.