The S phase is a phase in the cell cycle, in which DNA replication occurs. Not only does the DNA get replicated, but also the structure of chromatin; this means that proteins such as histones must also be replicated. The S phase is initiated by kinases called S-cyclin dependent protein kinase (Cdk), which is activated by S-cyclin. This protein then activates proteins which can begin to unwind the DNA and to begin replication. Cdk also inhibits proteins that would otherwise allow origins of replication to begin replication more than once. This ensures the DNA is replicated only once .
- ↑ Alberts et al (2008) Molecular Biology of the Cell, pages 1067-1071