Final answer:
S cyclins bind to Cdk to induce DNA replication, with high levels maintained during S phase as well as G2 and early M phase. They regulate cell cycle progression through phosphorylation and are decomposed after fulfilling their role in cell cycle checkpoints.
Step-by-step explanation:
S cyclins bind to Cdk and the complex directly induces DNA replication. The levels of S cyclins remain high throughout S phase and throughout G2 and early M phase.
Cyclins, a group of proteins, regulate the cell cycle by binding to cyclin-dependent kinases (Cdks). The Cdk/cyclin complex must be phosphorylated, which activates the complex to phosphorylate other proteins, thus advancing the cell to the next phase. Cyclin levels fluctuate and are specifically high when they need to guide the cell past a checkpoint, such as transitioning from the growth phase G1 into the DNA synthesis phase S. After a cyclin has served its purpose, it gets degraded by cytoplasmic enzymes, which marks the progression to the next cell cycle phase.
In cell cycle regulation, S cyclins bind to Cdk (cyclin-dependent kinase) to form a complex. This complex directly induces phosphorylation on specific proteins that are involved in advancing the cell to the next phase.
During the cell cycle, the levels of S cyclins remain high throughout S phase, as well as throughout G2 and early M phase.
For example, in the G1 phase, the levels of cyclin D increase, forming a complex with Cdk4 or Cdk6, which phosphorylates proteins that allow the cell to progress to the S phase.