Final Answer:
G1/S cyclins rise in late G1 phase and fall in early S phase. The Cdk-G1/S cyclin complex begins to induce the initial process of DNA replication, by arresting systems that prevent S phase Cdk activity in G1. The cyclins also promote other activities to progress the cell cycle, such as centrosome duplication. The rise in presence of G1/S cyclins is paralleled by a rise in G2/M cyclins.
Step-by-step explanation:
The cell cycle is a tightly regulated process, and the activity of cyclin-dependent kinases (Cdks) plays a crucial role in orchestrating its different phases. G1/S cyclins, as the name suggests, are present predominantly in the late G1 phase of the cell cycle. This marks a critical point where the cell decides whether to proceed with DNA replication or enter a resting state (G0). As G1/S cyclins rise, they form complexes with Cdks, initiating the process of DNA replication by overcoming inhibitory signals that prevent S phase Cdk activity in G1.
The subsequent fall of G1/S cyclins coincides with the entry into the S phase, where DNA synthesis occurs. This downregulation is necessary to avoid prolonged activation of Cdks and ensure proper progression of the cell cycle. The activities of G1/S cyclins extend beyond DNA replication, influencing processes like centrosome duplication, which is essential for proper cell division.
Furthermore, the rise in G1/S cyclins is paralleled by an increase in G2/M cyclins. This coordinated increase in G2/M cyclins prepares the cell for mitosis by activating Cdks that drive the cell into the G2 phase and subsequently into mitosis. This intricate regulation ensures that each phase of the cell cycle is executed accurately, preventing errors that could lead to genomic instability or uncontrolled cell proliferation.