Describe kinases and cyclins. How do they interact to cause cells to move through the cell cycle?

Kinases are enzymes that catalyze the transfer of a phosphate group from ATP to a target protein, typically at serine, threonine, or tyrosine residues. This process, called phosphorylation, is a common post-translational modification that can change the target protein's activity, stability, or localization. In the context of cell cycle regulation, kinases play crucial roles in controlling the progression from one phase to another.

Cyclins are a family of proteins that regulate the activity of cyclin-dependent kinases (Cdks) in the cell cycle. They are synthesized and degraded in a timely manner during different cell cycle phases, which governs the interactions with their binding partners, the Cdks, to ensure proper cell cycle progression.

The cell cycle is divided into four main phases: G1, S, G2, and M. The progression through these phases is controlled by periodic activation and inactivation of Cdks through their association with cyclins and post-translational modifications. The activated cyclin-Cdk complexes influence and coordinate multiple cellular events that facilitate cell cycle progression and maintain proper checkpoints.

Here's an overview of cyclin-Cdk interaction during the cell cycle: 1. In the G1 phase, cyclin D partners with Cdk4 and Cdk6, forming the cyclin D-Cdk4/6 complex. This complex phosphorylates the retinoblastoma protein (Rb), releasing E2F transcription factors, which promote the transcription of S-phase genes. 2. As cells enter S-phase, cyclin E levels rise and bind to Cdk2, creating the cyclin E-Cdk2 complex. This further phosphorylates Rb, enhancing the activation of E2F and promoting DNA synthesis. 3. In the S and G2 phases, cyclin A associates with Cdk2, forming the cyclin A-Cdk2 complex, which ensures the completion of DNA replication. Cyclin A also binds to Cdk1, forming the cyclin A-Cdk1 complex that prepares cells to enter mitosis. 4. Finally, in the M phase, cyclin B binds to Cdk1, activating the cyclin B-Cdk1 or M-phase promoting factor (MPF), which drives cells into mitosis. In late mitosis, cyclin B is degraded, causing MPF inactivation and exit from mitosis.

In summary, kinases (primarily Cdks) and cyclins interact closely to regulate cell cycle progression. Cyclins are synthesized and degraded in a timely manner, binding to and regulating Cdks activity, ensuring precise control over cell cycle events and transitions.