What are cyclins?

Cyclins are a special group of regulatory proteins that control the proper timing and order of the cell cycle. They are called "cyclins" because their concentration inside the cell rises and falls in a cyclical pattern during different phases of the cell cycle. They are not enzymes by themselves but become functional when they bind with another group of proteins called cyclin-dependent kinases (CDKs). These CDKs are enzymes that phosphorylate target proteins and this phosphorylation triggers specific events in the cell cycle like DNA replication or mitosis.

Cyclins are essential because they ensure that the cell cycle progresses in the correct sequence. Without them, cells would not know when to grow, when to copy DNA, or when to divide. Cyclins are produced at specific times during the cell cycle and once their function is complete, they are broken down by a system called the ubiquitin-proteasome pathway. This degradation prevents the cell from repeating the same phase and forces it to move forward in the cycle.

The balance of cyclins and CDKs is highly regulated. If cyclins are not produced or destroyed properly, it may lead to cell cycle problems, uncontrolled cell division, or even cancer. So, cyclins not only help the cell grow and divide at the right time but also protect it from making dangerous mistakes.

Types of Cyclins

There are four major types of cyclins, and each one works at a particular phase of the cycle:

1. Cyclin D (Active in G1 phase):

Cyclin D is the first cyclin to appear during the cell cycle. It is produced in response to external growth signals. Its main job is to help the cell pass the G1 checkpoint and move forward. It binds with CDK4 and CDK6, and this complex helps in the phosphorylation of the retinoblastoma (Rb) protein, which in turn releases E2F transcription factors, promoting entry into the S phase. Cyclin D acts like a sensor of growth signals and prepares the cell to commit to division.

2. Cyclin E (G1 to S transition):

Cyclin E is expressed later in the G1 phase and is very important for the G1/S transition. It binds with CDK2 and this complex helps in the initiation of DNA replication. Cyclin E ensures that all preparations for DNA synthesis are complete and that the cell is now ready to enter the S phase. It also helps the cell to pass through the restriction point, after which the cell becomes committed to division.

3. Cyclin A (S and G2 phase):

Cyclin A is active during both the S and G2 phases. It first binds with CDK2 in the S phase to promote DNA replication, ensuring that the entire genome is copied once and only once. Later, in the G2 phase, it switches to bind with CDK1 and helps prepare the cell for mitosis. Cyclin A helps regulate both DNA synthesis and the start of mitotic events.

4. Cyclin B (G2 to M transition):

Cyclin B is the last major cyclin in the cycle. It binds with CDK1 to form the well-known Maturation Promoting Factor (MPF), which is essential for the entry into mitosis. MPF triggers major events of mitosis like chromatin condensation, nuclear envelope breakdown, and spindle formation. Cyclin B accumulates in the G2 phase and reaches its peak during mitosis.





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