What is Cyclin-CDKs kinases? Write a brief note on the relation of cyclin with CDKs

Cyclin-CDK kinases are enzyme complexes that control the progression of the eukaryotic cell cycle. These complexes consist of two main components: a cyclin protein and a cyclin-dependent kinase (CDK). CDKs are serine/threonine protein kinases that are present in the cell in an inactive form. They require the binding of a regulatory protein, called a cyclin, to become active. Once a cyclin binds to a CDK, the complex becomes enzymatically active and can phosphorylate various target proteins involved in controlling key steps of the cell cycle, such as DNA replication, chromosome condensation and mitotic spindle formation.

The activity of cyclin-CDK complexes is regulated at multiple levels, including cyclin synthesis and degradation, phosphorylation and dephosphorylation of CDKs, and the presence of CDK inhibitors (CKIs). This regulation ensures that each phase of the cell cycle occurs only once and in the proper order, preventing genomic instability or uncontrolled cell division. Cyclin-CDK kinases play a key role in maintaining genome integrity and are considered essential guardians of normal cell proliferation.

Relationship of Cyclins with CDKs

The relationship between cyclins and CDKs is highly specific and tightly regulated. Cyclins are named so because their levels rise and fall cyclically during the cell cycle. Each phase of the cell cycle is controlled by a specific cyclin-CDK pair. The main types of relationships between cyclins and CDKs can be explained in the following steps:

1. Cyclins Activate CDKs:

CDKs are inactive on their own. They require a specific cyclin to bind and cause a conformational change that activates their kinase activity. Without cyclins, CDKs cannot function.

2. Stage-Specific Regulation:

Different cyclins are synthesized and degraded at different stages of the cell cycle, and each binds to specific CDKs to control that phase. For example:
  • Cyclin D-CDK4/6 regulates the early G1 phase.
  • Cyclin E-CDK2 controls the transition from G1 to S phase.
  • Cyclin A-CDK2 is active during S phase to support DNA replication.
  • Cyclin B-CDK1 is required for the G2 to M phase transition and mitosis initiation.

3. Sequential Activation:

The binding of cyclins to CDKs occurs in a sequential manner. One cyclin-CDK complex activates processes that lead to the formation or activation of the next complex. This ensures orderly progression through the cell cycle.

4. Destruction of Cyclins:

Once their function is complete, cyclins are degraded by the ubiquitin-proteasome pathway. This inactivates the CDK and stops that phase, preventing re-entry until the next proper signal arrives.
The relationship between cyclins and CDKs is highly specific and tightly regulated. Cyclins are named so because their levels rise and fall cyclically during the cell cycle. Each phase of the cell cycle is controlled by a specific cyclin-CDK pair. The main types of relationships between cyclins and CDKs can be explained in the following ...






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