Cyclins and cyclin-dependent kinases (CDKs) are very important in how cells grow and divide. They help make sure that cells divide at the right time and in the right way.

What Are Cyclins and CDKs?

• Cyclins: These are special proteins that control how the cell cycle moves along by activating CDKs. Their amounts change at different stages of the cell cycle, going up and down at certain times.

• Cyclin-Dependent Kinases (CDKs): These are enzymes that become active when they attach to cyclins. Once activated, they modify target proteins to help the cell move along in the cell cycle.

How Do They Work Together?

1. Activation: For a CDK to work, it needs to bind with a cyclin. For example, cyclin D attaches to CDK4 and CDK6 during the G1 phase, which helps the cell get ready for the next phase, S phase.

2. Phosphorylation: After a CDK is activated, it can change other proteins. For instance, during the G2 phase, the CDK-cyclin group changes proteins that the cell needs to prepare for dividing into two (mitosis).

3. Checkpoints: Cyclins and CDKs are very important at different checkpoints in the cell cycle:

   • G1 Checkpoint: Here, the cyclin D/CDK4 group looks at the cell’s size and checks if the DNA is okay.
   • G2 Checkpoint: At this checkpoint, cyclin B/CDK1 makes sure that DNA has been copied before the cell starts dividing.

Regulation

There are also other proteins called inhibitors that can control how cyclins and CDKs work. This helps stop cells from dividing too much, which is very important because if this process goes wrong, it can lead to cancer.

In short, cyclins and CDKs act like gatekeepers during the cell cycle, making sure that everything happens in order and keeping cells healthy.