Cyclins accumulate and decline in concentration during the cell cycle, and they activate CDKs by binding to them. Different cyclins are associated with different CDKs, and the specific cyclin-CDK complexes that are present in a cell determine what stage of the cell cycle the cell is in.

The production of cyclins and CDKs is directed by a variety of cellular signals, including growth factors, hormones, and DNA damage. When these signals are present, they activate transcription factors that induce the expression of cyclin and CDK genes. The levels of cyclins and CDKs in the cell then rise, leading to the formation of cyclin-CDK complexes and the progression of the cell cycle.

Here is a more detailed explanation of how cyclins and CDKs control the cell cycle:

1. G1 phase:

- The cell grows and prepares for DNA replication.

- Cyclin D levels rise and bind to CDK4 and CDK6.

- The cyclin D-CDK4/6 complex phosphorylates the retinoblastoma protein (Rb), which releases the transcription factor E2F.

- E2F activates the expression of genes involved in DNA replication.

2. S phase:

- DNA replication occurs.

- Cyclin E levels rise and bind to CDK2.

- The cyclin E-CDK2 complex phosphorylates several proteins involved in DNA replication, including the MCM proteins and DNA polymerase.

3. G2 phase:

- The cell checks for DNA damage and prepares for mitosis.

- Cyclin A levels rise and bind to CDK1.

- The cyclin A-CDK1 complex phosphorylates several proteins involved in mitosis, including the nuclear envelope and the spindle fibers.

4. M phase:

- Mitosis occurs, and the cell divides into two daughter cells.

- Cyclin B levels rise and bind to CDK1.

- The cyclin B-CDK1 complex phosphorylates several proteins involved in mitosis, including the nuclear envelope and the spindle fibers.