The initiation of the S phase (DNA replication) and M phase (mitosis) of the cell cycle depends on a pair of biochemicals called cyclins and cyclin-dependent kinases (CDKs).

Here's how they work:

* Cyclins: These are proteins that fluctuate in concentration throughout the cell cycle. They act as regulatory subunits for CDKs.

* Cyclin-dependent kinases (CDKs): These are enzymes that are always present in the cell but are only active when bound to the correct cyclin.

Together, cyclins and CDKs form cyclin-CDK complexes. These complexes act as checkpoint regulators that control the progression of the cell cycle by phosphorylating (adding a phosphate group to) specific target proteins. This phosphorylation can activate or inhibit these target proteins, thus controlling the cell cycle transitions.

Here's how they specifically relate to S and M phases:

* S phase initiation: The cyclin E-CDK2 complex is activated at the G1/S checkpoint, which allows the cell to enter S phase and begin DNA replication.

* M phase initiation: The cyclin B-CDK1 complex is activated at the G2/M checkpoint, driving the cell into mitosis.

In summary, cyclins and CDKs are essential for the orderly progression of the cell cycle. They act as a critical regulatory mechanism that ensures proper DNA replication and cell division.