i know that the cdks help the cell overcome the different checkpoints. But in different text books and online they refer them as G1/S cdk, and G2/M cdk. Are they the same as S cdk and M cdk? So what’s with the G1 and G2 at the beginning?
You have to remember to keep cdks in check with different organism models.
S. pombe has only one mitotic cyclin (Cdc13) and one cdk (Cdc2) that goes thru the whole cell cycle.
S. cerevisiae and Vertebrates have complex cyclins and cdks for each phase of the cell cycle: G1, S, G2, and M phases.
For example, there is Cyclin D with CDK4 and CDK6 in G1 to make sure there is no DNA damage before going on to the S phase. Thus CDK4 and CDK5 are Mid-G1 CDKs, for vertebrates. CDK2 is then used by vertebrates to make sure that all the DNA has been duplicated and contains no errors (before moving into the M phase), and classified as a Late-G1 and also the S phase cdk.