PATHOLOGY

Detailed explanation-1: -At the heart of the cell-cycle control system is a family of protein kinases known as cyclin-dependent kinases (Cdks). The activity of these kinases rises and falls as the cell progresses through the cycle.

Detailed explanation-2: -In human cells, there are 20 CDKs and 29 cyclins [17]. CDK1, CDK2, CDK3, CDK4, CDK6, and CDK7 directly regulate cell-cycle transitions and cell division, whereas CDK7–11 mediate gene transcription.

Detailed explanation-3: -Cyclins are a family of proteins that have no enzymatic activity of their own but activate CDKs by binding to them. CDKs must also be in a particular phosphorylation state-with some sites phosphorylated and others dephosphorylated-in order for activation to occur.

Detailed explanation-4: -Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases (Cdks). A lone Cdk is inactive, but the binding of a cyclin activates it, making it a functional enzyme and allowing it to modify target proteins.