AP BIOLOGY

Detailed explanation-1: -Cyclin D1 plays an important role in cell cycle progression through the association with CDK4 and CDK6, which phosphorylate and inactivate the retinoblastoma protein pRb, leading to the expression of a subset of proliferation-associated E2F target genes.

Detailed explanation-2: -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-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 are the regulatory subunits of holoenzyme CDK complexes that control progression through cell-cycle checkpoints by phosphorylating and inactivating target substrates. The cyclins associate with different CDKs to provide specificity of function at different times during the cell cycle (see Fig.

Detailed explanation-5: -The cell cycle is regulated by many cyclins and cyclin-dependent kinases (CDKs) that are a group of serine/threonine kinases. They form complexes with cyclins to stabilize, activate, and phosphorylate CDKs in the specific phases [6, 7].