AP BIOLOGY

Detailed explanation-1: -Cyclin D forms heterodimers with either CDK4 or CDK6, resulting in the activation of CDK4/6 and entry into the G1 phase of the cell cycle.

Detailed explanation-2: -During G1 phase, growth-dependent cyclin-dependent kinase (CDK) activity promotes DNA replication and initiates G1-to-S phase transition. CDK activation initiates a positive feedback loop that further increases CDK activity, and this commits the cell to division by inducing genome-wide transcriptional changes.

Detailed explanation-3: -The G1 cyclins are composed of the D-type cyclins that include cyclins D1, D2, and D3. Along with their partners, CDK4 and CDK6, G1 cyclins act early in the G1 phase of the cell cycle. The levels of G1 cyclin are low in G0 phase and increase progressively upon addition of growth factors or mitogens to the cells.

Detailed explanation-4: -Cyclin D1 plays a central role in the regulation of proliferation, linking the extracellular signaling environment to cell cycle progression [1]. The expression level of cyclin D1 is highly responsive to the action of proliferative signals including growth factor receptors, Ras, and their downstream effectors.

Detailed explanation-5: -(C) According to the classical model of cell cycle control, D-type cyclins and CDK4 or CDK6 regulate events in early G1 phase (not shown), cyclin E-CDK2 triggers S phase, cyclin A-CDK2 and cyclin A-CDK1 regulate the completion of S phase, and CDK1-cyclin B is responsible for mitosis.