What is the role of cyclin D in the cell cycle?

Cyclin D plays a crucial role in regulating the cell cycle, particularly in the transition from the G0 phase to G1 phase and then from G1 phase to the S phase. As cells prepare to enter the cell cycle, cyclin D levels rise, leading to its binding with cyclin-dependent kinases (CDKs). This complex then phosphorylates target proteins that are key for cell cycle progression.

These phosphorylations facilitate the movement of the cell from a quiescent or resting state (G0) into an active state (G1), and further into the S phase where DNA replication occurs. By promoting these transitions, cyclin D is essential for ensuring that a cell can properly respond to growth signals and begin the processes necessary for division.

The other choices do not reflect the primary functions of cyclin D. While some cyclins may be involved in other processes such as transcription activation or spindle assembly, cyclin D's specific role is accurately represented by its involvement in advancing through the cell cycle phases.