What can trigger the transition from the G1 to the S phase of the cell cycle?

The transition from the G1 phase to the S phase of the cell cycle is primarily regulated by specific proteins known as cyclins, along with their associated cyclin-dependent kinases (CDKs). The presence of specific cyclins activates these CDKs, which phosphorylate target proteins and propel the cell into the S phase for DNA synthesis.

In addition to cyclins, increased nutrient availability is also crucial for this transition. Sufficient nutrients and growth factors signal to the cell that conditions are favorable for growth and division, further promoting the transition into the S phase.

Together, the presence of specific cyclins and increased nutrient availability are both necessary for the proper regulation of the cell cycle. Thus, the option that encompasses both of these critical factors is the most accurate in describing what can trigger the transition from G1 to S phase.