Final answer:
Passage of the G1 checkpoint primarily involves assessing cell size, nutrient availability, and DNA integrity to prepare the cell for DNA replication in the S phase. It is different from the G2 and mitotic spindle checkpoints, which evaluate chromosome duplication and spindle fiber attachment, respectively.
Step-by-step explanation:
The G1 checkpoint is a crucial control mechanism of the cell cycle just before the cell enters the S phase. Its main function is to ensure that the cell is prepared for DNA replication and is large enough to divide. Passage of the G1 checkpoint entails cell growth and preparation for division. If conditions are not favorable, or if the cell is not big enough, it may enter a resting state known as G0.
At the G1 checkpoint, the integrity of the DNA is assessed. Unlike the G2 checkpoint, where proper chromosome duplication is evaluated, or the mitotic spindle checkpoint in metaphase, the G1 checkpoint does not involve completion of DNA synthesis (which occurs during the S phase), chromosome condensation, or attachment to spindle fibers (both of which are assessed at later checkpoints).
In summary, passage through the G1 checkpoint primarily involves ensuring that the cell is of adequate size and has sufficient nutrients, and that the DNA integrity is uncompromised, thus preparing the cell to enter the S phase.