Final answer:
Without the G1 and G2 phases, cells would not be prepared for DNA replication and mitosis, potentially leading to incomplete DNA replication or mitotic errors. The G1 checkpoint is where external forces have the greatest influence on the cell cycle.
Step-by-step explanation:
If a cell only underwent the M and S phases of the cell cycle, without the G1 and G2 phases, several critical issues would arise. First, during the G1 phase, a cell grows and accumulates the materials necessary for DNA replication and cell division. Without this phase, the cell might not have the necessary components to successfully replicate its DNA during the S phase. Second, skipping the G2 phase means the cell would not have a period to grow and prepare for mitosis. The G2 phase also allows for the repair of any DNA that was improperly replicated, and without this phase, these errors would go uncorrected. This situation could lead to issues such as incomplete DNA replication or errors during mitosis, resulting in cells that lack vital genetic material or with damaged DNA, which can cause cell malfunction or death.
Regarding the question about the outcome of blocking S-phase, the correct answer would be b. DNA replication would not occur. If the sister chromatids of a chromosome did not separate during mitosis, the result would likely be cells with unequal genetic material, leading to disorders in the daughter cells. In the case of an abnormal cyclin-dependent kinase at the G2/M checkpoint, it is likely that the cell would struggle to enter mitosis properly, which could result in cell cycle arrest or malfunctioning division processes.
External forces have the greatest influence at the G1 checkpoint, where a cell assesses factors such as nutrient availability and growth factors before committing to another round of cell division.