Final answer:
Blocking the S phase of interphase would result in no DNA replication, which is essential for cell division. External forces often have the greatest impact at the G1 checkpoint of the cell cycle.
Step-by-step explanation:
The outcome of blocking the S phase of interphase in the cell cycle would be that DNA replication would not occur. This phase is crucial for replicating the cell's genetic material so that each new daughter cell receives an exact copy of the DNA. Without DNA replication, the cell would not be able to proceed to mitosis, leading to a failure in cell division.
Concerning the greatest influence of external forces on the cell cycle checkpoints, these forces tend to have a significant impact at the G1 checkpoint. This checkpoint acts as a critical point for the cell to decide whether to proceed with the cell cycle and divide, repair damage, or enter a quiescent state.