A researcher fuses a cell in S-phase with a cell in G1. The G1 nucleus immediately begins DNA replication. In a separate experiment, an S-phase cell is fused with a G2 cell, and the G2 nucleus does not re-replicate.
Which conclusion is best supported?
- Acheck_circle
S-phase cells contain a diffusible factor that triggers replication, but G2 nuclei are licensed only once per cycle
- B
G1 nuclei cannot replicate DNA
- C
G2 nuclei always re-replicate when fused
- D
S-phase cells block replication
Explanation
The S-phase cell must contain a positive trigger (S-phase promoting factor) that activates replication in the receptive G1 nucleus. The G2 nucleus, having already replicated and not re-licensed, cannot duplicate again until passing through mitosis.