Final answer:
After a pulse-labeling experiment with [3H] thymidine and washing out the radioactive thymidine, roughly 50% of cells would be labeled. These would be the cells that were in the S phase during the pulse, as this is when DNA synthesis occurs and the thymidine would be incorporated into DNA.
Step-by-step explanation:
The question involves understanding a pulse-chase experiment with [3H] thymidine. In this case, cells are labeled with radioactive thymidine for a short period (the pulse), after which the radioactive medium is washed out and replaced with non-radioactive medium (the chase). During the pulse, only cells that are actively synthesizing DNA (those in the S phase of the cell cycle) will incorporate the radioactive thymidine. If we assume that the cell cycle is uniformly distributed and the S phase occupies roughly 50% of the cell cycle duration, then after the pulse and washing out the radioactive thymidine, about 50% of the cells would have incorporated the radioactive label into their DNA.
Let's analyze the timeline:
- Immediately after the pulse: Cells that were in the S phase would have incorporated [3H] thymidine and become labeled.
- After a 3-hour chase: The labeled cells remain labeled since the radioactive thymidine is now part of the DNA, and washing doesn't remove it.
- After longer chase periods (5 hours, 20 hours): The proportion of labeled cells would change as labeled cells proceed through the cell cycle and new cells enter S phase, but these will not be labeled since the radioactive thymidine has been removed.
Therefore, immediately after the pulse and washing out the radioactive medium, the fraction of cells that will be labeled is 50%, assuming a constant cell cycle length and a consistent S phase duration relative to the full cycle.