Question

Which of the following is the most radiosensitive phase of the cell cycle?

• A. G1
• B. S
• C. G2
• D. M

Hint:

Cells caught mid-division, with condensed chromatin and no time to repair, are hit hardest by radiation.

Answer 1

The Correct Option is D

Step 1: Recall the phases of the cell cycle.
A dividing cell passes through G1, the growth phase before DNA synthesis, S, where DNA is replicated, G2, the growth phase before division, and M, mitosis itself.

Step 2: Recall the law that governs radiosensitivity.
The law of Bergonie and Tribondeau states that cells are more radiosensitive when they divide rapidly, have a long dividing future, and are less differentiated. This same idea extends within a single cell cycle: the phase where the cell is busy dividing, with condensed, exposed chromatin and little time to repair damage, is the most vulnerable.

Step 3: Rank the phases by sensitivity.
M phase, mitosis, is the most radiosensitive because chromatin is tightly condensed and the cell has no opportunity to pause and repair DNA damage before the chromosomes must segregate. G2 is also quite sensitive since the cell is preparing to enter mitosis. Late S phase is the most radioresistant, since active DNA repair machinery, particularly homologous recombination using the sister chromatid, is available at that time. G1 sits at an intermediate level of sensitivity.

Step 4: Rule out the distractors.
G1, while some sensitivity is present especially late G1, is not the peak. S phase is actually the most resistant part of the cycle, the opposite of what the question asks. G2 is close to M in sensitivity but M is still ranked highest classically.

Step 5: Final answer.
\[ \boxed{\text{M phase}} \]