Radioactivity acts on which phase of cell cycle: March 2010

**Core Concept**
Radioactivity is a process by which unstable atomic nuclei lose energy through radiation, emitting alpha, beta, or gamma particles. In the context of cell biology, radioactivity can affect the cell cycle by causing DNA damage, leading to cell death or mutations. The cell cycle is a complex process consisting of four phases: G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis).

**Why the Correct Answer is Right**
Radioactivity primarily affects the S phase of the cell cycle, where DNA replication occurs. During this phase, the cell synthesizes new DNA strands, and any damage to the DNA template can result in mutations or cell death. Radioactive particles can directly interact with DNA, causing breaks, cross-links, or other types of damage that can be lethal to the cell. The S phase is particularly vulnerable to radioactivity due to the increased metabolic activity and the presence of replication forks, which are sensitive to radiation-induced damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because G1 is a preparatory phase where the cell grows and prepares for DNA replication, and it is not directly affected by radioactivity.
**Option B:** Incorrect because G2 is a phase where the cell prepares for mitosis, and while it is sensitive to DNA damage, it is not the primary phase affected by radioactivity.
**Option C:** Incorrect because mitosis (M phase) is the division of the cell into two daughter cells, and while it can be affected by DNA damage, it is not the primary phase affected by radioactivity.

**Clinical Pearl / High-Yield Fact**
Radioactivity can be used in cancer therapy, taking advantage of the preferential sensitivity of cancer cells to radiation-induced DNA damage, particularly during the S phase. This is the basis for radiation oncology, which uses ionizing radiation to kill cancer cells and shrink tumors.

**Correct Answer: C. S phase.**