Assertion: DNA synthesis occurs in `G_(1)` and `G_(2)` periods of cell cycle.
Reason: During `G_(1)` and `G_(2)` phase the DNA contents become double.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To analyze the assertion and reason provided in the question, we will break down the concepts step by step.

### Step-by-Step Solution:

1. **Understanding the Cell Cycle**:
- The cell cycle consists of two main phases: Interphase and M phase (mitosis).
- Interphase is further divided into three phases: G1 (first gap), S (synthesis), and G2 (second gap).

2. **G1 Phase**:
- The G1 phase is characterized by cell growth and an increase in the number of organelles.
- There is no DNA synthesis occurring during this phase; the DNA content does not double.

3. **S Phase**:
- The S phase is where DNA synthesis occurs. During this phase, the DNA is replicated, resulting in the doubling of DNA content.
- After the S phase, each chromosome consists of two sister chromatids.

4. **G2 Phase**:
- In the G2 phase, the cell continues to grow and prepares for mitosis.
- The DNA content is already doubled due to the synthesis that occurred in the S phase, but no additional DNA synthesis occurs in G2.

5. **Evaluating the Assertion**:
- The assertion states that "DNA synthesis occurs in G1 and G2 periods of the cell cycle." This is incorrect because DNA synthesis only occurs in the S phase.

6. **Evaluating the Reason**:
- The reason states that "During G1 and G2 phase the DNA contents become double." This is also incorrect because the DNA content only doubles during the S phase, not in G1.

7. **Conclusion**:
- Both the assertion and the reason are false. Therefore, the correct answer is that both statements are incorrect.

### Final Answer:
Both the assertion and reason are false.