Radioactive thymidine when added to the medium surrounding living mammalian cells gets incorporated into the newly synthesised DNA. Which of the following types of chromatin is expected to become radioactive if cells are exposed radioactive thymidine as soon as they enter the S-phase?

Options

A. Heterochromatin
B. Euchromatin
C. Both (a) and (b)
D. Neither heterochromatin nor euchromatin but only the nucleolus

Explanation

To solve the question regarding the incorporation of radioactive thymidine into chromatin during the S-phase of the cell cycle, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the S-phase**:
- The S-phase (Synthesis phase) of the cell cycle is when DNA replication occurs. During this phase, the entire DNA content of the cell is duplicated.

2. **Role of Radioactive Thymidine**:
- Radioactive thymidine is a modified nucleotide that can be incorporated into newly synthesized DNA. When cells are exposed to radioactive thymidine, it gets incorporated into the DNA strands being formed during replication.

3. **Types of Chromatin**:
- There are two main types of chromatin:
- **Euchromatin**: This is the less condensed form of chromatin that is transcriptionally active, meaning it is involved in gene expression.
- **Heterochromatin**: This is the more condensed form of chromatin that is generally transcriptionally inactive.

4. **Incorporation into Chromatin**:
- Since DNA replication occurs for both euchromatin and heterochromatin during the S-phase, radioactive thymidine will be incorporated into both types of chromatin.

5. **Conclusion**:
- Therefore, if cells are exposed to radioactive thymidine as soon as they enter the S-phase, both euchromatin and heterochromatin will become radioactive due to the incorporation of the radioactive thymidine into the newly synthesized DNA.

### Final Answer:
Both types of chromatin (euchromatin and heterochromatin) are expected to become radioactive.