If radioactive thymidine is used during S-phase of interphase, which part of the chromosome will become radioactive?

Options

A. Near the centromere
B. At both end of chromosome
C. Euchromatin region
D. Both euchromatin and heterochromatin region

Answer

To solve the question regarding which part of the chromosome will become radioactive if radioactive thymidine is used during the S-phase of interphase, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the S-phase of Interphase**:
- The S-phase, or synthesis phase, is the part of the cell cycle where DNA replication occurs. During this phase, each chromosome is duplicated, resulting in two identical sister chromatids.

**Hint**: Remember that the S-phase is crucial for DNA synthesis and replication.

2. **Role of Thymidine in DNA**:
- Thymidine is one of the four nucleotides that make up DNA. It pairs with adenine (A) in the DNA structure. If thymidine is radioactive, it means that any DNA synthesized using this radioactive thymidine will incorporate the radioactive label.

**Hint**: Consider how nucleotides are incorporated into the growing DNA strand during replication.

3. **Incorporation of Radioactive Thymidine**:
- During the S-phase, as the DNA strands are being synthesized, the radioactive thymidine will be incorporated into both strands of the newly formed DNA. This means that both the original (parental) DNA and the newly synthesized (daughter) DNA will contain radioactive thymidine.

**Hint**: Think about the process of DNA replication and how new nucleotides are added to the growing strands.

4. **Euchromatin vs. Heterochromatin**:
- DNA exists in two forms: euchromatin (loosely packed and transcriptionally active) and heterochromatin (densely packed and transcriptionally inactive). However, during DNA replication, both forms of chromatin are replicated.

**Hint**: Recall the characteristics of euchromatin and heterochromatin and their roles in DNA replication.

5. **Conclusion**:
- Since radioactive thymidine is incorporated into all newly synthesized DNA during the S-phase, both euchromatin and heterochromatin will become radioactive. Therefore, the entire chromosome will become radioactive as it undergoes replication.

**Hint**: Focus on the fact that DNA replication involves copying all parts of the chromosome, regardless of chromatin structure.

### Final Answer:
The entire chromosome will become radioactive when radioactive thymidine is used during the S-phase of interphase, as both euchromatin and heterochromatin are replicated.