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In S–phase, DNA is replicated in a mediu...

In S–phase, DNA is replicated in a medium containing radioactive thymidine, radioactivity will be observed in :

A

Euchromatin

B

Heterochromatin

C

Both

D

Nucleolus

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The correct Answer is:
**Step-by-Step Solution:** 1. **Understanding the S Phase**: The S phase, or synthesis phase, is a part of the cell cycle where DNA replication occurs. During this phase, each chromosome is duplicated, resulting in two identical copies. 2. **Role of Radioactive Thymidine**: Thymidine is one of the four nucleotides that make up DNA. When DNA is replicated in a medium containing radioactive thymidine, the radioactive thymidine will be incorporated into the newly synthesized DNA strands. 3. **Types of Chromatin**: There are two main types of chromatin in the nucleus: - **Euchromatin**: This is the loosely packed form of chromatin that is transcriptionally active, meaning it is involved in gene expression and is lightly stained. - **Heterochromatin**: This is the densely packed form of chromatin that is transcriptionally inactive, meaning it is not actively involved in gene expression and appears darkly stained. 4. **DNA Replication in Chromatin**: During the S phase, both euchromatin and heterochromatin undergo replication. This means that all regions of DNA, regardless of whether they are euchromatin or heterochromatin, will incorporate the radioactive thymidine. 5. **Conclusion**: Since both types of chromatin (euchromatin and heterochromatin) are replicated during the S phase, radioactivity will be observed in both types of chromatin. **Final Answer**: Radioactivity will be observed in both euchromatin and heterochromatin. ---
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