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

1. Heterochromatin
2. Euchromatin
3. Both heterochromatin and euchromatin
4. Neither heterochromatin nor euchromatin but only the nucleolus

Correct answer: Both heterochromatin and euchromatin

Solution

Both euchromatin and heterochromatin synthesize DNA during S phase, though euchromatin generally replicates earlier and heterochromatin later. Since the cells are exposed to radioactive thymidine from the start of S phase, newly synthesized DNA in both chromatin types can become labeled.

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