Barr body is formed by conversion of euchromatic regions of one X-chromosome into

Options

A. Constitutive heterochromatin
B. Facultative heterochromatin
C. Nascent heterochromatin
D. All the above.

Answer

**Step-by-Step Solution:**

1. **Understanding Barr Body Formation**: A Barr body is an inactivated X chromosome found in females. This process is known as X-inactivation, which ensures that females, who have two X chromosomes, do not produce double the amount of X-linked gene products compared to males, who have only one X chromosome.

2. **Euchromatin vs. Heterochromatin**: The X chromosome consists of regions that are euchromatic (active, gene-rich regions) and heterochromatic (inactive, gene-poor regions). During X-inactivation, one of the X chromosomes in females is converted from a euchromatic state to a heterochromatic state.

3. **Types of Heterochromatin**: There are different types of heterochromatin:
- **Constitutive Heterochromatin**: Always inactive and does not change its state.
- **Facultative Heterochromatin**: Can switch between active and inactive states depending on the cellular context.
- **Nascent Heterochromatin**: Refers to newly formed heterochromatin, which is not a standard term in this context.

4. **Identifying the Correct Type**: The Barr body is specifically formed by the conversion of the euchromatic regions of one X chromosome into facultative heterochromatin. This is because the inactive X chromosome can become active again under certain conditions, such as when it is passed to a male offspring.

5. **Conclusion**: Therefore, the correct answer to the question is that Barr body is formed by the conversion of euchromatic regions of one X chromosome into **facultative heterochromatin**.

**Final Answer**: Facultative heterochromatin.

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