Question

Barr body is an example of:

• A. Facultative euchromatin
• B. Constitutive heterochromatin
• C. Facultative heterochromatin
• D. Euchromatin

Hint:

Constitutive heterochromatin = Permanently condensed D (e.g., centromeres).
Facultative heterochromatin = Condensation is variable/reversible (e.g., Barr body ictivated X-chromosome).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to classify the chromatin state of a Barr body, which is formed during dosage compensation in female mammal somatic cells.

Step 2: Key Formula or Approach:
Chromatin is categorized based on structure and transcriptiol activity:
• Euchromatin: Uncondensed, transcriptiolly active chromatin.

• Constitutive Heterochromatin: Permanently condensed and genetically ictive across all cell types (e.g., centromeres and telomeres).

• Facultative Heterochromatin: Chromatin that can switch between a condensed (ictive) state and a transcriptiolly active state depending on cell development or metabolic context.

Step 3: Detailed Explation:

• Formation of a Barr Body: In female mammals (XX), one of the two X chromosomes is randomly ictivated early in embryonic development to equalize X-linked gene expression with males (XY). This process is known as Lyonization.

• Epigenetic Silencing: The ictivated X chromosome ($X_i$) condenses into a dense heterochromatic structure called a Barr body, visible against the inner surface of the nuclear envelope during interphase.

• Why Facultative? The $X_i$ chromosome contains functiol genes that were transcriptiolly active prior to ictivation, and it can become active again in germline cells (oocytes). Because this condensation is reversible across generations and selective per cell, it represents facultative heterochromatin.

Step 4: Fil Answer:
A Barr body is a classic biological example of facultative heterochromatin.