Which of the following is true of Barr bodies? Choose 1 answer:
A Barr body is typically found in the nucleus of female gametes.
Barr bodies are chromosomes that are condensed prior to mitosis.
Barr bodies are formed during embryonic development.
A cell that has three X chromosomes will have three Barr bodies.
The Correct Answer and Explanation is:
Correct Answer:
Barr bodies are formed during embryonic development.
Explanation
Barr bodies are inactivated X chromosomes found in the nuclei of cells in female mammals. They serve as a mechanism for dosage compensation to ensure that females, who have two X chromosomes (XX), do not produce twice the amount of X-linked gene products as males, who have one X chromosome (XY). The process of X-inactivation, also called Lyonization, occurs during early embryonic development, typically in the blastocyst stage. Hence, the correct answer is: “Barr bodies are formed during embryonic development.”
The inactivation of one of the X chromosomes is largely random in each somatic cell (excluding gametes), and once inactivated, the same X chromosome remains inactive in all the progeny cells of that cell. The inactivated X condenses into a compact structure known as a Barr body, which is visible under a microscope near the nuclear envelope.
Let’s analyze the incorrect options:
- “A Barr body is typically found in the nucleus of female gametes”: False. Female gametes (eggs) do not contain Barr bodies. Gametes are haploid and only have one X chromosome, so there is no need for inactivation.
- “Barr bodies are chromosomes that are condensed prior to mitosis”: Incorrect. While chromosomes condense before mitosis, Barr bodies refer specifically to inactivated X chromosomes, not all chromosomes preparing for cell division.
- “A cell that has three X chromosomes will have three Barr bodies”: False. In any cell, only one X chromosome remains active, and all the extra X chromosomes are inactivated. So a cell with three X chromosomes (XXX) will have two Barr bodies, not three.
Therefore, Barr bodies are a key example of epigenetic regulation and are crucial for maintaining gene dosage balance betwee
n sexes.