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Chapter 5: Problem 16

What is a Barr body, and where is it found in a cell?

Short Answer

Expert verified
Answer: A Barr body is an inactivated X chromosome present in the nuclei of female somatic cells. It forms as a result of X-chromosome inactivation during early development to ensure proper gene expression dosage. Barr bodies are found within the cell nucleus, usually attached to the nuclear envelope.

Step by step solution

01

Define Barr Body

A Barr body, also known as a sex chromatin, is an inactivated X chromosome present in the nuclei of female somatic cells. It appears as a dense, compact structure under a microscope, consisting of heterochromatin.
02

Explain the Formation of Barr Bodies

Barr bodies form during early development as a result of a process called X-chromosome inactivation (XCI). In female mammals, one of the two X chromosomes in each cell is randomly inactivated to ensure the proper dosage of X-linked gene expression in females (XX) and males (XY). The inactive X chromosome condenses into a small, dense structure called a Barr body.
03

Describe the Location of Barr Bodies

Barr bodies are found within the nucleus of the cell, usually attached to the nuclear envelope. The nuclear envelope is a double-membrane structure that separates the nucleus from the cytoplasm and provides a boundary for the cell's genetic material. Since one X chromosome is inactivated during XCI, only one Barr body is present in most female somatic cells.

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Most popular questions from this chapter

What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?

In chickens, a key gene involved in sex determination has recently been identified. Called \(D M R T 1\), it is located on the \(Z\) chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike \(S R Y\) in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co- discoverer of the human \(S R Y\) gene), Craig Smith and colleagues were able to "knock down" expression of \(D M R T 1\) in \(Z Z\) embryos using RNA interference techniques (see Chapter 16 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?

The genes encoding the red-and green-color-detecting proteins of the human eye are located next to one another on the X chromosome and probably evolved from a common ancestral pigment gene. The two proteins demonstrate 76 percent homology in their amino acid sequences. A normal-visioned woman with both genes on each of her two X chromosomes has a redcolor-blind son who was shown to have one copy of the greendetecting gene and no copies of the red-detecting gene. Devise an explanation for these observations at the chromosomal level (involving meiosis).

Review the Chapter Concepts list on p. \(83 .\) These all center on sex determination or the expression of genes encoded on sex chromosomes. Write a short essay that discusses sex chromosomes as they contrast with autosomes.

As related to sex determination, what is meant by (a) homomorphic and heteromorphic chromosomes; and (b) homogametic sex and heterogametic sex?
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