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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
Based on the solution above: A Barr body is an inactive X chromosome in the nucleus of a female cell, which ensures that females express a similar amount of X-linked genes compared to males. The process of inactivation is called X-chromosome inactivation or lyonization. The Barr body is located within the nucleus of a cell and is generally found at the periphery of the nucleus, near the nuclear envelope, as it remains condensed and is not involved in active transcription of genes.

Step by step solution

01

Define Barr body

A Barr body is an inactive X chromosome in the nucleus of a female cell. It was named after Murray Barr, a Canadian scientist who discovered it in 1949. Barr bodies are formed when one of the two X chromosomes in each female cell condenses and becomes inactive. This process of inactivation is called X-chromosome inactivation or lyonization, and it ensures that females express a similar amount of X-linked genes compared to males, who have only one X chromosome.
02

Location of Barr body in a cell

The Barr body is located within the nucleus of a cell. The nucleus is a double-membrane-bound organelle that contains the cell's genetic material (DNA). Since the Barr body is an inactive X chromosome, it is generally found at the periphery of the nucleus, near the nuclear envelope, as it remains condensed and is not involved in active transcription of genes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

X-chromosome inactivation
In every female cell, there are two X chromosomes. To ensure balance between males (who have one X chromosome) and females in terms of gene expression, nature employs a process known as X-chromosome inactivation. This scientific process ensures that only one of the X chromosomes in each cell is active.
  • During early development, each female cell randomly inactivates one X chromosome, making it a Barr body.
  • This process of silencing one X chromosome is called lyonization, named after geneticist Mary Lyon who proposed it.
  • Through this mechanism, females effectively operate with one functional X chromosome, similar to males.
This ensures that both genders end up with a comparable dose of X-linked gene products, preventing an overexpression of X chromosome genes in females.
nucleus
The nucleus acts as the control center of a eukaryotic cell. It is where the genetic blueprint of an organism, its DNA, resides. This compartment is enclosed by a double-membrane called the nuclear envelope, protecting its valuable content.
  • Within the nucleus, DNA is organized into chromosomes. In females, one of these is the inactive X chromosome or Barr body.
  • The Barr body is located along the edge of the nucleus, often near the nuclear envelope.
  • Though not actively involved in gene transcription, it remains a crucial structural feature of the nuclear genetic setup.
Although this X chromosome is inactive, its presence and position in the nucleus are significant for the regulation of genetic material in female cells.
genetic material
Genetic material refers to the molecular instructions that guide the development, functioning, and reproduction of organisms. Found primarily in DNA, it ensures that genetic information is passed down accurately from generation to generation.
  • DNA is packed into chromosomes, each containing numerous genes which correspond to particular traits or functions.
  • In females, one of the X chromosomes becomes inactive and forms what is known as a Barr body, which still contributes to the structural organization of DNA in the cell.
  • The process of inactivating one X chromosome ensures genes are not overrepresented in females.
Thus, the precise organization and regulation of genetic material in cells control everything from growth to development, illustrating the critical nature of genetic management through processes like X-chromosome inactivation.

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Most popular questions from this chapter

Can the Lyon hypothesis be tested in a human female who is homozygous for one allele of the X-linked G6PD gene? Why, or why not?

Describe how nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete.

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

In mice, the X-linked dominant mutation Testicular feminization (Tfm) eliminates the normal response to the testicular hormone testosterone during sexual differentiation. An XY mouse bearing the \(T f m\) allele on the \(X\) chromosome develops testes, but no further male differentiation occurs-the external genitalia of such an animal are female. From this information, what might you conclude about the role of the Tfm gene product and the \(\mathrm{X}\) and \(\mathrm{Y}\) chromosomes in sex determination and sexual differen- tiation in mammals? Can you devise an experiment, assuming you can "genetically engineer" the chromosomes of mice, to test and confirm your explanation?

It is believed that any male-determining genes contained on the Y chromosome in humans are not located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region?
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