Barr Body

The Barr body is a condensed, inactivated X chromosome present in the cells of females due to X-chromosome inactivation, a process essential for dosage compensation. This process, known as lyonization, occurs randomly in each cell during early development.

The Barr body in the cell nucleus is absent in males. Its formation helps equalize gene expression between males and females and prevents overexpression of X-linked genes in females. In this article, we will learn about the structure, formation, and function of Barr body.

What is a Barr Body?

Definition of Barr Body: The Barr Body is a condensed, inactivated X chromosome found in the cells of females as a result of X-chromosome inactivation.

They are small, dark-staining structures. Barr bodies are seen in the nuclei of female mammals. They are highly condensed X chromosomes visible in the cells of female mammals because their DNA is converted into heterochromatin. Their only function is to exist in the inactive form to mitigate the gene dosage effects. They are most commonly found at the periphery of the nucleus.

The presence of Barr bodies indicates that the organism has more than one X chromosome. A human female has only one Barr body per somatic cell, whereas a human male has no Barr bodies per somatic cell at all. In all cases, the number of Barr bodies is one fewer than the total number of X chromosomes in a person’s body.

Barr Body Anatomy

The Barr body, also known as the sex chromatin body, is a condensed, inactive X chromosome found in the nuclei of somatic cells in females. The structure of the barr bodies is as follows:

Location of Barr Bodies

Barr bodies are usually found near the nuclear envelope, and their position can vary within the nucleus.

• They can be seen in the interphase nucleus as a small mass that is in contact with the nucleus membrane.
• Barr bodies are not found in every type of cell. They are typically observed in somatic cells of females but not in germ cells (eggs and sperm) or certain other cell types.
• Barr bodies in female cell can be seen in 4 positions:
  • Barr bodies are found attached to the nucleus in nerve cells.
  • They are found attached to the nuclear membrane.
  • Found freely in the nucleoplasm.
  • Found in nuclear expansion.

Number of Barr Bodies

The number of Barr bodies in a person is one less than the total number of X chromosomes.

• In a normal female somatic cell with two X chromosomes, only one X chromosome is inactivated, leading to the formation of a single Barr body.
• In rare cases, additional X chromosomes may undergo inactivation, resulting in the presence of multiple Barr bodies.
• For example, a female with XX chromosomes will have one Barr body, while a male with XY chromosomes will have zero Barr bodies.

Barr Body Appearance

The term “Barr body” is named after Canadian researcher Murray Barr, who first observed these structures in the 1940s.

• Barr bodies are darkly staining masses that appear in the interphase nucleus of female mammals. They can be seen in neutrophils at the rim of the nucleus.
• Barr bodies have several distinct shapes, including wedge-shaped and plano-convex. The plane side of the Barr body rests against the nuclear membrane, while the convex part points toward the cytoplasm.

Formation of Barr Bodies

Barr bodies are small, well-defined bodies that stain intensely with nuclear dyes.  They are a condensed X chromosome that is inactive and does not participate in DNA replication or gene expression.

The formation of Barr bodies is a part of the process known as lyonization or X-inactivation, which ensures dosage compensation for the genes located on the X chromosomes in females. The process of formation of barr bodies is a given below:

• Random X-Chromosome Selection: In each cell, either the maternal or paternal inherited X chromosome is randomly selected for inactivation.
• Initiation of Inactivation: X-chromosome inactivation begins during early embryonic development, shortly after fertilization.
• Coating by XIST RNA: The X-inactive-specific transcript (XIST) gene on the chosen X chromosome is activated, producing XIST RNA. This RNA coats the X chromosome undergoing inactivation.
• Silencing of Genes: XIST RNA initiates changes in chromatin structure, leading to the compaction and transcription silencing of genes on the chosen X chromosome.
• Formation of Barr Body: The compacted, transcriptionally silent X chromosome takes on a condensed structure known as the Barr body. This dense structure is observable within the cell nucleus.
• Maintenance of Inactivation: Once established, X-chromosome inactivation and the presence of Barr bodies are maintained throughout cell divisions. It ensures consistent gene expression patterns in subsequent generations of cells.

Barr Body Diagram

The below is a diagram of the barr body:

Lyon’s Hypothesis and X-inactivation Method

Mary Lyon’s hypothesis, also known as Lyonization, explains the process of X chromosome inactivation in females. Lyon’s Hypothesis was proposed by Mary Lyon in 1961, and it provides a molecular explanation for the dosage compensation observed between males (XY) and females (XX). The key points of Lyon’s Hypothesis are:

• Random X Inactivation
  • Lyon proposed that early in embryonic development, one of the two X chromosomes in each somatic cell of a female mammal becomes randomly inactivated.
  • This inactivation occurs independently in each cell, leading to a mosaic pattern of X chromosome activity among the cells of an individual female.
• Formation of Barr Bodies
  • The inactivated X chromosome undergoes structural changes, transforming from euchromatin to heterochromatin.
  • The condensed, inactive X chromosome is visualized as a dark-staining structure called a Barr body in the nucleus.
• Balancing Gene Dosage
  • XCI balances the gene dosage of X-linked genes between males and females.
  • In males, who have only one X chromosome, all X-linked genes are expressed.
  • In females, one X chromosome is inactivated in each cell, resulting in roughly equal expression levels of X-linked genes compared to males.
• Selection of Active X Chromosome
  • Lyon’s Hypothesis suggests that the choice of which X chromosome to inactivate is random and occurs early in development.
  • Once the X chromosome is inactivated in a particular cell, the same X chromosome remains inactive in all descendant cells.
• Maintenance of XCI during Cell Division
  • XCI is maintained during cell division. If a cell undergoes division, the inactivated X chromosome is passed on to daughter cells.
  • This ensures that the dosage compensation mechanism is maintained throughout the life of the individual.
• Escape Genes
  • Lyon’s Hypothesis states that not all genes on the inactivated X chromosome are completely silenced. Some genes, known as escape genes, may continue to be expressed to varying degrees.

What is the Function of Barr Body?

The primary function of Barr bodies is related to the regulation of gene expression and dosage compensation. The other key functions of the barr bodies are as follows:

• Barr bodies help achieve dosage compensation between males (XY) and females (XX) by equalizing the gene expression levels of X-linked genes.
• Ensures equal gene expression from X-linked genes in both males and females, Barr bodies contribute to the overall cellular homeostasis. This balance is crucial for normal development and function of cells and tissues.
• X chromosome inactivation and the formation of Barr bodies contribute to the stability of the genome by preventing disruptions that could arise from having excess X-linked gene products in females.
• The process of X chromosome inactivation involves epigenetic modifications, such as DNA methylation and histone modifications, which play an important role in the stable silencing of genes on the inactivated X chromosome.
• Barr bodies can adapt to changes in cellular conditions. For example, if a cell undergoes division, the inactivated X chromosome is passed on to daughter cells which ensures that the dosage compensation mechanism is maintained.
• X chromosome inactivation and the formation of Barr bodies are essential for cellular differentiation during early embryonic development. This process helps establish distinct cell lineages with specific gene expression patterns.

X-Inactivation Example and Barr Body

In human females (XX), one of the X chromosomes in each somatic cell is inactivated, leading to the formation of a Barr body.  They are found in biological samples like blood, hair, and buccal cells. Some examples of Barr bodies are:

• Tortoiseshell cats: Female cats with black and tan color alleles on the X chromosome inactivate their two Xs during embryonic development. This results in a tortoiseshell coat pattern with alternating patches of black and tan fur.
• Klinefelter syndrome: People with Klinefelter syndrome (47, XXY) have a single Barr body.
• In the forensic science, detecting Barr bodies is a simple and noninvasive method for determining the sex of an unknown sample.

Conclusion – Barr Body

In conclusion, Barr bodies are condensed, inactive X chromosomes formed in the nuclei of somatic cells during X chromosome inactivation in females. This process ensures dosage compensation by randomly silencing one of the two X chromosomes, balancing gene expression levels with males. Barr bodies play a critical role in maintaining genomic stability, preventing the overexpression of X-linked genes in females, and aid in sex chromosome regulation.

FAQs on Barr Body

What is a Barr Body and What is its Function?

A Barr body is an inactive X chromosome in the somatic cells of female mammals. The function of Barr bodies is related to the regulation of gene expression and dosage compensation.

Why is a Barr Body only Found in Females?

A Barr body is only found in females due to X chromosome inactivation, a process that randomly silences one of the two X chromosomes in each somatic cell. This ensures dosage compensation, balancing gene expression with males, who possess one X and one Y chromosome.

Who Discovered the Barr Body?

Murray Llewellyn Barr (1908 – 1995) and graduate student Ewart George Bertram discovered the Barr body in 1948.

What is a Barr Body in a Nerve Cell?

In a nerve cell, a Barr body is a condensed, inactive X chromosome resulting from X chromosome inactivation in females, contributing to gene dosage balance.

Where is the Barr Body?

Barr bodies are small, dark-staining structures that are found in the nuclei of female somatic cells. They are most commonly located at the periphery of the nucleus, but some can be found in other parts of the nucleus.

What is the Barr Body Associated With?

Barr bodies are associated with sex determination. Barr bodies are inactivated X chromosomes that are found in female mammals. In humans, females have one Barr body per somatic cell nucleus, while males have none.

Which Phase is Associated With Barr Body?

The Barr body is found in the interphase nucleus of the cell cycle. It is a small, darkly staining mass that is in contact with the nucleus membrane.

Why do Males have no Barr Body?

Males lack a Barr Body because they inherit only one X chromosome, while females have two. The Barr Body forms from the inactivation of one X chromosome in females during early development to balance gene expression between the sexes.