Barr Body: Overview, Formation, Function And Examples

What is a Barr Body?

A Barr body is defined as a compact, inactive X chromosome that is, in normal conditions, observable inside the nuclei of cells, though usually only in females. The compact form was first discovered by Dr. Murray Barr and Ewart Bertram in 1949. It was named after Dr. Barr. In biological terms, a Barr body is an inactive X chromosome that is silenced, so that only one X chromosome can be active per cell.

Formation of Barr Bodies

The process of X-inactivation forms the Barr body. It involves one of the two X chromosomes in female cells becoming condensed and non-sensitised. The inactivated X chromosome compacts into a dense structure known as the Barr body. It makes gene expression between males and females well-balanced since a cell always has just one active X chromosome.

Lyon Hypothesis

The understanding of the existence of a Barr body commences with the Lyon hypothesis. The Lyon hypothesis is a hypothesis first proposed by geneticist Mary Lyon. According to the Lyon hypothesis, any cell in female mammals that has one of the two X chromosomes randomly becomes inactivated during the early stages of embryo development. The inactivated X chromosome becomes the Barr body. The process is termed as X-inactivation and ensures that females, like males, do not have more than one functional X chromosome per cell. The Lyon hypothesis explains why the Barr body is necessary for maintaining the balance of the genetic makeup of a female to that of a male.

Structure of the Barr Body

The Barr body is a small, dense structure found in the nucleus of female cells. It results from the inactivation of one of the two X chromosomes. In a microscope, it presents as a dark spot. DNA in the Barr body is highly compact, which means that its genes are not active. This makes sure that females and males have equal numbers of active X-chromosome genes.

Barr Body and Its Location

The Barr body exists in the nuclei of the cells of females, and sometimes it may appear in males with related genetic conditions, such as Klinefelter syndrome. In this condition, there happens to be an extra X chromosome, exposing an individual to various medical conditions, including X-linked recessive disorders (XXY). The presence of a Barr body in each somatic cell is found to occur in normal females. In the case of males who have a typical complement of XY chromosomes, it is not found in the nuclei of cells. The appearance of a Barr body, therefore, indicates that the person has more than one X chromosome.

What is the Function of the Barr Body?

The inactive X chromosome is called the Barr body and ensures that females do not produce double the amount of protein from X-linked genes as males. Since females carry two copies of the X chromosome, one of the X chromosomes must be inactivated in every cell to form the Barr body. The inactivation of an X chromosome takes place relatively early during development and is random for each cell.

Significance of Barr Body

The presence of the Barr Body helps to regulate gene expression by making sure that one of the two X chromosomes in female cells is always inactive. This helps to prevent the overproduction of proteins that might be harmful to the cell.

The Barr body helps in preventing human genetic disorders that occur due to the overproduction of certain proteins. Understanding the importance of the Barr body in disorders like Turner syndrome will help improve the diagnosis and treatment of these conditions.

The study of the Barr body gives an idea of the cellular process, such as the regulation of gene expression and also the mechanism of chromosome function. It is an important resource to better understand the relationships between chromosomes, gene expression, and diseases as well.

X-Inactivation Example and Barr Body

Females balance the number of active X chromosomes between males and females by X-inactivation. An example is with cats, which have the fur-colour gene on the X chromosome. If a female cat has one X chromosome with the gene for black fur and the other X chromosome with the gene for orange fur, X-inactivation simply means that some cells in her body will use the black gene while others will use the orange gene, creating black and orange patches, or tortoise shell. An inactivated X chromosome will form a dense structure called a Barr body, visible in the cell's nucleus under a microscope. This means that X-inactivation has occurred.

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MCQs on Barr Body

Q1. What is X chromosome inactivation?

Option 1: The process by which the expression of X-linked genes is balanced between males and females

Option 2: The process by which a gene's expression is regulated by the number of copies of the gene in the genome

Option 3: The process by which the X chromosome is replicated during the cell cycle

Option 4: The process by which a gene is mutated and loses its normal function

Correct answer: (1) The process by which the expression of X-linked genes is balanced between males and females.

Explanation:

The process by which one of the two X chromosomes in female cells is randomly inactivated early in embryonic development is known as X chromosome inactivation. The inactivated X chromosome condenses and forms a unique structure in the nucleus known as a Barr body, whereas the active X chromosome remains uncondensed and is responsible for X-linked gene expression in that cell. Because males only have one X chromosome and females have two, this process ensures that the dosage of X-linked genes is equalized between sexes. To ensure that the next generation has an equal chance of inactivating either X chromosome, the inactivated X chromosome is reactivated in cells that give rise to eggs or sperm.

Hence, the correct answer is Option (1) The process by which the expression of X-linked genes is balanced between males and females.

Q2. In a female human cell with one Barr body, how many active X chromosomes are present?

Option 1: One

Option 2: Two

Option 3: Three

Option 4: None

Correct answer: (1) One

Explanation:

In female human cells, one of the two X chromosomes is randomly inactivated, forming a Barr body. Therefore, in a female human cell with one Barr body, one X chromosome is active, and the other X chromosome is inactivated. The active X chromosome is responsible for the expression of X-linked genes in the cell, while the inactivated X chromosome is silenced and condensed into the Barr body.

Hence, the correct answer is Option (1) One

Q3. The Lyon hypothesis and Barr bodies are most relevant to which aspect of genetics?

Option 1: Genetic disorders caused by Y chromosome abnormalities

Option 2: Genetic disorders caused by mutations in autosomal genes

Option 3: Genetic disorders caused by mutations in X-linked genes

Option 4: Genetic disorders caused by mitochondrial DNA mutations

Correct answer: (3) Genetic disorders are caused by mutations in X-linked genes.

Explanation:

The Lyon hypothesis and Barr bodies are most relevant to the regulation of X-linked gene expression in females. In female cells, one of the two X chromosomes is randomly inactivated during early embryonic development, leading to the formation of a Barr body. This process ensures that the dosage of X-linked genes is equalized between males and females. However, this inactivation can influence the severity and manifestation of genetic disorders caused by mutations in X-linked genes. In these cases, the presence of a Barr body can indicate which X chromosome is inactivated and, therefore, which X-linked gene is affected. This information can be useful in diagnosing and treating X-linked genetic disorders in females.

Hence, the correct answer is Option (3) Genetic disorders are caused by mutations in X-linked genes.

Also Read:

• MCQ Practice on Barr Body

• Sex Determination in Drosophila

• Molecular Basis of Inheritance