Problem 11 - Biology: Concepts and Applications

Question

A cell with a Barr body is __________. a. a bacterium b. a sex cell c. from a female mammal d. infected by Barr virus

Step-by-Step Solution

Answer

A cell with a Barr body is from a female mammal.

Step 1: Understand the Barr Body Concept

A Barr body is a dense, inactivated X-chromosome found in the nucleus of female mammalian cells. This process is known as X-chromosome inactivation, which occurs to balance the dosage of X-linked genes between males (XY) and females (XX).

Step 2: Analyze Each Option

Let's consider the options: - Option a: A bacterium does not have any chromosomes like mammals; it is not applicable here. - Option b: Sex cells, or gametes, either carry an X or a Y chromosome, but they do not normally inactivate an X chromosome to form a Barr body. - Option c: Female mammals have two X chromosomes, and typically one of them is inactivated to form a Barr body. - Option d: Barr bodies are unrelated to any viral infection; the name 'Barr' comes from the scientist who discovered X-chromosome inactivation, not a virus.

Step 3: Identify the Correct Option

Given that a cell with a Barr body results from the inactivation of an X chromosome found in female mammals, the correct choice is option c: from a female mammal.

Key Concepts

X-chromosome inactivation

In female mammals, each cell carries two X chromosomes; however, having twice the number of X-linked genes compared to males could lead to an imbalance in protein production. To solve this, one of the X chromosomes in each cell becomes tightly condensed, forming what is known as a Barr body. This process is called X-chromosome inactivation (XCI). It's a random process, meaning that in some cells the X chromosome inherited from the mother is inactivated, while in others, it is the paternal X that becomes inactive. The chosen X chromosome ceases to express most of its genes, ensuring that the gene dosage is equalized between males and females. XCI starts early in embryonic development, and once an X chromosome is inactivated in a cell, all its daughter cells will have the same X inactivated. This is significant for maintaining balance in genetic expression, creating what is known as a "mosaic" pattern, where different cells can display active genes from either the maternal or paternal X chromosome.

female mammal

Female mammals are distinguished by their XX sex chromosome configuration, which contrasts with the XY configuration in males. This genetic setup leads to the formation of a Barr body in female cells, as a result of one of these X chromosomes undergoing inactivation. This inactivation is crucial because it ensures that females only have one active X chromosome per cell, just like their male counterparts, who possess only one X chromosome. The presence of a Barr body is a reliable marker that a cell is from a female mammal. Various factors can influence which X chromosome is inactivated, but generally, it is a random choice in each embryo cell. Interestingly, the discovery of Barr bodies has been instrumental in the recognition of the XCI process and its broader implications for genetics and cell biology, particularly in understanding conditions that arise from abnormal X chromosome numbers or inactivity.

gene dosage compensation

Gene dosage compensation is a critical concept in genetics. It ensures that organisms with differing numbers of specific chromosomes or genes do not experience imbalances in gene product levels. In mammals, this concept is primarily associated with X-chromosome inactivation. By inactivating one of the two X chromosomes in females, the organism equalizes the "dose" of X-linked gene products with that of males, who only have one X chromosome. Without gene dosage compensation, females could produce twice as many X-linked gene products as males, which could interfere with cellular processes and cause various biological issues. By forming a Barr body through XCI, females maintain balance, ensuring they express X-linked genes at levels comparable to males, despite having an extra X chromosome. This process demonstrates nature's ingenuity in preserving genetic stability and preventing harmful conditions that could emerge from gene dosage imbalances.