Now we are going to discuss a special topic in genetics, barr body and Lyon’s hypothesis which gives us an general overview about what is barr body and how they are formed along with examples of barr body.
What is Barr Body:
In human female all cell have two allosome are leveled as ‘X’ so in human female each cell have two ‘X’ chromosome but the somatic cell also have two ‘X’ chromosome in human female. But the somatic cell do not take part in reproduction and among the two ‘X’ chromosome in human female become inactive by lyonization.
The one ‘X’ chromosome in human female which becomes inactive as a result of lyonization is know as Barr body and this phenomenon was first observed by a British genetics Marry F. Lyon.
The main reason for the inactivation of one ‘X’ chromosome is to prevent expression of both ‘X’ chromosomes in human females and unnecessary genetic information should not transfer to the next generation so one ‘X’ chromosome remains in inactive state.
Due to the inactivation of one ‘X’ chromosome in both male and female expression for ‘X’ chromosome become equal, otherwise in male and female it would show unequal expression.
Main difference between the active ‘X’ chromosome and the inactive ‘X’ chromosome is in their chromatin structure because active ‘X’ chromosome is enclosed by euchromatin which can express by the method of transcription which form complementary RNA and the complementary RNA is further used in translation process for producing protein to express active ‘X’ chromosome.
But the inactive ‘X’ chromosome is enclosed by heterochromatin which does not allow the transcription process for production of complementary RNA which do not allow all of them to express. As the inactive ‘X’ chromosome is enclosed by heterochromatin so they appear like a dense structure which gives them the name Barr body.
Barr Body and Lyon’s Hypothesis:
Now we will understand the relation between Barr body and Lyon’s Hypothesis, postulates of Lyon’s Hypothesis give us a more clear concept about the Barr body in mammalian female cells.
- In mammalian female somatic cells have two ‘X’ chromosomes but two chromosomes do not present in functional state only one among the two ‘X’ chromosomes remain in the active stage while another one remains in the inactive stage.
- Inactivation occurs in one ‘X’ chromosome in a mammalian female somatic cell randomly that means the possibility of an ‘X’ chromosome to become inactive among the two ‘X’ chromosomes is equal.
- This inactivation of one ‘X’ chromosome by the process of lyonization which causes one ‘X’ chromosome enclosed by heterochromatin occurs during the development of the mammalian female.
- When an ‘X’ chromosome becomes inactive it continues to remain in an inactive stage for generations to generations for that particular cell.
Formation of Barr Body:
Now we will discuss different genes present on the ‘X’ chromosome which controls the inactivation of one ‘X’ chromosome or formation of Barr body during embryonic development in mammalian females.
On the ‘X’ chromosome in mammalian females there is a region known as XIC or X inactivation centre and there are two different genes one is Xist or X inactivation specific transcript while another gene is reverse Xist or Tsix.
The two genes Xist and Tsix control the ‘X’ chromosome inactivation in opposite ways, the Xist is responsible for inactivation of the ‘X’ chromosome but the other gene Tsix prevents the inactivation of ‘X’ chromosome.
And this inactivation occurs randomly in any ‘X’ chromosome which depends upon the expression of the two genes on ‘X’ chromosome Xist and Tsix during the embryonic development of mammalian female in their somatic cells because the somatic cells do not take part in reproduction.
Read More: Differences Between Autosomes and Allosomes
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