After that read the text aloud, trying to imitate the intonation. “SEX CHROMATIN” (“Barr body”) is the substance of genetically inactivated X-chromosome detected in normal conditions only in somatic (diploid) cells in

“SEX CHROMATIN” (“Barr body”) is the substance of genetically inactivated X-chromosome detected in normal conditions only in somatic (diploid) cells in females. In granular neutrophilic leukocytes “sex–chromatin” appears as a “drumstick”. The number of X–chromosomes is equal to the number of bodies of sex-chromatin plus one. In the cells of different tissues the ocurrence of “sex chromatin” is different. Thus, 80 – 90% of neuron nuclei contain “sex chromatin”, while only 20 – 40% of cell nuclei of the buccal epithelium contain it. “Sex chromatin” is localized in the nucleus, mostly attached to the nuclear membrane by thin filaments. In the case when “sex chromatin” is attached to the nuclear membrane, it has a triangle shape with the apex directed towards the nucleus centre. All organisms with two X-chromosomes in their karyotypes contain only one body of “sex- chromatin”. In the case of trisomy of X – chromosome the nucleus contains two bodies of “sex chromatin”.

There is a reverse correlation between the mitotic activity of the cell population and the frequency of sex–chromatin occurrence.

On the fifth and sixth days of embryonic development in a female organism inactivation of one of the two “X–chromosomes” takes place. The mechanism of this phenomenon is conditioned by methylation of the DNA in the inactivated chromosome. There is a special site Xq27.3 (CpG islet) containing multiple cytosine-guanine repeats which can be methylated. They are methylated in the Barr body but are not methylated in the second (active) female Х-chromosome, nor are they methylated in male X–chromosome.

Fig.. Barr body.

3Answer the questions.

1. What is sex chromatin?

2. What is the number of X–chromosomes?

3. Does the occurrence of sex chromatin in the cells depend on the type of the tissue?

4. Where is sex chromatin localized?

5. What shape does sex chromatin have in the case when it is attached to the nuclear membrane?

6. How many bodies of sex- chromatin are contained in the cell nuclei of chromosomes in normal female karyotypes?

7. How many bodies of sex- chromatin does a cell nucleus contain in the case of X-trisomy?

8. What sort of correlation is there between the mitotic activity of the cell population and the frequency of sex–chromatin occurrence?

9. What happens on the fifth and sixth days of embryonic development in a female organism?

10. By what is the mechanism of this phenomenon conditioned?