Work in pairs. Ask and answer the questions above but not in the order they are listed in exercise 3.
Listen to the recording. Repeat each medical term during the pause in the recording. Mind the stress.
| Barr body | тельцеБарра |
| buccal | 1) буккальный, относящийся к щеке; щечный; трансбуккальный (о методе введения лекарственного средства); 2) внутриротовой |
| chromosome | хромосома |
| cytosine-guanine | цитозин-гуаниновый |
| diploid | диплоидный, с двойным набором хромосом |
| filament | филамент, нить |
| epithelium (pl.: epithelia) | эпителий, эпителиальнаяткань |
| karyotype | кариотип (совокупность особенностей числа и формы хромосом клетки) |
| leukocyte | лейкоцит, белое кровяное тельце |
| methylation | метилирование |
| mitotic | митотический |
| nucleus (pl.: nuclei) | ядро, мн.: ядра |
| somatic | соматический |
| trisomy | трисомия (наличие в клетке лишней хромосомы) |
| sex chromatine | половойхроматин |
| neutrophilic | нейтрофильный, характеризующийся наличиемнейтрофилов |
Without looking into the text listen to the recording.
Say what information you have gathered.
Listen to the text again.
Now, read the text silently, trying to grasp all the details of the contents. Then, read it simultaneously with the speaker, trying to catch up with the tempo. 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 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 occurrence 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 (CpGislet) 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 X-chromosome, nor are they methylated in male X-chromosome (see fig. 4).
Fig. 4. Barr body
Answer 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?
11. Describe the figure "Barr body" above?
Work in pairs. Ask and answer the questions above but not in the order they are listed in exercise 3.