This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “X-Chromosomes Inactivation Occurs in Female XX Animals”.
1. X-chromosome inactivation is known as ______________
a) Dominance
b) Epistasis
c) Dosage compensation
d) Complementation
View Answer
Explanation: In placental mammals, dosage compensation of X-linked genes is achieved by the inactivation of one of the female X-chromosomes. This mechanism was first proposed by Mary Lyon in 1961.
2. The proposal for the concept of X-chromosome inactivation in female XX-chromosomes was experimentally shown in ___________
a) Drosophila
b) Mice
c) Human
d) Monkey
View Answer
Explanation: The mechanism of X-chromosome inactivation was proposed by Mary Lyon in 1961 who inferred it from the studies on mice. Subsequent research by Lyon and others has shown that the inactivation event occurs when the mouse embryo consists of a few thousand cells.
3. The maternal X-chromosome is inactivated during Dosage compensation.
a) True
b) False
View Answer
Explanation: At a few thousand cell stage of the embryo the inactivation of one X chromosome occurs. The X-chromosome is chosen at random, though once chosen it remains inactivated in all the descendents of that cell.
4. Female mammals are called genetic mosaics.
a) True
b) False
View Answer
Explanation: The female mammals are called genetic mosaics as they contain two types of cell lineages. The maternally inherited X-chromosome is inactivated in roughly half of these cells and the paternal inherited X-chromosome is inactivated in the other half.
5. The best genetic mosaicism is observed in which of the following?
a) Tobacco
b) Fruitfly
c) Dragonfly
d) Cat
View Answer
Explanation: The best genetic mosaicism is observed in the study of fur coloration of cats and mice. A female that is heterozygous for an X-linked gene is therefore able to show two different phenotypes.
6. Barr body is the ___________
a) Active X chromosome
b) Inactive X chromosome
c) Active Y chromosome
d) Inactive Y chromosome
View Answer
Explanation: Chemical analysis shows that the inactivated X chromosome condenses into a dark staining structure. This structure was called the Barr body after the scientist Murray Barr who discovered it.
7. What changes occur in the chromosome to make it inactive?
a) Methylation
b) Glycosylation
c) Acetylation
d) Phosphorylation
View Answer
Explanation: An X chromosome that has been inactivated does not look or acts like other chromosomes. Chemical analysis shows that the DNA is modified by the addition of numerous methyl groups. This provides the structure a highly condensed darkly stained structure called the Barr body.
8. The X-chromosome once inactivated remains inactive throughout the life in all the cells of the body.
a) True
b) False
View Answer
Explanation: The inactivated X chromosome remains in this altered state in all the somatic cells. However, in germ cells it is reactivated because two copies of some X linked genes are needed for the successful completion of oogenesis.
9. All genes on the inactivated X chromosome are transcriptionally silent.
a) True
b) False
View Answer
Explanation: All genes on the inactivated X chromosome are not transcriptionally silent. One that remains active is called X-inactive specific transcript or XIST.
10. The functional product of the Xist gene is ___________
a) mRNA
b) rRNA
c) RNA devoid of ORF
d) Protein
View Answer
Explanation: The Xist gene in human beings encodes a 17 kb transcript devoid of any significant open reading frame. It therefore seems unlikely to code protein. Instead the RNA itself is probably the functional product of the Xist gene.
11. The one which is able to repress the production of the Xist transcript is the active X chromosome.
a) True
b) False
View Answer
Explanation: In mammalian dosage compensation system the X chromosome that is able to repress the production of Xist transcript remains active. This is because the production of high amount of Xist RNA leads to the higher rate of methylation of DNA leading it to condense into the inactivated Barr body.
12. In which stage of the cell cycle the Barr body can be identified?
a) Interphase
b) Prophase
c) Metaphase
d) Anaphase
View Answer
Explanation: Inactivated X chromosome or the Barr body is readily identified in mammalian cells. During interphase, they condense into a darkly stained mass associated with the nuclear membrane.
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