Multiple Choice
Why is the normal range for the mass of DNA in a male cat cell lower than that in a female cat?
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3. Extensions to Mendelian Inheritance
X-Inactivation
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Which of the following statements about the Xist gene is true?
A
Xist is required for dosage compensation in autosomes.
B
Xist is only expressed from the active X chromosome.
C
Xist is a non-coding RNA that coats the inactive X chromosome and initiates its silencing.
D
Xist encodes a protein that directly degrades X-linked mRNAs.
Verified step by step guidance1
Understand the biological context: The Xist gene plays a crucial role in X chromosome inactivation, a process that ensures dosage compensation between males (XY) and females (XX) by silencing one of the X chromosomes in females.
Recall that dosage compensation is necessary because females have two X chromosomes, and without inactivation, they would produce twice the amount of X-linked gene products compared to males.
Recognize that Xist does not encode a protein; instead, it produces a long non-coding RNA that coats the X chromosome from which it is transcribed, leading to its inactivation.
Note that Xist RNA is expressed only from the inactive X chromosome, not the active one, and it initiates silencing by recruiting other factors that modify chromatin structure.
Conclude that the true statement is the one describing Xist as a non-coding RNA that coats the inactive X chromosome and initiates its silencing, distinguishing it from statements about autosomes or protein-coding functions.
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