Which statement about the Xist gene is true?

• A. It is located in the Xic region of the X chromosome and it encodes an RNA that coats an X chromosome to promote compaction
• B. It is located on the Y chromosome and encodes a protein involved in sex determination
• C. It encodes a tRNA involved in translation
• D. It is not involved in X inactivation

Answer: (A)

X inactivation in female mammals is driven by Xist, a long noncoding RNA produced from the X chromosome’s X-inactivation center (Xic). Once transcribed, Xist RNA spreads along the chromosome from which it is expressed, coating that X in cis and recruiting chromatin-modifying complexes that convert it into a compact, transcriptionally silent form. This coating and remodeling lead to the silencing of most genes on that chromosome, effectively inactivating one X copy in each cell. The Xist gene itself does not code for a protein and is located on the X chromosome, not the Y. It is also not a tRNA. Therefore, the statement describing it as located in the Xic region and producing an RNA that coats an X chromosome to promote compaction is the correct one.