For X-chromosome inactivation to occur, where are the Xist and Tsix genes expressed? Note: Xi is the inactive X chromosome and Xa is the active one.

Question

BlackTom AI · Accepted Answer

Xist RNA coats the inactive X chromosome (Xi) and triggers silencing, while Tsix is the antisense transcript expressed from the active X (Xa) to repress Xist.

1. Relevant knowledge points and concepts
- X-chromosome inactivation (XCI) equalizes X-linked gene dosage between XX females and XY males.  
- Xist (X-inactive specific transcript) is a long noncoding RNA transcribed from the X inactivation center that spreads in cis over the chromosome and recruits silencing machinery to form Xi.  
- Tsix is an antisense transcript that overlaps Xist and functions as a negative regulator of Xist; Tsix expression prevents Xist upregulation on that chromosome, keeping it active (Xa).  
- Choice and maintenance: mutually exclusive expression of Xist and Tsix on homologous Xs establishes which X will be inactive.

2. Step-by-step reasoning
- Two X chromosomes initially express factors that allow a choice. For the chromosome that will remain active, Tsix is expressed. Tsix transcription across the Xist locus represses Xist there (through transcriptional interference and chromatin changes).  
- On the chromosome that will become inactive, Tsix expression is downregulated or absent, permitting Xist transcription to rise. Xist RNA then coats that same chromosome in cis, recruiting Polycomb complexes and other chromatin modifiers to silence gene expression.  
- Thus, the pattern is reciprocal: Xi expresses Xist (and not Tsix); Xa expresses Tsix (and not Xist).

3. Calculations
- Not applicable (no numerical calculations required).

4. Why the correct answer is correct
- The molecular roles directly predict the pattern: Xist must be produced from the chromosome that becomes inactive to spread silencing locally; Tsix must be produced from the chromosome that remains active to block Xist there. Empirical studies show Xist RNA coating on Xi and Tsix transcription on Xa.

5. Why the other options are incorrect
- Option 1 (Xist on Xa, Tsix on Xi): reverses known functions; if Xist were on Xa it would inactivate the active X, contradicting its role.  
- Option 3 (both on Xa): impossible because Xist expression on Xa would silence it; Tsix cannot coexist with Xist on the same chromosome in stable XCI.  
- Option 4 (both on Xi): both being on Xi would conflict—Tsix would prevent Xist and thus prevent inactivation; experimental data do not support both being expressed on Xi.

6. Conclusion
- Therefore, Xist is expressed only on the inactive X (Xi), and Tsix is expressed only on the active X (Xa), matching the provided correct answer.

For X-chromosome inactivation to occur, where are the Xist and Tsix genes expressed? Note: Xi is the inactive X chromosome and Xa is the active one.单项选择题

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