Question textThis diagram is depicting Answer 1 Question 14[select: , Dust on a G-banded chromosome, The location of PAR on chromosomes, Xist RNA coating the X chromosome, GFP probes binding to a chromosome, A colour blindness test]Which would result in Answer 2 Question 14[select: , Identification of colour blindness individuals, Incorrect G banding analysis, Fluorescent identification of the chromosome, X and Y chromosome pairing, Silencing of the X chromosome]

Question

BlackTom AI · Accepted Answer

Xist RNA coating the X chromosome is a hallmark of X‑chromosome inactivation (XCI), so the diagram represents that process and its consequence — silencing of the X chromosome.

1) Relevant knowledge points and concepts
- Xist (X‑inactive specific transcript) is a long noncoding RNA expressed from the future inactive X chromosome.
- Xist spreads in cis along the X, coating it and recruiting chromatin‑modifying complexes (e.g., Polycomb repressive complexes).
- Recruitment leads to histone modifications (H3K27me3), histone deacetylation, DNA methylation and chromatin compaction (Barr body formation), producing stable transcriptional silencing.
- The pseudoautosomal regions (PAR) and GFP probes are unrelated concepts; G‑banding is a DNA stain pattern used for karyotyping.

2) Step‑by‑step reasoning
- The image shows one chromosome (or chromatin fiber) covered by numerous small squiggles; this visually matches RNA molecules coating chromatin.
- Coating of an X chromosome by RNA is specifically characteristic of Xist during XCI rather than a staining artifact or fluorescent probe pattern.
- Xist coating leads to recruitment of repressive complexes → chromatin compaction → loss of transcription from that X.

3) Calculations / intermediate steps
- Not applicable (no numeric calculation required).

4) Why the correct answer is correct
- Xist RNA physically coats the inactive X and initiates epigenetic silencing; therefore the depicted event (Xist coating) directly results in silencing of that X chromosome. Mechanistically, Xist triggers histone modifications and DNA methylation that shut down gene expression across the chromosome.

5) Why other options are incorrect (brief)
- Dust on a G‑banded chromosome: would be an artifact, not organized coating by RNA.
- The location of PAR on chromosomes: PAR are small homologous regions on X/Y ends, not an RNA coating pattern.
- GFP probes binding: would give localized fluorescent signals, typically used experimentally, not continuous RNA coating.
- A colour blindness test: unrelated (a visual clinical test, not cytogenetic).

6) Conclusion / confirmation
Therefore the diagram depicts "Xist RNA coating the X chromosome" and this would result in "Silencing of the X chromosome," matching the provided answer.

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