GATE 2023 BT – Question 40

Methylation of CpG islands near the promoter of a gene can inhibit transcription by

Show answer and explanation

Correct answer: (A) preventing RNA polymerase binding; (B) facilitating repressor binding; (C) facilitating heterochromatin formation

Explanation

Methylation at CpG islands in a promoter silences transcription in several ways.

**A. Preventing RNA polymerase binding:** the methyl groups on cytosines can directly block the binding of transcription factors and the polymerase machinery. ✓
**B. Facilitating repressor binding:** methyl-CpG-binding proteins (such as MeCP2 and MBD1 to 4) bind the methylated DNA and recruit repressor complexes. ✓
**C. Facilitating heterochromatin formation:** these complexes bring histone deacetylases and methyltransferases, which compact the chromatin into transcriptionally silent heterochromatin. ✓
**D. Inducing euchromatin formation:** euchromatin is the open, active state, so this is the opposite effect. ✗

Answer **A, B and C**.