Question

One of the DNA fragments is found to have a region rich in CpG repeat sequences. Further analysis reveals that most of the cytosine residues have been methylated to 5-methylcytosine. What will occur?

a) Gene silencing
b) Gene activation
c) Increased transcription
d) Enhanced replication

Answer 1

DNA methylation in CpG islands, specifically when cytosine residues are methylated, leads to gene silencing by making the DNA inaccessible to transcription machinery.

Step-by-step explanation:

When DNA within CpG islands is methylated, the most common outcome is gene silencing. These CpG rich areas are often found in the promoter regions of genes, where methylation of cytosine residues to 5-methylcytosine has been shown to inhibit transcription. This epigenetic modification of DNA changes its interaction with proteins such as histones, leading to tightly coiled DNA that cannot be easily accessed by transcription machinery. Consequently, the transcription of the associated gene is reduced or completely shut down.

Epigenetic changes, like histone deacetylation in conjunction with DNA methylation, are hallmarks of silenced genes in various conditions, including cancer. Therefore, in the case of the student's DNA fragment with a high degree of methylated cytosines in a CpG rich region, the most likely result would be gene silencing.