True about DNA methylation –
**Core Concept**
DNA methylation is an epigenetic modification that plays a crucial role in regulating gene expression by adding a methyl group to the 5th carbon atom of the cytosine residue in a CpG dinucleotide, typically in a CpG island. This process is catalyzed by DNA methyltransferases (DNMTs) and is essential for various cellular processes, including development, differentiation, and cancer.
**Why the Correct Answer is Right**
DNA methylation is a key mechanism of gene silencing, and its dysregulation has been implicated in various diseases, including cancer. The addition of a methyl group to CpG islands in promoter regions can prevent transcription factor binding and thereby repress gene expression. This process is reversible, and changes in DNA methylation patterns can be used as biomarkers for disease diagnosis and prognosis. The enzymes responsible for DNA methylation, such as DNMT1 and DNMT3A, are critical for maintaining genome stability and regulating gene expression.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because DNA methylation primarily involves the addition of a methyl group to cytosine, not adenine. Adenine methylation is not a well-characterized epigenetic modification.
**Option B:** Incorrect because histone modification, another type of epigenetic regulation, involves the post-translational modification of histone proteins, not DNA methylation.
**Option C:** Incorrect because mutations in DNA repair genes can lead to genomic instability, but they are not directly related to DNA methylation.
**Clinical Pearl / High-Yield Fact**
DNA methylation patterns can be used as biomarkers for disease diagnosis and prognosis, and changes in these patterns have been implicated in various cancers, including colon, breast, and lung cancer.
**Correct Answer:** D.