What effect does methylation have on genes?

Methylation involves the addition of a methyl group to DNA, specifically at the cytosine base in the context of CpG dinucleotides. This process is a key aspect of gene regulation and typically serves to repress gene expression. When methylation occurs in the promoter region of a gene, it can prevent the binding of transcription factors necessary for gene activation, thereby stopping the transcription process.

By inhibiting transcription, methylation effectively silences the gene, leading to a scenario where the gene is not transcribed into mRNA, and consequently, no protein is produced. This silencing mechanism is crucial for various biological processes, including development, X-chromosome inactivation, and genomic imprinting. Thus, the effect of methylation on genes predominantly results in their inability to be transcribed, supporting the notion that genes are not transcribed when they are methylated.