What is DNA methylation and how does it influence gene expression?

What is DNA methylation and how does it influence gene expression?

• A. Addition of methyl groups to cytosine residues, often repressing transcription
• B. Addition of phosphate groups to DNA backbone
• C. Mutations in promoter region cause activation
• D. DNA methylation increases transcription globally

Answer: (A)

DNA methylation is the addition of a methyl group to cytosine bases in DNA, usually at CpG sites, performed by DNA methyltransferases. When this mark sits in a gene’s promoter, it often blocks transcription factor binding and recruits proteins that recognize methylated DNA (such as methyl-CpG–binding domain proteins). These proteins bring in other factors that modify chromatin to a closed, repressive state, making the DNA less accessible to RNA polymerase and other transcriptional machinery. The result is reduced or silenced gene expression. This mechanism is important in development, genomic imprinting, and X-chromosome inactivation. It’s distinct from adding phosphates to the DNA backbone, which is not methylation, and from promoter mutations that alter sequence rather than epigenetic regulation. In general, methylation at promoters tends to repress transcription rather than increase it globally.

DNA methylation is the addition of a methyl group to cytosine bases in DNA, usually at CpG sites, performed by DNA methyltransferases. When this mark sits in a gene’s promoter, it often blocks transcription factor binding and recruits proteins that recognize methylated DNA (such as methyl-CpG–binding domain proteins). These proteins bring in other factors that modify chromatin to a closed, repressive state, making the DNA less accessible to RNA polymerase and other transcriptional machinery. The result is reduced or silenced gene expression. This mechanism is important in development, genomic imprinting, and X-chromosome inactivation. It’s distinct from adding phosphates to the DNA backbone, which is not methylation, and from promoter mutations that alter sequence rather than epigenetic regulation. In general, methylation at promoters tends to repress transcription rather than increase it globally.