Final answer:
DNA methylation decreases gene expression by adding a methyl group to cytosine at CpG islands, altering nucleosome spacing and making DNA less accessible.
Step-by-step explanation:
DNA methylation has been linked to a decrease in gene expression. It most often occurs at position 5 of cytosine within CpG islands in the promoter regions of genes. DNA methylation is an epigenetic modification that leads to gene silencing by adding a methyl group to the DNA molecule, specifically to the cytosine in a cytosine-guanine (CG) pair.
This modification changes how DNA interacts with proteins, including histone proteins, altering the spacing of nucleosomes and thus affecting gene accessibility. Highly methylated DNA regions with deacetylated histones become tightly coiled and transcriptionally inactive, contributing to gene silencing.