DNA methylation and histone modification are two crucial epigenetic mechanisms that play a significant role in regulating gene expression.

DNA Methylation
DNA methylation involves the addition of a methyl group to the cytosine nucleotide in DNA. This modification can lead to the repression of gene expression by hindering the binding of transcription factors to the DNA, thereby preventing the initiation of transcription. Typically, DNA methylation is associated with long-term gene silencing and is integral to processes such as X-chromosome inactivation and genomic imprinting.

Histone Modification
In contrast, histone modification refers to the addition or removal of chemical groups to the histone proteins that comprise the nucleosome. These modifications can alter the chromatin structure, influencing its accessibility to transcription factors and RNA polymerase. For instance, the acetylation of histones is generally linked to a more open chromatin structure, which promotes increased gene expression. Conversely, histone methylation can lead to either the activation or repression of gene expression, depending on which specific amino acid is modified.

In summary, both DNA methylation and histone modification are vital mechanisms for regulating gene expression and maintaining cellular identity. Although they are distinct processes, they often collaborate to achieve precise control over gene expression patterns.