What is the Difference Between DNA Methylation and Histone Methylation

The main difference between DNA methylation and histone methylation is that DNA methylation is the addition of methyl groups to DNA molecules, whereas histone methylation is the addition of methyl groups to histone protein in the nucleosome.

In general, DNA methylation and histone methylation are two types of methylation events that occur in genetic material. Both types of methylation events affect gene expression.

Key Areas Covered

1. What is DNA Methylation
   • Definition, Characteristics, Importance
2. What is Histone Methylation
   • Definition, Characteristics, Importance
3. Similarities Between DNA Methylation and Histone Methylation
   • Outline of Common Features
4. Difference Between DNA Methylation and Histone Methylation
   • Comparison of Key Differences               

Key Terms

DNA Methylation, Histone Methylation

What is DNA Methylation

DNA methylation is the addition of methyl groups to the DNA molecule. This is due to the changing of the activity of a DNA segment without changing its sequence. When DNA methylation occurs in a promoter, the gene undergoes transcription repression. In DNA, methylation occurs in the nitrogenous base, cytosine, and CpG dinucleotides. Around 75% of CpG dinucleotides are methylated in somatic cells.

Figure 1: DNA Methylation

Furthermore, DNA methylation is an epigenetic mark involving the covalent transfer of a methyl group to the C-5 position of the cytosine ring of DNA by DNA methyltransferases (DNMTs). However, DNA methylation is essential for normal development and is associated with a number of key processes including genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis in mammals.

What is a Histone Methylation

Histone methylation is the methylation of the amino acids of the histone proteins in the nucleosome. Here, the nucleosome contains DNA double helix wraps around, forming chromosomes. Moreover, histone methylation can either increase or decrease the transcription of genes. However, it depends on the type of methylated amino acids of histones. Methylation weakens chemical attractions between histone tails and DNA, thereby allowing transcription factors and other proteins to access the DNA.

Figure 2: Histone Methylation

When the histone tails encompass DNA, transcription factors lack the accessibility to DNA, decreasing the expression. Therefore, methylation and demethylation of histones turn the genes in DNA “off” and “on,” respectively.

Similarities Between DNA Methylation and Histone Methylation

• DNA methylation and histone methylation are two types of methylation events in which methyl groups are added to DNA or histone groups.
• Both types of methylation events affect gene expression.

Difference Between DNA Methylation and Histone Methylation

Definition

DNA methylation refers to a biological process by which methyl groups are added to the DNA molecule, while histone methylation refers to a process by which methyl groups are transferred to amino acids of histone proteins that make up nucleosomes.

Occurrence

In DNA methylation, methyl groups incorporate into cytosine while in histone methylation, methyl groups incorporate into amino acids of the histone protein.

Activation of DNA

Usually, DNA methylation represses gene expression while histone methylation activates or represses gene expression.

Function

DNA methylation is important in genomic imprinting, X-chromosome inactivation, repression of transposable elements, aging, and carcinogenesis, while histone methylation is important in “off” and “on” DNA for expression.

Conclusion

In brief, DNA methylation and histone methylation are two events of chromosomes, in which a methyl group is added to either CpG sites or amino acids, respectively. DNA methylation represses the expression of genes. Therefore, it is important in events such as genomic imprinting, X-chromosome inactivation, etc. Histone methylation, on the other hand, can either activates or repress gene expression. In DNA methylation, methyl groups incorporate into cytosine while in histone methylation, methyl groups incorporate into amino acids of the histone protein. Therefore, the main difference between DNA methylation and histone methylation is their ability to activate and repress genes.

References:

1. Jin B, Li Y, Robertson KD. DNA methylation: superior or subordinate in the epigenetic hierarchy? Genes Cancer. 2011 Jun;2(6):607-17. doi: 10.1177/1947601910393957. PMID: 21941617; PMCID: PMC3174260.
2. Histone methylation. An overview | ScienceDirect Topics. (n.d.). Retrieved September 8, 2022.

Image Courtesy:

1. “DNAme landscape ” By Mariuswalter – Own Work (CC BY-Sa 4.0) via Commons Wikimedia
2. “Epigenetic mechanisms ” By National Institutes of Health – Own Work (Public Domain) via Commons Wikimedia