Methylation is one type of epigenetic change that can occur to DNA. The enzyme DNA methyltransferase is responsible for DNA methylation. The methyltransferase attaches a methyl group (-CH3) to cytosine, one of four nucleotides in the DNA sequence. The cytosines that become methylated are next to guanines, another nucleotide. The strings of cytosines (C) and guanines (G) are called CpG islands, where "p" stands for the phosphate attaching the two nucleotides. This process results in blocks of methylation when there is a long string, or island, of CpGs.
As it is currently understood, the presence of the methyl groups results in condensed chromosomes, a structure called heterochromatin, tightly wound DNA around protein structures called histones. This condensation makes it difficult for transcription factors, other proteins, to come into contact with the DNA, essentially turning genes off by eliminating their expression.
Gene Expression in a Nutshell
By helping the DNA wind more tightly, DNA methylation controls gene expression. The expression of a gene is the translation of protein from the mRNA, which is coded by the DNA sequence. Double-stranded DNA is usually unzipped to expose the sequence to transcription factors and RNA polymerase. Once the factors bind, the polymerase lays down the RNA in the sequence matching the DNA. This RNA sequence is then used to lay down the amino acid sequence of the resultant protein. The tight structure can prevent the DNA from unzipping, or the methylation blocks on unzipped DNA may affect the binding of transcription factors. The lack of RNA transcription results in a lack of protein, which is a lack of gene expression.
Because of the high level of replication and gene expression needed during embryonic development, DNA methylation is virtually absent during that stage. As cells become differentiated, genes start to get shut off to eliminate unnecessary proteins. Altered methylation patterns are thought to play a role in the development of some cancers, which tend to result from increased or aberrant gene expression.
Studying DNA Methylation in Genetics
Epigenetics is a relatively new field studying the epigenome, the pattern of chemical alterations on chromosomes. In recent years, these alterations have been found to be inherited to some extent and environmentally affected during embryonic development. The full extent of this process is still being studied.
The DNA methylation patterns are not identical in everyone. Genetic imprinting is the process of inheriting the genetic patterns of one parent over the other. Recent studies have found that the DNA sequence and epigenome are inherited independently of one another, which may explain differences in identical twins. This difference is being considered the next step to understanding the inheritance component of diseases like hypertension, diabetes, and heart disease.