How does DNA methylation affect gene expression?

Hint: DNA methylation is defined as the biological process which includes the addition of methyl groups to the DNA. Methylation plays an important role in altering gene expression without causing any change to the gene sequence.

Complete answer:
DNA comprises four bases- adenosine, guanine, cytosine and thymine, out of the four two of them can be methylated. They are cytosine and adenine. Methylation of cytosine converts it to 5-methylcytosine which in turn can get deaminated to form thymine. This causes a T-G mismatch which is recognized by the repair machinery of the cell. The repair machinery can either reverse and form the original GC-G base pair or can convert G to A forming A-T base pair, thus introducing a mutation. Cytosine methylation is highly found in both eukaryotes as well as prokaryotes.

Since DNA methylation can change the base sequences in the DNA it is also capable of altering gene expression. DNA methylation is known to affect transcription of genes in two possible ways- by blocking the binding of transcriptional proteins to the promoter of the gene and by causing methylated DNA to bind to certain proteins that causes chromatin remodeling leading to formation of heterochromatin. In both the cases no transcription takes place as a result of which expression of the gene also does not take place.

Note: DNA methylation being an important process that can affect gene expression it is a highly researched area and methylation can be detected by various means few of which are-mass spectrometry, whole genome bisulfite sequencing, methylation specific PCR, ChIP assay, etc.