DNA methylation is the addition of the methyl group to cytosine and adenine nucleotides of the DNA. This causes the silencing of the genes and controls gene expression for transcription.

If DNA encoding a specific gene is to be transcribed into RNA, the nucleosomes surrounding that region of DNA can slide down the DNA to open that specific chromosomal region and allow for the transcriptional machinery ( RNA polymerase ) to initiate transcription. Nucleosomes can move to open the chromosome structure to expose a segment of DNA but do so in a very controlled manner.

The DNA molecule itself can also be modified. This occurs within very specific regions called CpG islands. These are stretches with a high frequency of cytosine and guanine dinucleotide DNA pairs (CG) found in the promoter regions of genes. When this configuration exists, the cytosine member of the pair can be methylated (a methyl group is added). This modification changes how the DNA interacts with proteins, including the histone proteins that control access to the region. Highly-methylated (hypermethylated) DNA regions with deacetylated histones are tightly coiled and transcriptionally inactive. These changes to DNA are inherited from parent to offspring, such that while the DNA sequence is not altered, the pattern of gene expression is passed to the next generation. This is known as epigenetic modification.

Practice Questions

MCAT Official Prep (AAMC)

Section Bank C/P Section Passage 8 Question 59

Section Bank C/P Section Passage 8 Question 63

Practice Exam 3 C/P Section Passage 8 Question 43

Key Points

• Modifications such as acetylation or methylation of the histones can alter how tightly DNA is wrapped around them, while methylation of DNA changes how the DNA interacts with proteins, including the histone proteins that control access to the region.

• This type of genetic regulation is called epigenetic regulation (“above genetics”) as it does not change the nucleotide sequence of the DNA.

Key Terms

epigenetics: the study of heritable changes caused by the activation and deactivation of genes without any change in a DNA sequence

histone: any of various simple water-soluble proteins that are rich in the basic amino acids lysine and arginine and are complexed with DNA in the nucleosomes of eukaryotic chromatin

DNA methylation: the addition of the methyl group to cytosine and adenine nucleotides of the DNA

nucleosome: the basic structural unit of DNA packaging in eukaryotes

RNA polymerase: an enzyme that turns DNA into RNA

CpG islands:  stretches with a high frequency of cytosine and guanine dinucleotide DNA pairs (CG) found in the promoter regions of genes

promoter: a region of DNA where transcription of a gene is initiated

methylated: the addition of a methyl group