DNA contains our genetic code for who we are. When we need new proteins, enzymes, and cells, specific areas of the DNA called genes are copied to produce these new products.
When DNA is methylated (the addition of a carbon and three hydrogen molecules to the DNA), the ability for the gene to express or be copied is blocked. This prevents unwanted proteins and cells from being replicated.
The methyl groups bind to areas on the DNA strand called CpG sites. CpG stands for Cytosine-phosphate-Guanine. The CpG site acts like a plug. If the CpG site is filled with a methyl group, the gene can’t replicate. If there is no methyl group attached to the CpG site the gene can be copied.
I’ll use an analogy to make this easier to understand.
When I want to use my vacuum cleaner I need to plug it into electricity. There are specific spots in my wall where I can plug the cord in called outlets. Our DNA is a long molecule. There are specific areas on the DNA where replication can occur. The CpG sites signify the areas where replication can occur.
When a gene needs to replicate to produce new proteins, messengers molecules called “transcription factors” (the plug of my vacuum) are sent to the DNA to bind to the CpG sites (the outlet). When the two combine, the gene is able to be copied.
When my kids were little, we put protective covers on our electrical outlets when we weren’t using them to prevent the kids from sticking things in the outlet and getting an unwanted shock.
Methyl groups act like protective covers on our CpG sites to prevent unwanted replication of genes.
If there are child protective outlet covers on the outlets in my home, I can’t plug the cord in. If I can’t plug the cord in, I can’t vacuum. On the DNA, methyl groups act like outlet protectors preventing the transcription factors from binding to the CpG sites, inhibiting replication of the gene. This is called DNA methylation.
When a gene needs to replicate, the methyl group must be pulled off of the CPG site. This allows the transcription factor to “plug in” to the CpG site, triggering the replication of the gene. When replication of the gene is completed, a methyl group is added back to the CpG site to stop the cell from continuing to replicate. Again, the addition of the methyl group is called, DNA methylation.
Without methylation of DNA, cells could excessively replicate producing problems like tumors and cancer. Methylation is a form of epigenetic control of gene expression. When someone develops cancer there is both hypo and hyper-methylation occurring. That discussion is a bit more complex and needs it’s own separate post.