Methylation is a process by which a gene's behavior is altered, but the gene itself isn't changed. This is almost like following all the directions in baking a batch of cookies correctly, except for the oven temperature. Even though all of the ingredients are the same, those cookies won't bake -- or behave -- the same way when baked a couple hundred degrees lower than they should be.
In methylation, though, the wrong oven temperature is actually other factors, such as environmental exposures or different lifestyle choices. These factors have the potential to cause methyl groups, which are groups of one carbon and three hydrogens, to land on top of our genes and change how they are expressed. This, in turn, changes the ability of our genes to share the directions they contain for making our bodies' proteins.
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