You’re adding on another carbon to your molecule, thus turning it into something else. The result depends on the chemical. There’s no universal result. Methylating DNA in certain patterns, for instance, is a method used by the body to tell maternal strands from paternal strands, while some bacteria do it to tell their own DNA from foreign DNA.
That’s one biochemical example. Chemically, if you were to methylate ethanol, you’d get propanol.
It’s done to create new chemicals. A new chemical will have different chemical properties that may provide you with something more functional than the original one. The implications are nothing more than a change in physical properties…kind of like lighting a match changes the match stick cellulose into heat, water vapor, carbon, carbon dioxide, etc.
If you need to know the mechanisms of methylation, google it or buy an old Organic Chemistry textbook-- cuase its been years and I forgot.
Here’s a really nice simple lay intro to organic you should look at. After reading the beginning just remember that methylation just means adding another methane building block and I think it’ll become clear what methylation does.
Here is a book called the methylation miracle that would probably go into depth. Another book is methyl magic. I don’t know how it works exactly but I am quite sure methylation is necessary for neuron and DNA functioning among other things. I remember reading that there are a billion methylations going on each second in the body.
The only example I can think of offhand of methylation are anabolic steroids. Either the 1st or 17th carbon has the hydrogen replaced with a methyl group. As a result it is harder for the liver to break down the testosterone and it remains active in the body long enough to have an effect. Normal testosterone has a half life of about 10 minutes, but methylated testosterone (dianabol for example) has a half life of 4.5 hours.