DNA Methylation

[hamsterboy121]

As it relates to restriction enzymes...

Background:

In prokaryotes, restriction enzymes cut up foreign DNA to protect the cell from inadvertantly copying outside DNA (i.e., from a phage). The cell protects its own DNA by methylation. Methyl groups are added to the 6N of A and the 5C of C in its own DNA.

Question:

Why are the methyl groups added only to A and C? What about the chemical structure of A and C makes them receptive to methyl groups, and what about T and G makes them unreceptive?

I'm still in highschool and just learning the beginnings of biochemistry, so please feel free to dumb things down. I am atleast familiar with functional groups and have a basic knowledge of Lewis dot structures, which I'd guess is vital to understanding this phenomena.

Anything to shed some light on this question would be appreciated.
Thanks,

Michael

[vladkara]

You have brought up an excellent question. I cannot answer your question directly based stricly on biochemistry. I believe that it is very much a question of evolution. There are many methytransferases (enzymes) which are also somewhat organism specific. I believe that the evolution is going in somewhat unpredictable ways although, there are many general patterns. So somewhere along the way the first methyltransferase was fitting better to methylate A or C base.
It has been said that biology is all about evolution. This statement is compounded with every new discovery in biology.
You might be interested to read a very interesting book: The Epigenetics Revolution by Nessa Carey
This book shows how epigenetics is all about the control of many biological functions.
Sincerely
Vladimir karasek