Question: I have read a small amount about methylation patterns and how they affect genes in a few magazines and I got the impression that the proteins you referred to in your lecture switched genes off, rather than on, am I right?, I realize that you may have been speaking figuratively.

Karolina Chocian answered on 19 Dec 2013:

Hi,
As I haven’t seen Alison’s lecture yet I can only give you a general answer…
So methylation of DNA occurs on Cysteine, especially when it’s close to Guanine (so called CpG dinucleotides). methylation is usually found to genes that are not actively expressed in the particular cell as it helps to form an inactive form of chromatin: heterochromatin.
However it’s biology so nothing is simple. to make matters slightly more complicated there are regions of DNA we call CpG islands – clusters of cysteine and Guanine that are NOT methylated- and they usually are present at the promoters of the genes (regions of DNA that regulate switching gene on/off).
So methylation usually is a sign of repression and lack of methylation a sign of activation.

But then there is cancer- in which everything is deregulated- things that were supposed to be methylated (and therefore off) get demethylated and the CpG islands that were supposed to keep genes switched on, become methylated and cause the genes to be downregulated.
There are particular enzymes that can methylate and demethylate DNA and probably these are the ones Alison referred to but I will be able to tell you more when I see it on TV 😉
hope it clears some stuff up