methylation, irreversible? - (Mar/01/2007 )
it is thought methylation is irreversible, so far no DNA demethylase was found, so the methylation can only be diluted after DNA replication, if the methytransferase did not add methy during replication.
but during epegenetic reprogramming for gametogenesis and fertilization, primordial germ cells undergo two times active demethylation followed by passive demethylation. (Epigenetic reprogramming in mammals, human molecular genetics, 2005)
i am confused, please post your opoinin.
it is true there is active demethylation.
no one has found the demethylation responsible for it yet, that is the holy grail and a nature/science paper if found.
People have speculated that MBD2 could be it or DNA glycosylases but they have now proven to be wrong (becuase it can't be replicated)
This is a theory for the case that there is no DNA demethylase - it presumes that "methylation is irreversible".
At least it is a theory that can be used until a DNA demethylase is found (if ever), which would immediately invalidate the theory.
Because it has been shown that DNA is demethylated without cell replication during gametogenesis,
scientists presume that there must be a mechanism of active demethylation, a DNA demethylase.
Unfortunately, there is no proof until now, so it remains a presumption.
I have allowed myself to combine and slightly rephrase your statements, maybe this will help:
"It is theoretically possible that methylation is reversible, and processes during gametogenisis indicate this.
However, an active DNA demethylase to prove this has not been found, so active reversal of methylation remains a theory.
The only thing we can definitely say about demethylation is that there is a passive pathway which works by "dilution" of methylation during replication"