DNA methylation

[m25]

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Is the effect of DNA methylation to prevent the transcription of the gene, thereby silencing the gene? Is there an instant where DNA methlylation actually activates the gene for transcription?

 

[Czarcasm]

Is the effect of DNA methylation to prevent the transcription of the gene, thereby silencing the gene? Is there an instant where DNA methlylation actually activates the gene for transcription?

My molecular biology is a little bit rusty, but that seems true. I believe there are two major instances where methylation is involved in gene regulation. On the one hand, you have methylation of cytosine residues (typically), generally in the promoter sequence, which acts to suppress transcription activity. This is one way several genes are kept permanently off in certain specialized tissues, or within the organism as a whole (as can be illustrated by genomic imprinting).

However, there's a level of regulation in Eukaryotes just prior to transcription and that's at the chromatin level; more specifically, the methylation of histone proteins. By methylating histone proteins, it promotes a more condensed form of DNA which in most cases prevents transcription by hiding the promotor or even the entire gene. But an entirely different modification of histones: acetylation, apparently loosens up the DNA wound around histone proteins (collectively, the nucleosome), thereby making transcription more available. These are just two ways modification of histone proteins can affect transcription - there are still others too (ie. phosphorylation, etc).

Bare in mind these are generalities and there can be exceptions to these trends.

 

[jamcat]

Is the effect of DNA methylation to prevent the transcription of the gene, thereby silencing the gene? Is there an instant where DNA methlylation actually activates the gene for transcription?

Yes. No. (Careful here though; if you methylate the gene for some sort of inhibitor, then the inhibitor won't get transcribed and whatever it inhibits will no longer be inhibited and thus appear to be activated. But this is several steps down the line; the methylated gene is not transcribed.)

Like Czarcasm said, the situation is different when it comes to protein modifications.

This is simplistic but I don't think you need to go deeper than that.