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Sometimes general questions are the best.
Here is a very general answer: post translational modification of histones controls whether or not transcription factors can access binding sites and gene regulatory elements in the DNA, so it effectively adds another layer of control to whether or not certain genes are transcribed.
Alright, so is chromatin directly responsible for binding sites in gene regulation?
The promoters and enhancers in the DNA - so called "cis regulatory elements" - are recognized by transcription factors specific for each gene. They are the binding sites directly responsible for gene regulation.
That said, the conformation adopted by the DNA - how it's packed into nucleosomes and those nucleosomes are condensed into chromatin - determines which recognition sequences are accessible to the transcription factors.
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I see. Apparently, on cellular differentiation, "bivalent chromatin structures" derepress gene expression. I understand everything in this sentence except what bivalent means with respect to chromatin.
Correction. I mean to say the bivalent chromatin structures apparently poise, but suppress, gene expression in ESC. Upon differentiation, they derepress certain important genes.
My question is what does genetic bivalence have to do with chromatin.
Ah, wait, never mind. I got confused. Thanks, blues!