Why does acetyl have negative charge? "Acetylation (adding an acetyl group) and phosphorylation (adding a phosphate group) make the histones more negatively charged because acetyl and phosphoryl groups are negative. They are "glass is half empty" molecules. By making histones more negatively charged, their grip on DNA will be much looser because DNA is also negatively charged. Similar charges (negative and negative) repel one another". 


I know oxygen and carbon have different electronegativities but that generates a partial negative charge and a partial positive charge. So why do we consider the negative charge prevails over the positive charge in order to histones "repel" DNA?
The acetyl group itself does not have any significant charge. But the residue that gets acetylated is lysine in this case, which has a positive charge. The acetylation results in a neutral amide, removing the positive charge.



From Wikimedia Commons

I'd add that the effect of histone acetylation is likely more subtle than just the charge effect. This kind of posttranslational modification often affects the protein structure in some way, or is recognized by other proteins interacting with that site.