histone acetylation
"Two major tags can be added to DNA that generally have opposing effects on information exposure. Using a stoplight as an analogy for these DNA tags, DNA methylation is like a red light, while histone acetylation is like a green light. Thus, instructions that are "methylated" are not read or carried out by the cell (red light). On the other hand, histone acetylation, which is the addition of an acetyl group to the tennis ball-like histone proteins that DNA is wrapped around inside cells, actually allows the "string" to be more loosely wrapped around the "tennis balls", making it easier for the instructions to be read and executed by the cell (green light)."
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