Histone Modification
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Histone modification, such as adding an acetyl group, changes how tightly DNA wraps around histones, loosening or tightening chromatin and altering gene accessibility.
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Adding acetyl groups to histones loosens chromatin's packing, generally making the underlying genes easier for RNA polymerase to reach.
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A histone modification changes DNA's accessibility, not its underlying sequence — the gene itself is unaltered by this regulatory layer.