DNA is wound round histone proteins. This is called chromatin. Heterochromatin is tightly wound and euchromatin is loosely wound. The genes in euchromatin can be transcribed but RNA polymerase cannot access the genes in heterochromatin.The addition of acetyl groups or phosphate groups to histones reduces their positive charge. This makes DNA coil less tightly allowing genes to be transcribed. Methylation makes histones more hydrophobic so they bind together more tightly causing DNA to coil tightly. The control of gene expression by chromatin remodelling through histone modification is sometimes known as epigenetics.