Inside the cell nucleus, the DNA molecule is found in a densely packed DNA-protein complex known as chromatin. Here the DNA is wrapped around a core of histone proteins and densely packed to form nucleosomes. The structure of the nucleosomes determines which genes are accessible and active and therefore plays an important role in gene regulation. To respond to metabolic signals, changed environmental conditions, and developmental processes, the nucleosomes must undergo repeated dynamic modifications with the aid of enzymes.