The substance of which eukaryotic chromosomes are composed. It consists of proteins (principally histones), DNA, and small amounts of RNA. The DNA molecule is wrapped around the histones to form a series of linked globular nucleosomes, resembling beads on a string. This is itself coiled to form a highly condensed solenoid arrangement, constituting a form of chromatin called heterochromatin, which stains densely with basic stains. The genes in the solenoid can only be transcribed if the solenoid unfolds to some extent, forming an expanded chromatin (euchromatin), which is lighter staining. The degree of condensation in any particular region is regulated by reversible acetylation of the histones: the greater the degree of acetylation, the less condensed the chromatin, and hence the greater the availability of genes for transcription.

http://www.accessexcellence.org/RC/VL/GG/ecb/chromatin_packing.html Diagram showing levels of chromatin packing