Histones combine with DNA to form nucleosomes, the fundamental structural units of chromatin.  Nucleosomes pack DNA in a stable coiled form in eukaryotic nuclei.  The total DNA complement of the nucleus, with associated histone and non-histone proteins, is broken into individual lengths; these are the chromosomes.

The coiling of DNA around nucleosomes and the further winding of nucleosomes into chromatin fibers greatly compact the DNA of the eukaryotic nuclei.  Winding the DNA into nucleosomes and chromatin fibers is estimated to shorten its length by at least a factor of 10,000.

Because packing of nucleosomes in chromatin fibers probably prevents access to DNA by the enzymes of transcription and replication, the fibers probably unwind for these activities to take place. The increased sensitivity of active chromatin to endonuclease digestion indicates that even nucleosomes probably unfold or dissassemble from DNA as it enters transcription or replication.  All of this points to the conclusion that chromatin in intact nuclei is highly dynamic, with different folding conformations that reflect its activity.  [The figure on the left shows DNA structure from unpacked double helix (top) to fully compacted chromosome (bottom)]