Length of DNA double helix molecule, in a typical mammalian cell is approximately 2.2 meters. (This can be worked out simply by multiplying the total number of base pairs with distance between the two consecutive base pairs). Approximate size of a typical nucleus 10-6m. How to long DNA molecule can be accommodated in such a small nucleus? It is therefore must be condensed and coiled and super coiled to fit in the nucleus.
In eukaryotes, this packaging or organization of DNA is rather complex. Histones are required for the packaging of DNA. Histones are proteins that are rich in the basic amino acid residues lysines and arginines which carry positive charge in their side chains. Eight molecules of histones (two each of H2A, H2B, H3 and H4) get organized to form histone octamer. DNA is negatively charged and it is wrapped around the positively charged histone octamer to form nucleosome.
Under the electron microscope, nucleus shows chromatin network. The nucleosomes in chromatin are seen as 'beads-on-string'. Around the octamer, DNA molecule is wrapped as 1 and 3/4th turn. This DNA is called core DNA and it consists of about 146bp (base pairs). Adjacent nucleosomes are linked with small segments of DNA called linker DNA; of about 54bp. (Thus the string is, DNA- 2nm or 20A° in diameter). This 'beads-on-string' which is 10 nm (100A°) in diameter. H1 histone is present in the linker region, and as DNA makes two complete turns it is present where the DNA starts wrapping the octamer and leaves it. Each nucleosome contains 200bp of DNA helix.
The thin and long nucleosome fiber is coiled like a telephone wire to make solenoid fiber with diameter 30 nm or 300A°. Nucleosome and solenoid fibers are characteristic of nucleus at interphase. During prophase of cell division, there is formation of chromatin fibers of 200 nm and then a chromatid of 700 nm. At metaphase of cell division a chromosome with two chromatids joined by common centromere can be seen under the compound microscope.
The packaging of chromatin at higher levels need additional set of proteins that are collectively called Non-Histone Chromosomal (NHC) proteins. A loosely packed region of chromatin that strains light, is called euchromatin and densely packed region that stains dark, is called heterochromatin. Euchromatin is considered as transcriptionally active chromatin, while heterochromatin is inactive.