The average distance between two nucleotide in DNA is 3.4 A. Length of DNA for a human diploid is 6.6x10 power of 9 bp. The DNA is acidic due to presence of large number of phosphate group. Compaction occurs by folding and attachment of DNA with basic proteins polyamine in prokaryotes and histones in eukaryotes.
DNA packaging in Prokaryotes
DNA is found in cytoplasm in supercoiled state. The coils are maintained by non histone basic proteins like polyamines. RNA may also be involved. This compact structure of DNA is called nucleoid/genophore.
DNA packaging in eukaryotes/Nucleosome
The word nucleosome was proposed by oudet. In eukaryotes there is a set of positively charged, basic proteins called histones. Histones are five types H1, H2A, H2B,H3 and H4. Histones are rich in the basic amino acid residues lysines and arginine with positive charges in their side chains. Histones are organized to form a unit of a structure in nucleus called chromatin, thread like stained bodies seen in nucleus. The nucleosomes in chromatin are seen as "beads on string" structure when viewed under electron microscope. The packaging of chromatin at higher level required additional set of proteins that collectively referred as Non Histone chromosomal Protein(NHP).
Chromatin
The beads in sting structure in chromatin is packaged to form chromatin fiber that are further coiled and condensed at metaphase stage of cell division to form chromosomes. In a typical nucleus, some regions of chromatin are loosely packed and are referred to as euchromatin. The chromatin that is more densely packed and stains dark are called heterochromatin and transcriptionally active chromatin or loosely packed called euchromatin.