2 PACKAGING OF DNA HELIX  The average distance between two adjacent base pairs is 0.34 nm (0.34 × 10 –9 m or 3.4 Å).  Length of DNA for a human diploid cell is 6.6 × 109 bp × 0.34 × 10–9 m/bp = 2.2 m.  This length is far greater than the dimension of a typical nucleus (approximately 10–6 m).  The number of base pairs in Escherichia coli is 4.6 × 10 6. The total length is 1.36 mm.  The long sized DNA is accommodated in small area (about 1 µm in E. coli) only through packing or compaction. 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 histone in eukaryotes. DNA packaging in Prokaryotes :  DNA is found in cytoplasm in supercoiled state.  The coils are maintained by non histone basic protein like polyamines.  RNA may also be involved. This compact structure of DNA is called nucleoid or genophore. Fig: Genetic Material in a Prokaryotic Cell DNA packaging in Eukaryotes :  It is carried out with the help of lysine and ariginine rich basic proteins called histones.  The unit of compaction is called nucleosome.  There are five types of histone proteins H1, H2A, H2B, H3 and H4.  Four of them occur in pairs to produce histone octamer (2 copies of each -H2A, H2B, H3 and H4), called nu body or core of nucleosome.  Their positively charged ends are directed outside.  They attract negatively charged strands of DNA.  About 200 bp of DNA is wrapped over nu body to complete about 1¾ turns.  This forms a nucleosome of size 110 × 60 Å (11 × 6 nm).  DNA present between two adjacent nucleosome is called linker DNA.  It is attached to H1 histone protein. Fig: DNA packaging in Eukaryotes  Length of linker DNA varies from species to species.  Nucleosome chain gives a beads on string appearance under electron microscope.  The nucleosomes further coils to form solenoid.  It has diameter of 30 nm as found in chromatin.  The beads-on-string structure in chromatin is packaged to form chromatin fibres that are further coiled and condensed at metaphase stage of cell division to form chromosomes.  The packaging at higher level requires additional set of proteins (acidic) that collectively are referred to as non-histone chromosomal (NHC) proteins. Non-Histone Chromosomal proteins are of three types: (i) Scaffold or Structural NHC protein. (ii) Functional NHC protein, e.g., DNA polymerase, RNA polymerase. (iii) Regulatory NHC protein, e.g., HMG (High mobility group proteins that controls gene expression). Various steps in the folding and superfolding of basic chromatin components to generate an eukaryotic chromosome:  In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. • The chromatin that is more densely packed and stains dark is called as heterochromatin, specifically euchromatin is said to be transcriptionally active and heterochromatin is transcriptionally inactive. Chemical Composition of Chromosome  A chromosome consists of following chemical compositions: (a) DNA 40% (b) RNA 1.2% (c) Histone protein 50% (d) Acidic proteins 8.5% (e) Lipid traces (f) Ca+2, Mg+2, Fe+2 traces