16.     DNA packing in eukaryotes is carried out with help of lysine and arginine 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 (H₂A, H₂B, H₃ and H₄) occur in pairs to produce histone octamer, called nu body or core of nucleosome. Their positively charged ends are towards the outside. They attract negatively charged strands of DNA. DNA over nu body forms 1¾ turns to form nucleosome core. A typical nucleosome contains 200 bp of DNA helix. DNA connecting two adjacent nucleosomes is called interbead or linker DNA. It bears H₁ histone protein.Nucleosome chain gives a beads-on-string appearance