• The nucleosome model was proposed by Roger Kornberg for the DNA packaging in eukaryotic chromosome.
• Under electron microscopes, chromosomes were found to look like 'beads on a string' because of the series of spherical particles connected by a fine filaments.
• These 'beads' were termed as nucleosomes.
• Nucleosome is a complex of DNA and Histone proteins to condense the DNA into smaller volume inside the nucleus.

Important Points

• Histones are positively charged, basic proteins which are rich in Lysine and Arginine residues.
• The histones H2A, H2B, H3 and H4 are involved in the nucleosome structure, while the H1 protein acts like a linker molecule between two nucleosomes.
• 2 molecules of H3 and H4 form a core of tetramer (as a pair of dimers).
• This (H3)₂.(H4)₂ tetramer is associated with 2 independent dimers of H2A-H2B.
• DNA being negatively charged, wraps around the histone octamer and forms the nucleosome.
• DNA enters and leaves the same side and the two turns are sealed by the H1 protein.
• A nucleosome contains about 200bp of DNA, out of which 146bp is involved in the coiling of octamers and the rest is associated with the linker molecule.
• It forms the 11nm wide interphase fibre.

Additional Information

• Nucleosomes are the first level of DNA condensation involved in DNA packaging.
• Nucleosomes are further condensed into the 30nm solenoid which is again organized into the tightly packed metaphase chromosomes.
• Non-histone proteins (NHP) can also bind to DNA and are present in the nucleus, but they do not play any role in nucleosome formation.
• NHPs may have various functions related to higher level of DNA packaging.