Packaging of DNA Helix

Nucleoid is the region in prokaryotic cells where DNA is organized and associated with proteins, despite the absence of a true nucleus.

• In eukaryotes, chromosomes are gene carriers, and each chromosome consists of a single long DNA molecule associated with histone proteins (unineme model).
• DNA is packaged into repeating units called nucleosomes, where DNA is wrapped around a histone octamer made of H₂A, H₂B, H₃, and H₄ proteins.
• Nucleosomes form a 10 nm beaded fibre, which coils further into a 30 nm solenoid structure with the help of histone H₁.
• Higher-level folding condenses chromatin into looped domains attached to a protein scaffold, finally forming metaphase chromosomes with the help of non-histone chromosomal proteins.
• Chromatin exists as euchromatin (loosely packed, transcriptionally active) and heterochromatin (densely packed, transcriptionally inactive).

The process by which a very long DNA molecule is compactly organised inside the cell nucleus so that it fits within the limited nuclear space and remains functional is called DNA packaging.

Positively charged basic proteins rich in lysine and arginine that associate with DNA to help in its packing in eukaryotic cells are called histones.

A structural unit composed of eight histone protein molecules around which DNA is wrapped is called histone octamer.

The basic repeating unit of chromatin formed by DNA wrapped around a histone octamer is called nucleosome.

The thread-like complex of DNA and proteins present in the nucleus of eukaryotic cells is called chromatin.

Proteins other than histones that are associated with chromatin and help in higher-order DNA packaging and regulation are called non-histone chromosomal (NHC) proteins.