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.

• DNA is extremely long (about 2.2 m in humans) and needs to be highly compacted to fit inside very small cells.
• In prokaryotes, DNA is present in a nucleoid without a true nucleus and is organised into loops and supercoils with the help of HU proteins, DNA gyrase, and topoisomerase enzymes.
• In eukaryotes, DNA is associated with positively charged histone proteins rich in lysine and arginine, which help in packaging.
• DNA wraps around a histone octamer made of H2A, H2B, H3, and H4 to form nucleosomes, which are the basic repeating units of chromatin.
• Nucleosomes further coil into higher-order structures like 10 nm fibres, 30 nm solenoids, and looped domains with the help of histone H1 and non-histone chromosomal proteins, ultimately forming chromosomes.
• Chromatin exists in two forms: euchromatin, which is loosely packed and transcriptionally active, and heterochromatin, which is densely packed and 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.