Lesson 3 of 13 · 7 min
Packaging of the DNA helix
NCERT § "The DNA"
Stretch out all the DNA in one of your cells and it would run about 2.2 metres — yet it fits inside a nucleus roughly a millionth of a metre across. Something has to fold it, hard.
The lesson in notes
In short
Human diploid DNA is about 6.6 × 10⁹ bp; multiplied by 0.34 × 10⁻⁹ m per bp, the total length is about 2.2 m, far longer than a nucleus of about 10⁻⁶ m, so it must be packed tightly.
In prokaryotes like E. coli there is no defined nucleus. The negatively charged DNA is held with some positively charged proteins in a region called the nucleoid, organised in large loops.
In eukaryotes the DNA is packaged by histones, which are positively charged basic proteins rich in lysine and arginine, both basic amino acid residues.
A histone octamer is made of eight histone molecules. Wrapping of the negatively charged DNA around it forms a nucleosome, which typically holds 200 bp of DNA.
Nucleosomes are the repeating units of chromatin and appear as 'beads-on-string' under the electron microscope. Chromatin is coiled further into chromatin fibres, which condense at metaphase into chromosomes.
Packaging beyond the nucleosome level needs another set of proteins, the non-histone chromosomal (NHC) proteins.
Euchromatin is loosely packed, stains light and is transcriptionally active. Heterochromatin is densely packed, stains dark and is inactive.