Simulation · Biology · Class 12
Packing 2.2 m of DNA into a nucleus
From the lesson Packaging of the DNA helix in Molecular Basis of Inheritance. Change the values and watch what happens.
Packing 2.2 m of DNA into a nucleusBiology · Class 12
The idea behind it
NCERT § "The DNA"
- 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.