Page 3 - CH6 (1)

Packaging of DNA Helix

● Distance between two consecutive base pairs in a DNA = 0.34 nm =
0.34 × 10​ m
−9​

● Total number of base pairs in a human DNA = 6.6 × 10​bp
9​
9
−9​
● Total length of human DNA = 0.34 × 10​ × 6.6 × 10​
= ~ 2.2 m
● 2.2 m is too large to be accommodated in the nucleus (10​ m).
−6​
● Organisation of DNA in prokaryotes:
○ They do not have nucleus. DNA is scattered.

○ In certain regions called nucleoids, DNA (negatively charged) is
organised in large loops and is held by some proteins (positively
charged).

● Organisation of DNA in eukaryotes:
○ They have positively charged basic proteins called histones

(positive and basic due to presence of positive and basic amino
acid residues, lysine and arginine).

○ Histone octamer − Unit of eight molecules of histone
○ DNA (negatively charged) winds around histone octamer

(positively charged) to form nucleosome.

○ 1 nucleosome has approx. 200 bp of DNA.
○ Nucleosomes in a chromatin resemble beads present on strings.

○ Beads on string structure in chromatin are further packaged to
form chromatin fibres, which further coil and condense to form

chromosomes during metaphase.