Write the length of DNA double helix, if the total numbers of base pairs are 6.6 × 10⁹ in a typical mammalian cell. How do nucleosomes help in packaging this long molecule in a cell?

Let the distance between two consecutive base pairs be 0.34nm = 0.34×10^-9m

Given the total numbers of base pairs is 6.6 × 10⁹ in a typical mammalian cell.

Then the length of DNA double helix = total number of base pairs × the distance between two consecutive base pairs.

i.e., length of DNA double helix = 6.6 × 10⁹ × 0.34×10^-9m

or, length of DNA double helix = 2.24 m

Hence the length of DNA double helix in a typical mammalian cell is 2.24 m.

Histone proteins act like molecular spools that coil the strands of DNA into bead-like units called nucleosomes.

A nucleosome contains eight histones wrapped by DNA, and serves as the repeated primary unit for organizing the higher levels of chromatin structure.

There are two copies of four different types of histones in a nucleosome.

These nucleosomes coil and stack together to form fibers called chromatin. Chromatin, in turn, loops and folds with the help of additional proteins to form chromosomes. Condensing DNA into chromosomes prevents DNA tangling and damage during cell division.