Question

a diploid human cell contains approximately 6.4 billion base pairs of dna. how many histone proteins are complexed to this dna? I know the answer is 2.9 x 10^8 but do not know how to start it.

Answer 1

In eukaryotes, DNA is packaged into nucleosome units which consist of DNA wound over a core of octameric histone proteins. Two nucleosomes are linked together by means of a segment of DNA known as 'linker DNA'. This gives the nuclear DNA it's 'string on beads' appearance.

The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA wrapped in 1.67 left-handed superhelical turnsaround a histone octamer consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. Core particles are connected by stretches of "linker DNA", which can be up to about 80 bp long. Thus, each nucleosome unit consists of approximately 225 bp DNA associated with 8 histone proteins. Hence, if 225 bp DNA is assoxiated with 1 histone octamer, then 6.4 billion bp DNA will be associated with following number of histones:

225 bp DNA = 1 histone octamer

Therefore, 6.4 x 10¹⁰bp DNA = (1/225) x 6.4 x 10¹⁰ = 2.9 x 10⁸ histone octamers.

Hence, 6.4 billion base pairs of DNA will have 2.9 x 10⁸ histone octamers complexed with it.