Question

A ‘beads on a string’ like chromatin with 200 beads or nucleosomes, contains ____ bps of DNA.
A) 80,000
B) 40,000
C) 20,000
D) 10,000

Answer 1

Hint: A nucleosome core consists of approximately 146 bp of DNA. That adds up to a very high number of base pairs of DNA in the chromatin.

Complete Answer:
Let us start with knowing what are nucleosome-
1) Nucleosomes are fundamental organizational units of chromatin which appear as “beads-on-a-string” arrangements.
2) Each bead in the nucleosome has 200 base pairs of DNAs. The DNA is wound 1.65 times around histone octamer (having the following types of histone proteins - H2A, H2B, H3 and H4). The bead plus linker DNA (20-40 bp; average 18 bp) leads to the next bead and forms the nucleosomes. So, the beads on string structure with 200 beads contains $218 \times 200= approx. 40000 bp$ DNA on an average.

Additional information:We can learn about the structure of genes which are mostly regarded as the hereditary units of organism-
Genes are the functional unit of heredity that determines a biological trait in an organism and is present in the linear arrangement on chromosomes. Nucleic acids (DNA and RNA) are the polymers of nucleotides each of which is made up of sugar, the nitrogenous base and the phosphate moiety.
The hydrophilic backbones of alternating sugar and phosphate groups are present on the outside of the double helix facing the surrounding water.
The bead plus linker DNA leads to the next bead and forms the nucleosomes.

Thus, the correct answer is option B (40,000).

Note:The nitrogenous bases, purine and pyrimidine, are stacked inside the double helix. Each nucleotide base of one strand is paired in the same plane with a base of the other strand and appears as steps of the staircase.