What are the net electric charges on DNA and histone proteins?

The correct answer is Option (3) → Negative and positive

In eukaryotes the long DNA molecule is packed with the help of proteins called histones. DNA carries a negative charge because of its phosphate groups. Histones are rich in the basic amino acids lysine and arginine, whose side chains carry positive charges, so histones are positively charged. The opposite charges attract, and DNA wraps around a histone octamer to form a nucleosome.

Option (1) Both negative — Incorrect. Histones are basic proteins with positive charges. If both were negative they would repel each other, and DNA could not wrap around them.

Option (2) Both positive — Incorrect. DNA is negatively charged because of its phosphate groups, so it cannot be positive like the histones.

Option (3) Negative and positive — Correct. DNA is negative and histones are positive. This attraction lets the negatively charged DNA wind around the positively charged histone octamer to form nucleosomes.

Option (4) No charge — Incorrect. Both molecules carry a net charge. Their opposite charges are what hold DNA and histones together in chromatin.

NCERT Extract:
Packaging of DNA Helix – “… There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in the basic amino acid residues lysine and arginine. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome (Figure 5.4 a).”