Q.The net electric charge on DNA and histones is:

The net electric charge on DNA and histones is determined by their chemical composition. DNA is a nucleic acid with a sugar-phosphate backbone. Each phosphate group carries a negative charge at physiological pH, giving the entire DNA molecule a net negative charge.

Histones are basic proteins rich in positively charged amino acids like lysine and arginine. These amino acids have side chains that carry a positive charge at physiological pH, giving histones a net positive charge. …

The question touches on a fundamental property of DNA and its packaging proteins — one that explains how these two molecules interact so intimately inside the nucleus. To understand the charge, you need to look at the chemical structure of each.

DNA is a long polymer made of nucleotides. Each nucleotide has three components: a sugar (deoxyribose), a nitrogenous base, and a phosphate group. It is this phosphate group that gives DNA its character. At the pH found inside a cell, the phosphate groups are ionised, carrying a negative charge. Since every nucleotide in the DNA strand contributes one phosphate, the entire DNA molecule is strongly negatively charged. Think of it as a long, negatively charged thread.

Histones, on the other hand, are proteins. What makes them special is their amino acid composition. Histones are rich in basic amino acids — particularly lysine and arginine. These amino acids have side chains that carry a positive charge at cellular pH. Because histones contain many such residues, the overall charge on a histone protein is positive.

Now, why does this matter? In the nucleus, DNA is not floating around as a naked thread. It is tightly wrapped around histone proteins to form a compact structure called chromatin. The attraction is electrostatic: the negative DNA is drawn to the positive histones, much like opposite poles of a magnet. This natural pull allows DNA to coil around histones, forming nucleosomes — the fundamental unit of chromatin packaging. …

Work backwards from the known outcome instead of listing charges first: DNA must wrap tightly around histones to fit inside the nucleus (chromatin packaging is the known result), and a tight, stable wrap between two molecules requires an attractive force between them. Since like charges repel, the only way DNA and histones could attract each other this strongly is if their charges are o …