Read the given passage and answer the questions that follow:
In prokaryotes such as E. coli, though they do not have a defined nucleus the DNA is not scattered throughout the cell. The DNA is held with some proteins in a specific region. In eukaryotes this organisation is much more complex. There is a set of positively charged basic proteins called histones. Histones are organised to form a unit of 8 molecules called histone octamer. The negatively charged DNA is wrapped around the positively charged histone octamer.
Histones are rich in which amino acid residues, whose side chains carry positive charges?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Lysine and arginine
The passage says histones are positively charged basic proteins, but it does not say where the charge comes from. A protein gets its charge from the side chains of its amino acid residues. Basic amino acids carry a positive charge on their side chains, and acidic amino acids a negative one. Histones are positive, so they bind the negatively charged DNA.
Option (1) Serine and threonine — Incorrect. Serine and threonine have side chains with a hydroxyl group, which carry no charge. A protein rich in them would not become positively charged.
Option (2) Lysine and arginine — Correct. Lysine and arginine are basic amino acids, and both carry positive charges in their side chains. Histones are rich in these two, which makes them positive. This lets the negatively charged DNA wrap around the histone octamer to form a nucleosome.
Option (3) Aspartic acid and glutamic acid — Incorrect. These are acidic amino acids, and their side chains carry a negative charge. A protein rich in them would be negative and would repel DNA, not hold it.
Option (4) Glycine and alanine — Incorrect. Glycine and alanine have small side chains (a hydrogen atom and a methyl group) that carry no charge. They cannot make a protein basic.
NCERT Extract:
Packaging of DNA Helix – “… In eukaryotes, this organisation is much more complex. 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.”