Choose the incorrect pair.

The correct answer is Option (3) → Thread-like, colourless unit of structure — Chromatin in nucleus

In eukaryotes, DNA is packed with the help of histones, basic proteins rich in lysine and arginine. Eight histone molecules form a histone octamer, and DNA wraps around it to make a nucleosome. Nucleosomes repeat along the DNA to form chromatin. The book describes chromatin as thread-like stained (coloured) bodies seen in the nucleus. Calling it colourless makes the pair wrong.

Option (1) Basic amino acid residues in histones — Lysine and arginine — A correct pair. Lysine and arginine are the basic amino acid residues in histones. Their side chains carry positive charges.

Option (2) Negatively charged DNA wrapped around positively charged histone octamer — Nucleosome — A correct pair. DNA is negative and the histone octamer is positive, so the DNA wraps around the octamer. This unit is the nucleosome, with about 200 bp of DNA.

Option (3) Thread-like, colourless unit of structure — Chromatin in nucleus — The incorrect pair. Chromatin is thread-like, but it takes up stain and is seen as coloured bodies in the nucleus, not colourless ones.

Option (4) Unit of 8 molecules in histones — Histone octamer — A correct pair. Histones are organised into a unit of eight molecules, the histone octamer.

NCERT Extract:
Packaging of DNA Helix – “… 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). A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread-like stained (coloured) bodies seen in nucleus.”

Figure 5.4a Nucleosome