Arrange the given steps of DNA packaging in correct sequence?
(A) Histones are organized to form a unit of eight molecules called histone octamer
(B) Nucleosomes form a structure of repeating unit called chromatin
(C) A set of positively charged, basic proteins are called histones
(D) A negatively charged DNA is wrapped around the histone octamer to form a structure known as nucleosome
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C), (A), (D), (B)
(C) Positively charged basic proteins called histones — first. Histones carry a positive charge because they are rich in the basic amino acids lysine and arginine.
(A) Histones form a histone octamer — second. Eight histone molecules come together as one unit.
(D) DNA is wrapped around the octamer to form a nucleosome — third. Negatively charged DNA winds round the positively charged octamer. A typical nucleosome holds about 200 bp of DNA.
(B) Nucleosomes form chromatin — fourth. Many nucleosomes repeat along the DNA and are seen as ‘beads-on-string’; this is chromatin.
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. 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, threadlike stained (coloured) bodies seen in nucleus.”
