Consider the following statements about the structure of proteins.
(A) The secondary structure of a protein arises from the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between >C=O and –NH– groups of the peptide bond.
(B) In the α-helix, the polypeptide chain twists into a left handed screw.
(C) In the β-pleated sheet structure, the stretched peptide chains laid side by side are held together by intramolecular hydrogen bonds.
(D) The spatial arrangement of the sub-units of a protein with respect to each other is known as its quaternary structure.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (C) only
The secondary structure of a protein is the shape a long polypeptide chain takes; it exists as the α-helix or the β-pleated sheet. Both shapes are held by hydrogen bonds between the >C=O and –NH– groups of peptide bonds.
(A) The secondary structure of a protein arises from the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between >C=O and –NH– groups of the peptide bond. — Correct statement. This is how both the α-helix and the β-pleated sheet arise.
(B) In the α-helix, the polypeptide chain twists into a left handed screw. — Incorrect statement. The chain twists into a right handed screw, with the –NH group of each residue hydrogen bonded to the >C=O of an adjacent turn.
(C) In the β-pleated sheet structure, the stretched peptide chains laid side by side are held together by intramolecular hydrogen bonds. — Incorrect statement. The hydrogen bonds join different chains lying side by side, so they are intermolecular.
In the α-helix, by contrast, the bonds are within one chain.
(D) The spatial arrangement of the sub-units of a protein with respect to each other is known as its quaternary structure. — Correct statement. Only proteins made of two or more polypeptide chains (sub-units) have a quaternary structure.
NCERT (2026-27 edition):
10.2.3 Structure of Proteins – “…These structures arise due to the regular folding of the backbone of the polypeptide chain due to hydrogen bonding between >C=O and –NH– groups of the peptide bond.”
“α-Helix is one of the most common ways in which a polypeptide chain forms all possible hydrogen bonds by twisting into a right handed screw (helix) with the –NH group of each amino acid residue hydrogen bonded to the >C=O of an adjacent turn of the helix…”
“In β-pleated sheet structure all peptide chains are stretched out to nearly maximum extension and then laid side by side which are held together by intermolecular hydrogen bonds.”
“(iv) Quaternary structure of proteins: Some of the proteins are composed of two or more polypeptide chains referred to as sub-units. The spatial arrangement of these subunits with respect to each other is known as quaternary structure.”