Which structure of the protein remain intact after denaturation?

The correct answer is Option (2) → Primary

Proteins are described at four levels:

• Primary structure: the sequence of amino acids, held by covalent peptide bonds.

• Secondary structure: the α-helix or β-pleated sheet, held by hydrogen bonds between the >C=O and –NH groups of the chain.

• Tertiary structure: the further folding of the chain into fibrous or globular shapes, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces.

• Quaternary structure: the arrangement of two or more polypeptide subunits with respect to each other.

Denaturation breaks only the weak forces (mainly hydrogen bonds) that hold the folded shapes; the covalent peptide bonds are not broken. So the secondary and tertiary structures are lost, while the primary structure remains intact.

Why the other options are incorrect

• Option (1) Quaternary — Incorrect. The subunits are held together by the same kind of weak forces, so this arrangement is also lost.

• Option (3) Secondary — Incorrect. The α-helix and β-pleated sheet are held by hydrogen bonds, which are the very bonds disturbed in denaturation.

• Option (4) Tertiary — Incorrect. The folding of the chain depends on weak forces and is lost when the globules unfold.

NCERT (2026-27 edition):
Denaturation of Proteins

Note: The same/similar question has been asked in different years by NTA as per the details given below: