During denaturation of proteins

The correct answer is Option (2) → Secondary and tertiary structures are destroyed.

Heat or a change of pH disturbs the hydrogen bonds that hold the folds and helices of a globular protein, so the globules unfold and the helix uncoils, and the protein loses its biological activity.

The peptide bonds are not broken, so the sequence of amino acids is unchanged.

Why the other options are incorrect

• Option (1) — Incorrect. These two structures are the ones that are lost.

• Option (3) — Incorrect. The primary structure remains intact.

• Option (4) — Incorrect. The tertiary structure is destroyed along with the secondary structure.

NCERT (2026-27 edition):
Denaturation of Proteins – “Protein found in a biological system with a unique three-dimensional structure and biological activity is called a native protein. When a protein in its native form, is subjected to physical change like change in temperature or chemical change like change in pH, the hydrogen bonds are disturbed. Due to this, globules unfold and helix get uncoiled and protein loses its biological activity. This is called denaturation of protein. During denaturation secondary and tertiary structures are destroyed but primary structure remains intact. The coagulation of egg white on boiling is a common example of denaturation.”