The parts 'P' and 'Q' of the molecule are held by.

The correct answer is Option (4) → Disulphide bonds

P and Q are the two heavy chains of the antibody. The figure shows them joined by small S–S links: disulphide bonds between cysteine residues. The same bonds join each light chain to a heavy chain.

Option (1) Covalent bonds — Incorrect. A disulphide bond is a kind of covalent bond, but “covalent” also covers the peptide bonds inside each chain; the bond that links the chains is named specifically.

Option (2) Ionic bonds — Incorrect. The chains are not held by attraction between charges.

Option (3) Phosphodiester bonds — Incorrect. These link nucleotides in DNA and RNA, not proteins.

Option (4) Disulphide bonds — Correct. The parts P and Q of the antibody are its chains, the light and heavy chains. The four chains of an antibody are held together by disulphide (S–S) bonds.