DNA’s double helix features two antiparallel strands running 5′ to 3′ in opposite directions, a key feature discovered by Watson and Crick. This MCQ tests the molecular basis of that polarity, fundamental for replication and transcription understanding.​

Correct Answer: 1. Phosphodiester bonds

Phosphodiester bonds link the 3′ hydroxyl of one deoxyribose sugar to the 5′ phosphate of the next nucleotide, creating the sugar-phosphate backbone with inherent 5′-3′ directionality. In the double helix, one strand’s 5’→3′ orientation opposes the other’s 3’→5′, enforcing antiparallelism. This polarity is crucial as DNA polymerase synthesizes only 5’→3′.

Why Not the Other Options?

• 2. Hydrogen bonds
  Hydrogen bonds (2 between A-T, 3 between G-C) hold complementary bases together across strands but don’t dictate directionality; they allow parallel or antiparallel alignment equally.

• 3. Disulphide bonds
  Disulfide bonds form between cysteine residues in proteins, irrelevant to nucleic acids or DNA structure; DNA lacks sulfur-containing amino acids.

• 4. Glycosidic bond
  N-glycosidic bonds attach nitrogenous bases to the 1′ carbon of deoxyribose but are identical on both strands, contributing nothing to antiparallel polarity.​

Structural Implications

Antiparallel strands enable semiconservative replication, with leading/lagging strand synthesis. This design stabilizes the helix via backbone interactions and base stacking. For students, visualize: phosphodiester bonds create polarity arrows pointing oppositely in the ladder-like model.​