In the DNA double helix model proposed by Watson and Crick, the two strands of DNA run:

Correct answer: (b) Antiparallel

Explanation

In the Watson–Crick double helix model (1953), the two strands of DNA run in opposite directions. They are antiparallel: one runs 5′ → 3′ and the other 3′ → 5′.

The strands are held together by hydrogen bonds between the bases (A with T by two bonds, G with C by three), and the sugar–phosphate backbones are on the outside. The antiparallel arrangement lets the bases pair correctly, and it is why DNA polymerase, which works only 5′ → 3′, makes one strand continuously and the other in pieces (Okazaki fragments).