Final answer:
DNA chains in a double helix have opposite polarity meaning one strand runs 5' to 3' while the other runs 3' to 5', with the two strands being antiparallel and connected by complementary base pairing.
Step-by-step explanation:
When we say that the DNA chains in a double helix have opposite polarity, we are referring to the orientation of the two strands relative to each other. Each strand of DNA has a directionality because one end has a 5' phosphate group and the other end has a 3'-hydroxyl (3'-OH) group. This means that if one strand runs in a 5' to 3' direction (top strand), the other runs in a 3' to 5' direction (bottom strand), hence they are antiparallel.
This antiparallel nature is crucial for the function of DNA, particularly during DNA replication and protein synthesis. Watson and Crick proposed the antiparallel double helix model where the two DNA strands are held together by complementary base pairing, with adenine (A) always pairing with thymine (T), and cytosine (C) with guanine (G). These pairs are like the steps of a spiral staircase, where the sugar-phosphate backbone forms the outer edges and the nitrogenous bases are the steps.