The opposite directions of the two DNA strands in the Watson–Crick model are described as antiparallel. One strand runs from its 5′ end to its 3′ end, while the complementary strand runs from 3′ to 5′.
This arrangement follows from the chemistry of the sugar-phosphate backbone. Nucleotides are linked through the 3′ carbon of one deoxyribose sugar and the 5′ carbon of the next, giving every strand a directional polarity. The two strands can pair correctly only when their backbones face opposite ways.
Antiparallel orientation was a crucial part of the 1953 double-helix model developed by James Watson and Francis Crick. It places complementary bases inside the helix, where adenine can pair with thymine and guanine with cytosine through precise hydrogen-bonding patterns.
A common mix-up is to call the strands “parallel” because they lie alongside each other. They are parallel geometrically, but chemically they point in opposite directions. This polarity also matters during DNA replication, when enzymes copy a template strand and synthesize new DNA only in the 5′-to-3′ direction.