The two strands of a DNA duplex each possess an inherent chemical directionality, because phosphodiester linkages join the 5′ carbon of one deoxyribose to the 3′ carbon of the next, so that every backbone runs unambiguously from a 5′ end to a 3′ end. In the duplex the two strands are complementary but oppositely oriented — one running 5′→3′ where the other runs 3′→5′ — a relationship termed antiparallel and visible structurally as the inverted orientation of the sugar rings on the two backbones. This belongs to molecular biology and structural biochemistry; the duplex is held together by hydrogen bonds between complementary nitrogenous bases while the negatively charged phosphate groups repel one another and drive the backbones apart, and the antiparallel geometry it produces is the constraint from which the directional behavior of replication and transcription follows.
The two strands of a DNA duplex each possess an inherent chemical directionality, because phosphodiester linkages join the 5′ carbon of one deoxyribose to the 3′ carbon of the next, so that every bac…