DNA goes in which directions?

• A. Opposite directions
• B. Same directions
• C. Both directions at once
• D. Unknown

Answer: (A)

DNA strands in a double helix are antiparallel: one runs from its 5' end to its 3' end in one direction, while the other runs from 3' to 5' in the opposite direction. This arrangement happens because DNA polymerases add new nucleotides to the 3' end, so synthesis proceeds in the 5' to 3' direction. As a result, the two strands must face in opposite directions relative to each other. In replication, this creates a leading strand made continuously toward the fork and a lagging strand made in short fragments away from the fork, but the molecule itself remains oriented in opposite directions on the two strands.