antiparallel strands

Imagine two cars parked side by side, but facing in opposite directions — one pointing north, the other south. The two strands of a DNA double helix are arranged just like that. They lie alongside each other, but each runs in the opposite direction, a property called being antiparallel.

The direction comes from the sugar-phosphate backbone, which has two distinct ends labeled 5' and 3'. In a double helix, one strand runs 5'-to-3' from left to right while its partner runs 5'-to-3' from right to left. So at any point along the helix, you are next to the 5' direction of one strand and the 3' direction of the other.

This arrangement is not just a quirk of geometry; it is required for the bases to pair correctly and for the helix to hold its regular shape. The base-pairing chemistry only fits when the two strands are oriented head-to-tail rather than head-to-head.

The antiparallel layout has real consequences for how DNA is copied. The enzymes that build new DNA can only add bases in the 5'-to-3' direction, so the two strands cannot be copied in quite the same smooth way — one is made continuously and the other in short pieces that are later joined.