antiparallel strands
/ AN-tee-PAIR-uh-lel STRANDZ /
Imagine a two-lane road where the lanes run side by side but the traffic flows in opposite directions — one lane heading north while the one right beside it heads south. The two strands of DNA are arranged just like that. They lie next to each other and pair up, but they point in opposite directions. That opposite orientation is what 'antiparallel' means.
Each DNA strand has a built-in direction, because its sugar-phosphate backbone is not symmetric: one end is called the 5' (five-prime) end and the other the 3' (three-prime) end, named after carbon positions in the sugar. In a double helix, one strand runs 5'-to-3' going left-to-right, while its partner runs 5'-to-3' going right-to-left. So at any point, one strand's 5' end sits across from the other strand's 3' end. They are not parallel like two arrows pointing the same way; they are anti-parallel.
This matters enormously because the cell's machinery only works in one direction: enzymes that build new DNA or RNA can only add letters to the 3' end, reading the template from 3' toward 5'. Antiparallel geometry is why DNA copying is lopsided — one new strand is made smoothly and continuously, while the other has to be stitched together in short backward pieces. Far from a quirky detail, directionality shapes how DNA is replicated, read, and repaired.
Write a short DNA strand as 5'-ATGC-3'. Its antiparallel partner reads 3'-TACG-5' — meaning, listed in the standard 5'-to-3' direction, it is 5'-GCAT-3'.
Pairing happens with the strands head-to-tail, not head-to-head.
A common slip is to assume the two strands run the same way; remember they are mirror-flipped, which is exactly why one is called the 'leading' strand and the other the 'lagging' strand during replication.