antiparallel strands

Meaning: In double-stranded DNA the two strands are complementary and oriented oppositely, so the 5' end of one strand sits beside the 3' end of the other. This geometry lets A–T (two hydrogen bonds) and G–C (three) pair along the helix. The same word is used for β-pleated sheets, where neighboring strands can run N→C in opposite directions (antiparallel) or in the same direction (parallel); both occur.

On the MCAT: You may be asked to write the complementary strand. For 5'-ATGC-3', the partner is 3'-TACG-5', which reads 5'-GCAT-3'. Antiparallel orientation also explains replication and transcription. Polymerases add nucleotides only to the 3' end of the growing chain, so synthesis always proceeds 5'→3' and the template is read 3'→5'. Because the two template strands run opposite ways at a replication fork, one new strand is made continuously (leading) and the other in Okazaki fragments (lagging). A new RNA is antiparallel to its template and has the same sequence as the coding strand, with U in place of T.

Common confusion: Students write the complement in the same left-to-right direction and forget to flip it. The complement of 5'-ATGC-3' is 3'-TACG-5', so to give the answer 5'→3' you must also reverse it. Antiparallel describes direction, not a mirror-image handedness; the DNA helix is right-handed in its common B form.