Which statement about DNA strand orientation is correct?

• A. Parallel in the same direction
• B. Antiparallel, opposite directions
• C. Random orientation
• D. One strand runs downward only

Answer: (B)

DNA strands in a double helix are antiparallel, meaning they run in opposite directions. Each strand has a 5' end with a phosphate group and a 3' end with a hydroxyl group, so one strand runs 5' to 3' while the other runs 3' to 5'. This orientation is essential because DNA polymerases add new nucleotides only to the 3' end, so synthesis proceeds 5' to 3' on each strand, but the two strands must be oriented in opposite directions to allow proper base pairing (A with T, C with G) and coordinated replication. The idea of parallel orientation would disrupt how bases pair and how enzymes operate, and the notions of random orientation or one strand extending in only one direction don’t describe the actual, functionally required arrangement of the DNA molecule.