Which term best describes the orientation of the two DNA strands?

Which term best describes the orientation of the two DNA strands?

• A. Parallel arrangement
• B. Antiparallel arrangement
• C. Complementary arrangement
• D. Symmetric arrangement

Answer: (B)

DNA has two strands that run in opposite directions, with one 5' to 3' and the other 3' to 5'. This opposite orientation is what antiparallel describes. It’s essential because DNA polymerases synthesize new DNA in the 5' to 3' direction while reading the template strand from 3' to 5', so the two strands must be oriented in opposite directions to be copied correctly. If the strands ran parallel, both would have the same direction, which isn’t how replication and the structure of the double helix work. The term complementary describes which bases pair with which (A with T, G with C), not the overall direction of the strands. Symmetric arrangement isn’t a standard descriptor for DNA strand orientation. So the orientation best described here is antiparallel.

DNA has two strands that run in opposite directions, with one 5' to 3' and the other 3' to 5'. This opposite orientation is what antiparallel describes. It’s essential because DNA polymerases synthesize new DNA in the 5' to 3' direction while reading the template strand from 3' to 5', so the two strands must be oriented in opposite directions to be copied correctly. If the strands ran parallel, both would have the same direction, which isn’t how replication and the structure of the double helix work. The term complementary describes which bases pair with which (A with T, G with C), not the overall direction of the strands. Symmetric arrangement isn’t a standard descriptor for DNA strand orientation. So the orientation best described here is antiparallel.