DNA strands are oriented in opposite directions, a property known as what?

• A. Antiparallel
• B. Parallel
• C. Co-linear
• D. Circular

Answer: (A)

DNA strands are oriented in opposite directions, which is called antiparallel arrangement. In the double helix, one strand runs 5' to 3' while the other runs 3' to 5'. This opposite orientation is crucial because base pairing (A with T, G with C) occurs between complementary nucleotides on these antiparallel strands, and DNA polymerases synthesize new DNA in the 5' to 3' direction, using the opposite strand as a template. The antiparallel setup makes replication efficient, with the leading strand synthesized continuously and the lagging strand formed as Okazaki fragments.

If the strands were parallel, or co-linear, or circular, they would not reflect the actual orientation and geometry of the DNA double helix or the way enzymes interact with the template during replication. Parallel would imply both strands go the same way, which would disrupt base pairing along a helical structure; co-linear suggests a straight-line arrangement rather than the real twisted, paired configuration; circular describes a closed loop shape, which is a different structural characteristic and not about directionality of the two strands.