DNA strands are antiparallel. What does antiparallel mean?

• A. One strand is RNA and one is DNA
• B. Strands run in the same direction
• C. Strands run in opposite directions
• D. Both strands are parallel

Answer: (C)

Antiparallel describes the orientation of the two DNA strands relative to each other. Each strand has a direction, from the 5' end to the 3' end, based on the sugar's phosphate group arrangement. In the double helix, one strand runs 5' to 3' in one direction while the complementary strand runs 3' to 5' in the opposite direction. This opposite alignment not only supports stable base pairing (A with T, C with G) but also fits how DNA polymerase adds nucleotides only to the 3' end, enabling continuous synthesis on the leading strand and fragment synthesis on the lagging strand. So, antiparallel means the two strands are oriented in opposite directions, not the same direction or parallel.