What does anti-parallel mean in the structure of DNA?

What does anti-parallel mean in the structure of DNA?

• A. Both strands run in the same direction
• B. Strands rotate to form a helix
• C. Strands are identical sequences
• D. Strands run in opposite directions with 5' and 3' ends

Answer: (D)

Antiparallel arrangement means the two DNA strands run in opposite directions. Each strand has a distinct directionality: one runs 5' to 3', while the other runs 3' to 5'. This opposite orientation, tied to the sugar-phosphate backbone ends (5' end vs 3' end), is essential for proper base pairing and for enzymes to synthesize DNA in the 5' to 3' direction. It also explains why bases pair complementarily (A with T, C with G) and how the double helix is stabilized. The other ideas miss the directional relationship: running in the same direction would be parallel, the helix is about shape rather than direction, and strands would not be identical in sequence if they are complementary.

Antiparallel arrangement means the two DNA strands run in opposite directions. Each strand has a distinct directionality: one runs 5' to 3', while the other runs 3' to 5'. This opposite orientation, tied to the sugar-phosphate backbone ends (5' end vs 3' end), is essential for proper base pairing and for enzymes to synthesize DNA in the 5' to 3' direction. It also explains why bases pair complementarily (A with T, C with G) and how the double helix is stabilized. The other ideas miss the directional relationship: running in the same direction would be parallel, the helix is about shape rather than direction, and strands would not be identical in sequence if they are complementary.