Which statement best describes the orientation of DNA strands in the double helix?

• A. They run in opposite directions and are designated by 5' and 3' ends
• B. They run parallel in the same direction
• C. They run in opposite directions and share the same ends
• D. They have identical sequences

Answer: (A)

DNA strands in the double helix are antiparallel: one runs from 5' to 3' while the other runs from 3' to 5'. This orientation comes from the way nucleotides are linked by phosphodiester bonds, connecting the 5' phosphate of one sugar to the 3' carbon of the next. With opposite directions, the bases on opposite strands can pair: A with T and G with C, forming the hydrogen bonds that hold the helix together. The antiparallel arrangement also aligns with how DNA polymerases add nucleotides only to the growing strand’s 3' end, which reinforces why the strands run in opposite directions. Running parallel in the same direction wouldn’t allow the bases to pair properly, and saying they share the same ends or have identical sequences doesn’t describe the spatial orientation of the strands.