Question

The two strands in a DNA double helix are antiparallel. This means

• A. The phosphate groups at the start of two DNA strands are in opposite position (pole).
• B. One strand turns clockwise.
• C. One strand turns anti-clockwise.
• D. The phosphate groups of two DNA strands, at their ends, share the same position.

Answer 1

The correct option is A The phosphate groups at the start of two DNA strands are in opposite position (pole).

Antiparallel strands of a DNA molecule means that the phosphate groups at the start of two DNA strands are in opposite position (pole).

The DNA molecule is composed of two strands held together by hydrogen bonds. A single strand is different at its two ends. One end is called 5' (5 prime), the other is called 3' (3 prime). The names come from the notation for the two sugar carbon atoms which participate in the phosphodiester bonds. The different strands in the helix run in opposite (anti-parallel) directions. At each end of the double helix, one strand is 5' and the other is 3'.

The anti-parallel strands allow for the formation of hydrogen bonds. If the strands were not antiparallel these non-symetrical hydrogen bonding interactions would not be possible. It is important that CG interactions have three H- bonds, and AT has two H-bonds, to keep these pairings specific.