Question

Antiparallel strands of a DNA molecule means that

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

Answer 1

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

Both A- and B-DNAs are right-handed helix which means that it is turned clockwise; Z-DNA is left-handed which is twisted anti-clockwise, the direction of twisting does not impart antiparallel nature to it. This makes options A and B incorrect. The same orientation of all phosphodiester linkages along the chain imparts each linear nucleic acid strand a specific polarity and distinct 5’ and 3’ ends. The 5’ end lacks a nucleotide at the 5’ position thereby having a free phosphate group and the 3’ end lacks a nucleotide at the 3’ position thereby having a free hydroxyl group. The two DNA strands are antiparallel means that 5’ end of one strand faces 3’ end of other strand and vice versa. This means that 5’ phosphate of one strand faces 3’ hydroxyl group of other strand and that the 5’ phosphate groups of two strands are present in opposite position; option C is incorrect and D is correct.

So, the correct answer is 'The phosphate groups at the start of two DNA strands are in the opposite position (pole)'.