Question

what is meant by the description \antiparallel\ regarding the two strands of nucleic acids that make up dna?
the twisting nature of dna creates nonparallel strands.
the 5 to 3 direction of one strand runs counter to the 5 to 3 direction of the other strand.
base pairings create unequal spacing between the two dna strands.
one strand contains only purines and the other contains only pyrimidines.

Explanation:

To determine the correct answer, we analyze each option:

• The first option is incorrect because the twisting (double helix) doesn't make the strands non - parallel in the sense of direction; the strands are parallel in terms of their overall structure but antiparallel in direction.
• The second option is correct. DNA strands have a 5' to 3' direction. In the double - helix, one strand runs in the 5'→3' direction and the other runs in the 3'→5' direction (counter to the 5'→3' direction of the first), which is the definition of antiparallel.
• The third option is incorrect. Base - pairing (A - T, G - C) actually creates equal spacing between the two DNA strands, forming a uniform double helix structure.
• The fourth option is incorrect. Both DNA strands contain a mix of purines (adenine, guanine) and pyrimidines (thymine, cytosine), not one strand with only purines and the other with only pyrimidines.

Answer:

B. The 5' to 3' direction of one strand runs counter to the 5' to 3' direction of the other strand.