Question

what is meant by the description \antiparallel\ regarding the two strands of nucleic acids that make up dna?
one strand contains only purines and the other contains only pyrimidines.
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.

Explanation:

• Option 1: Both DNA strands have a mix of purines (adenine, guanine) and pyrimidines (cytosine, thymine). So, this option is incorrect.
• Option 2: The "antiparallel" description is not about the twisting (which is about the double - helix structure in general). The twisting is a separate aspect of the DNA's three - dimensional shape. So, this option is incorrect.
• Option 3: In DNA, one strand has a \(5'\) to \(3'\) orientation and the complementary strand has a \(3'\) to \(5'\) orientation (or vice - versa when looking at the two strands). This opposite \(5'\) to \(3'\) directionality is what "antiparallel" means.
• Option 4: Base pairings (A - T and G - C) create a relatively uniform spacing (about \(0.34\space nm\) between base pairs) between the two strands. So, this option is incorrect.

Answer:

The \(5'\) to \(3'\) direction of one strand runs counter to the \(5'\) to \(3'\) direction of the other strand.