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:

• Option 1: The twisting nature of DNA (double - helix) is not the reason for the antiparallel nature. The double - helix is a three - dimensional structure, while antiparallel refers to the orientation of the strands.
• Option 2: In DNA, one strand runs from \(5'\) to \(3'\) and the other runs from \(3'\) to \(5'\) (or vice - versa in terms of the complementary description). This opposite \(5'\) to \(3'\) orientation is what defines the strands as antiparallel.
• Option 3: 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, unequal spacing due to base pairings is incorrect.
• Option 4: Both strands of DNA contain a mix of purines (\(A\) and \(G\)) and pyrimidines (\(T\) and \(C\)). Base pairing rules (\(A - T\) and \(G - C\)) ensure that there is a complementary mix on each strand.

Answer:

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