Question

"The two strands of a DNA double helix are antiparallel". It means-

• A. The two strands are not parallel.
• B. The two strands are parallel but their orientations are opposite in direction.
• C. The two strands are parallel in an alternative form of DNA.
• D. The two strands are opposite to parallel in Z form.

Hint:

Always associate the term "antiparallel" with opposite chemical directions of parallel backbones, namely one strand running 5'-to-3' and the other running 3'-to-5'.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The DNA double helix consists of two polynucleotide chains wound around a central axis to form a helical structure.
The term "antiparallel" describes the spatial alignment and relative directional polarity of these two complementary strands.

Step 2: Detailed Explanation:
Each strand of DNA has a distinct chemical polarity determined by the orientation of the sugar-phosphate backbone.
One end of a strand terminates with a free 5'-phosphate group, while the opposite end terminates with a free 3'-hydroxyl group.
In a double-stranded DNA molecule, the two strands run parallel to each other in space, maintaining a constant distance between their backbones.
However, their chemical directions are oriented in opposite ways.
One strand runs in the 5'-to-3' direction, while its complementary partner strand runs in the 3'-to-5' direction.
This parallel yet head-to-tail orientation is biologically essential, as it allows the complementary nitrogenous bases to form stable hydrogen bonds with correct geometric alignment.

Step 3: Final Answer:
Therefore, "antiparallel" means the two strands are parallel but their orientations are opposite in direction, which corresponds to option (B).