Nucleic acid chemistry — The 5′ and 3′ designations of a DNA (or RNA) strand refer specifically to what?

Introduction:
Nucleic acids are directional polymers. The terms 5′ and 3′ indicate chemical polarity, which is essential for replication, transcription, and many enzymatic processes. This question asks what these numbers refer to at the molecular level.

Given Data / Assumptions:

• DNA/RNA sugars are pentoses with labeled carbons 1′–5′.
• Backbone linkages connect the 3′-OH of one sugar to the 5′-phosphate of the next.
• Polymerases synthesize in the 5′→3′ direction.

Concept / Approach:
Associate strand polarity with sugar-carbon numbering: 5′ corresponds to the phosphate-bearing carbon; 3′ corresponds to the hydroxyl that accepts the next nucleotide. Orientation underlies antiparallel duplex formation and enzyme directionality.

Step-by-Step Solution:

Verification / Alternative check:
Antiparallel duplexes are described as 5′→3′ and 3′→5′, consistent with sugar-carbon references, not base counts.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing 5′/3′ with base-pairing or with the number of phosphates in a nucleotide triphosphate.

Final Answer:
They refer to the numbered carbons (5′ and 3′) of the pentose sugar.