Nucleic acid chemistry — The 5′ and 3′ designations of a DNA (or RNA) strand refer specifically to what?
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AThe total length of the DNA strand
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BThe numbering of carbon atoms in the pentose sugar (5′-C and 3′-C)
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CThe number of phosphates present in the backbone
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DThe Watson–Crick base-pairing rule
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EThe count of helical turns per 10 base pairs
Answer
Correct Answer: The numbering of carbon atoms in the pentose sugar (5′-C and 3′-C)
Explanation
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:
1) Identify the pentose sugar ring and its numbered carbons.2) Recognize phosphodiester bonds: 3′-OH to 5′-phosphate.3) Conclude that 5′ and 3′ denote specific sugar carbons, not strand length or base rules.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:
a,c,d,e) These do not define strand polarity; only sugar-carbon numbering does.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.