In a DNA strand, a phosphate connects a 3’ carbon atom in one deoxyribose to

Question

BlackTom AI · Accepted Answer

A phosphodiester bond links the 3' carbon of one deoxyribose to the 5' carbon of the next deoxyribose in the DNA backbone.

1) Relevant knowledge points and concepts
- Deoxyribose sugar in DNA is a five‑carbon ring labeled 1' through 5'. The base is attached to the 1' carbon. The 3' carbon bears a hydroxyl (–OH) and the 5' carbon has a phosphate group attached in nucleotides. 
- Adjacent nucleotides are joined by phosphodiester bonds: a phosphate forms two ester linkages, one to the 3' hydroxyl of one sugar and one to the 5' carbon of the next sugar. This creates the sugar‑phosphate backbone with intrinsic directionality (5' → 3').

2) Step‑by‑step reasoning
- Identify functional groups: the 3' carbon has an –OH capable of forming an ester; the 5' carbon is linked to the phosphate group of the incoming nucleotide.
- When nucleotides polymerize, the phosphate group connects the 5' carbon of the new nucleotide to the 3' hydroxyl of the growing chain, producing a phosphodiester bond between 3' and 5' carbons of adjacent sugars.
- This repeated linkage yields the continuous backbone and defines the 5' and 3' ends of a strand.

3) Intermediate structural points (no numerical calculations required)
- Phosphate forms two ester linkages: O–P–O connecting two deoxyribose residues (3'–O–P–O–5').
- Polymerases add nucleotides to the 3'‑OH, reinforcing the 5'→3' growth orientation.

4) Why the correct answer is correct
- The chemical connectivity and enzymatic mechanism require the phosphate to bridge 3' of one sugar to 5' of the adjacent sugar; this is the canonical phosphodiester linkage that defines DNA structure.

5) Why the other options are incorrect
- "a 3’ carbon in an adjacent deoxyribose.": A 3'–3' linkage is not the standard backbone connection in DNA and would not produce the normal 5'→3' polarity. 
- "a base in an adjacent nucleotide.": Bases attach to the 1' carbon via glycosidic bonds; bases are not linked into the backbone by phosphate groups. 
- "none of the above.": Incorrect because a standard, well‑known 3'–5' phosphodiester linkage does exist.

6) Conclusion
The correct answer is a 5’ carbon in an adjacent deoxyribose. This matches the provided answer: "a 5’ carbon in an adjacent deoxyribose."

In a DNA strand, a phosphate connects a 3’ carbon atom in one deoxyribose to单项选择题

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