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.

Relevant concepts:
- Deoxyribose sugar carbons are numbered 1' through 5'; the 1' carbon is attached to the nitrogenous base, the 3' carbon bears a hydroxyl (–OH), and the 5' carbon is where the phosphate group attaches.
- Nucleotides polymerize by forming phosphodiester bonds between sugars via phosphate groups, creating a sugar–phosphate backbone with directionality (5'→3').

Step-by-step reasoning and intermediate steps:
1. A free nucleotide (as a nucleoside triphosphate) has phosphates attached to its 5' carbon. The incoming nucleotide presents a 5'-triphosphate.
2. The 3' hydroxyl (3'-OH) of the growing chain performs a nucleophilic attack on the α-phosphate of the incoming nucleotide.
3. This reaction forms a phosphodiester linkage between the 3' oxygen of one deoxyribose and the 5' phosphate of the adjacent deoxyribose, releasing pyrophosphate (PPi).
4. The repeating linkages produce a backbone with alternating sugar and phosphate groups and an inherent 5' to 3' polarity.

Why the correct answer is correct:
- The phosphate physically connects the 3' carbon (through its oxygen) of one sugar to the 5' carbon (phosphate esterified to the 5' oxygen) of the next sugar—this is the canonical phosphodiester bond in DNA.

Why other options are incorrect:
- “a 3’ carbon in an adjacent deoxyribose”: a 3'–3' linkage is not how standard DNA is assembled and would not confer normal 5'→3' polarity.
- “a base in an adjacent nucleotide”: bases attach to the sugar at the 1' carbon via an N-glycosidic bond; they are not connected by phosphates.
- “none of the above”: incorrect because the phosphate does connect to a 5' carbon as described.

Conclusion: The correct answer is “a 5’ carbon in an adjacent deoxyribose,” which matches the provided answer.

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

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