In DNA, which covalent linkage joins adjacent nucleotides to produce the repeating sugar–phosphate backbone of each strand?

Introduction / Context:The structure of DNA depends on two kinds of bonds: covalent bonds within each strand and noncovalent hydrogen bonds between complementary bases. This question asks which covalent bond links nucleotides together along a single DNA strand.

Given Data / Assumptions:

• Standard deoxyribonucleotides with deoxyribose sugars.
• Backbone alternates sugar and phosphate groups.
• Bases project inward and pair by hydrogen bonds, not covalent bonds.

Concept / Approach:Each nucleotide contributes a 3' hydroxyl on its sugar and a phosphate attached to the 5' carbon of the incoming nucleotide. A dehydration reaction forms a phosphodiester linkage connecting the 3' hydroxyl and the 5' phosphate, yielding a 3'–5' phosphodiester bond and a directional (5'→3') polymer.

Step-by-Step Solution:

Verification / Alternative check:Enzymatic synthesis by DNA polymerases proceeds strictly 5'→3', consistent with formation of 3'–5' phosphodiester linkages and requirement for a free 3' OH primer terminus.

Why Other Options Are Wrong:

• 2', 5' or 2', 3': The 2' carbon lacks the required hydroxyl in deoxyribose and these linkages are not used for DNA backbones.
• 3', 4': No standard biochemical mechanism makes a 3'–4' backbone in nucleic acids.
• Glycosidic bonds between adjacent bases: Glycosidic bonds join a base to its own sugar, not to neighboring nucleotides.

Common Pitfalls:Confusing glycosidic base–sugar bonds with phosphodiester bonds, or forgetting polarity (5' phosphate to 3' OH).

Final Answer:3', 5' phosphodiester bonds to form a repetitive sugar–phosphate chain.