The complementarity of DNA strand is based on

Question

BlackTom AI · Accepted Answer

Base pairing between purines and pyrimidines — A with T, and G with C — is the molecular basis of DNA strand complementarity.

Relevant concepts: Watson–Crick base pairing, hydrogen bonding patterns, purine–pyrimidine geometry, and Chargaff’s rules. Adenine (a purine) pairs specifically with thymine (a pyrimidine) via two hydrogen bonds; guanine (a purine) pairs with cytosine (a pyrimidine) via three hydrogen bonds. The complementary shapes and hydrogen-bond donor/acceptor positions make only those pairs geometrically and energetically favorable within the double helix.

Step-by-step reasoning:
1. Nucleotide structure: each base has specific sites that can donate or accept hydrogen bonds. When two bases face each other with the correct orientation, their donor/acceptor groups align to form stable hydrogen bonds.
2. Purine–pyrimidine requirement: a purine paired to a pyrimidine preserves uniform helix width; purine–purine would be too wide, pyrimidine–pyrimidine too narrow.
3. Specificity: the pattern of hydrogen bond donors and acceptors for A and T matches (2 H-bonds), and for G and C matches (3 H-bonds). This chemical complementarity enforces sequence-specific pairing.
4. Chargaff’s empirical observation (A≈T and G≈C in double-stranded DNA) supports that complementarity follows the AT/GC rule.

Why the correct answer is correct: The AT/GC rule directly describes the specific base-pairing (A–T and G–C) that underlies strand complementarity; hydrogen bonding and geometry are the chemical basis for this rule.

Why other options are incorrect:
- a. Phosphodiester linkages form the sugar–phosphate backbone (connectivity), not the base-specific complementarity between strands.
- b. Protein binding can stabilize or recognize sequences but does not determine the fundamental base-pairing rules.
- d. “None of the above” is incorrect because option c states the correct basis.

Conclusion: The complementarity of DNA strands is based on the AT/GC rule (answer c).

The complementarity of DNA strand is based onSingle choice

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