DNA's Antiparallel Double Helix

Steps:

• Recall DNA structure: two polynucleotide strands form a double helix, linked by hydrogen bonds between complementary bases.
• Note strands are antiparallel, running in opposite 5' to 3' directions for proper base pairing and helix formation.
• Evaluate options: A confuses intra-strand bonds; B matches antiparallel requirement; C misstates hydrogen bonds; D ignores base pairing rules.
• Confirm B as correct via standard Watson-Crick model.

Why B is correct:

• DNA double helix requires antiparallel strands for base alignment and stability, as defined in the Watson-Crick model.

Why the others are wrong:

• A: Phosphodiester bonds link nucleotides within each strand; hydrogen bonds join strands.
• C: Purine-pyrimidine pairs form 2 H-bonds (A-T) or 3 (G-C), not 3 for all purine-involved pairs.
• D: Chargaff's rules state cytosine equals guanine, not thymine.

Final answer: B