What is the form of native DNA where two adjacent polynucleotide strands wind into a spiral?

• A. Double Helix
• B. Single Strand
• C. Triple Helix
• D. Quadruple Helix

Answer: (A)

Two polynucleotide strands wound into a spiral describes the double helix. In native DNA, the strands run antiparallel, with sugar-phosphate backbones on the outside and the bases paired inside. A pairs with T through two hydrogen bonds and G pairs with C through three, giving stable, specific base pairing that holds the two strands together. The helix is right-handed and typically has about 10 base pairs per turn with a diameter of roughly 2 nanometers. This arrangement explains how genetic information is stored and how replication can proceed by using one strand as a template for the other. A single strand lacks a partner to form the helical structure, and triple or quadruple helices are not the usual native forms in cells, though they can occur under special conditions or in certain constructs.