The strongly acidic nature of DNA poses a problem. Why do Watson and Crick think that the DNA "dyad", formed by hybridization of two negatively charged molecules, is stable?

Negatively charged phosphate groups of two strands are aligned distantly from each other in a DNA double helix.

Explanation:

DNA double helix is formed when the complementary bases of two single DNA strands pair with each other. The formation of double-stranded structures places the negatively charged phosphate groups of two single DNA strands away from each other. This is because complementary base pairs are present between the sugar-phosphate backbones of two DNA strands of a double helix. The double-helical structure also concentrates the nitrogenous bases away from the surrounding watery medium. Altogether, these factors stabilize DNA dyad.