The double helical structure of DNA is primarily determined by two factors:
1. Hydrogen bonds between complementary nitrogenous bases: Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C). These base pairs form the rungs of the DNA ladder.
2. The sugar-phosphate backbone: The alternating sugar (deoxyribose) and phosphate molecules form the sides, or the rails, of the DNA ladder.
These interactions between the nitrogenous bases and the sugar-phosphate backbone create the characteristic double helical structure of DNA, as first described by James Watson and Francis Crick in 1953.
