A molecule of dna has been described as a twisted ladder where the rungs of ladder are the

A molecule of DNA has been described as a twisted ladder where the rungs of the ladder are the chemical bonds, specifically hydrogen bonds, which occur between the two strands and involve a base from one strand and a base from the second in complementary pairing.

Individually, these hydrogen bonds are quite weak, but collectively they are quite strong. One unique thing about these bonds is that they are base specific, that is the base Adenine (A) will only form a hydrogen bond with the base Thymine (T) and Cytosine (C) will only form a hydrogen bond with Guanine (G). This means if you know the sequence of bases of one strand, you know the sequence of bases of the other strand.