Hydrogen bonding in biological systems

We cannot leave the topic of hydrogen bonding without mentioning its important role in biological systems, one of the best known being the formation of the double helical structure of DNA (deoxyribonucleic acid). The structures of adenine and thymine are exactly matched to permit hydrogen bonding between them, and they are referred to as complementary bases; guanine and cytosine form the second base-pair (Figure 1.1). The hydrogen bonding  between these base-pairs in the strands of DNA leads to the assembly of the double helix.