Let’s take another look at those bases. They come in two flavors: those that have just one ring and those that have two. The one-ringed bases we call pyrimidines, and the double-ringed bases we call purines. If you line two strands of DNA up against each other, you can maintain the same distance between the backbones if you alternate the bases such that each purine is opposite a less bulky pyrimidine.
 
But the process of base pairing is more specific than just pyrimidine-purine. In Part I, your Old Quantum Butcher introduced you to the concept of the hydrogen bond, in which hydrogens bound to oxygen lose part of their electron cloud and take on a positive charge that forms the basis for a weak electrostatic bond between atoms. It just so happens that the weak bases in DNA and RNA form hydrogen bonds with each other in a very particular way, known as Watson-Crick Base Pairing.

 

Figure 8. Watson Crick Base Pairing. Hydrogen bonds are shown in yellow. Atoms of the sugar-phosphate backbone are to the left and right of each picture. Can you pick out the single- and double-ringed bases?