Well, if we actually examine the bases within our double helix structure, some of the bases can act as acids. So let's take a look at Guanine for example. This hydrogen attached to this nitrogen can actually act as an acid. And what that means is at a certain particular PH it will begin to dissociate and give off that age. And when it dissociates it forms the depronative version of Guanine in which this age has been removed. And the two electrons in that covalent bond have now gone on to that nitrogen.