Spontaneous mutagenesis describes how the genetic code of DNA can incorporate errors without the influence of external factors. Generally, these errors disrupt the canonical (or Watson-Crick) base pairings, adenine-thymine (A-T) and guanine-cytosine (G-C). The root idea that the tautomerisation of DNA may be a mechanism promoting genetic mutation dates back to a short, tentative hypothesis from Watson and Crick's second paper of 1953, where the process of mitosis (the self-replication of DNA) was first theorised. Since then, many studies have investigated the validity of Watson and Crick's claim. The process of tautomerisation proceeds as follows. Each hydrogen bond between the canonical base pairs of DNA depends on the bonding strength of a hydrogen atom to the more electronegative atom on the opposite base (either nitrogen or oxygen). This hydrogen atom is preferentially associated with its donor base, but, arising from either classical or quantum phenomena, the proton (of the hydrogen atom) may travel along a minimum energy pathway to associate with the acceptor of the opposite base, leaving its electron with the base it was initially covalently bonded to. In this case, each base becomes charged, and the product is a zwitterionic base pair. This mechanism can be described as: A-T ↔ A + -T -and G-C ↔ G --C + .