"Mac Dónaill argues that the nucleotides' pairings are a kind of code. Each hydrogen bond has two components: chemical groups called donors and acceptors. If we denote a donor as 1 and an acceptor as 0, then C encodes the pattern 100, and G is 011.
In other words, each nucleotide can be represented as a short sequence of binary code, like the 1's and 0's used to record information in computers.
There is one more element in this code. A and G belong to a class of molecule called purines, and T and C are pyrimidines. Each pairing involves a purine and a pyrimidine. We can denote a purine by 0 and a pyrimidine by 1. Then C becomes 100,1 and G is 011,0.
Represented in this way, says Mac Dónaill, the permissible combinations of A,C,T and G correspond to what computer scientists call a parity code. Each nucleotide has an even number of 1's - it is said to have an even parity.
This makes it easier to spot errors such as non-natural nucleotides. If the error changes any one digit in a nucleotide, its parity changes from even to odd. Odd-parity nucleotides are clearly wrong. "