A few days ago Kea wrote post #66 in her long and fascinating series of M-theory posts. Seems like she’s giving away the text to a book here. She wrote:
I was quite intrigued when a mathematical biologist at a conference told me recently that no one really knew why DNA had four bases rather than two. Apparently it isn’t clear why self-replicating molecules fail to adopt a binary code in X and Y.
The context of the problem needs some explaining. DNA is a long chain molecule that is built from a series of nucleotides. The strange things is that exactly four nucleotides are used. This is strange because there are at least a dozen different nucleotides, why use just four?
[edit Feb 8, 2009]This question is discussed in the scientific literature. For example, I just now found the article: Why Are There Four Letters in the Genetic Alphabet?, which describes the question generally, but I think the reader will find the chemical and information content argument given here to be more precise.[/edit]