Fox and Hedgehog Seminar Series - 'A cosmologist looks at the genetic code'

At early times, the universe was an almost perfectly uniform plasma of elementary particles in almost perfect thermal equilibrium. Then, at a later point, it "came alive" (at least in one region, on Earth) – it began evolving and learning about itself, first unconsciously and later deliberately. How did that transition – from non-living to living matter – happen? We normally think of life as the domain of the biologists, but this sounds more like a problem for physicists and mathematicians. After discussing this problem from various angles, I will focus on a particular puzzle: the origin of the genetic code.

The genetic code is the rule by which genes (written in the 4-letter alphabet of DNA bases) are translated into proteins (written in the 20-letter alphabet of amino acids). It is succinctly summarized by the table which maps each of the 64 triplets of DNA bases to one of the 20 amino acids. This same table, with minor modifications, is shared be all species of life on Earth, and dates back to the very beginning of life, predating the last common ancestor of all known species. Despite the remarkable simplicity of this code, which has been known since the 1960s, the most basic questions about it remain unanswered: e.g. why 4 DNA bases? why triplets? why 20 amino acids? how were the triplet assignments determined, and in which order? where do the obvious patterns in the table come from?

I will explain some ideas about how to make progress on this puzzle. There are reasons to think the current triplet code was preceded by a doublet code, but a famous argument due to Crick claims to rule out this possibility. I will introduce the idea of "compatible comma-free codes," and explain how they provide a natural way around Crick's argument, and can provide new answers to the above questions.