[MUSIC] Last week we discussed the paper of Benzer on Gene Topography and The Genetic Fine Structure. Today we are going to discuss the paper by Craig, Adal, on the genetic code. But, as a transition, I just would like to spend a few minutes on a paper by Champ and Benzer, which is an introduction, basically to the Craig paper. So, in this paper, it's becoming clear now that amino acids sequence and base sequence have a simple relationship. We'll see later that this is a collinear relationship. So in that paper, what Benzer did was to test a hypothesis. The hypothesis is the following. If a deletion covers the R2a and R2b region, the phage have no R2a function, have no R2b function, and so cannot grow on a lambda. Now what happens with internal deletions in the locus? On this slide you see internal deletions such as these which cover part of the R2a gene for the first two and R2b gene for the last three. These are internal deletion. As expected, this deletion the first two don't have R2a function but they can provide b function. The last three cannot provide B function but can provide A function. So, the prediction from this is that, if you had a deletion that would cover the divide the limit between R2a and R2b gene or sees from the way Benzer used to call them. Such deletion should lose both a and b function. And so, he checked among the several hundred deletions that he had, and he found four, these four, that cover the divide between the two genes. That's here. And leave a little bit of R2a and a little bit of R2b on both sides. So, these are internal deletion covering the two genes. The prediction is that these deletion would be incapable of providing R2a function and R2b function. And this is exactly what he saw with the first three. But 1589, which we will see later again, for several this weekend. Next week 1589 is exceptional because it can provide R2b function. And this is surprising because usually we should delete part of a gene, you delete, you remove the function. Now, it's even more surprising because in the R2b gene, on the part covered by 5089 here, there are point mutation sites. So, how come you can have a mutation, a point mutation, a single nucleotide change, that will abolish the function and you can delete the region and keep the function. They're essentially two ways of dealing with this paradox, or apparent paradox. One way is to say that the R2a portion of 1589, somewhere here, can compensate for the absence of the corresponding region in b. The other more intriguing possibility is that in fact the beginning of the R2b region, this region, is not essential for the protein per se, for the protein function but is essential to make the protein.