[MUSIC] We've spoken about going in the obtuse in these crosses. Now the second phenomenon that was quickly discovered by Jacob and Bowman was something that is officially known as zygotic induction. Of course, Bowman and Jacob had called this phenomenon Erotic induction. And again, this boom of censorship of the establishment transformed the erotic into zygotic, which was much more respectable as a scientific form. What is zygotic induction? Well, among the markers that they wanted to mark on the e.coli chromosome was the prophage, the lambda prophage. Where is the lambda prophage on the chromosome? And they found that to map the land that professionally, they have to do crosses. Now all these crosses are directional. Remember they are directional. You cross an HFr with an F- and the DNA flies from the HFr into the F-. So if you take an HFr that is non lysogenic, and you cross it with an F- that is lysogenic and for instance, gal-. You can select gal+ and ask how many gal+ have retained the lambda and how many have lost the lambda by recombination. Simple, very trivial. And you find that most of the gal+ have lost the lambda so we have no lambda. And you find have no lambda, about 80% have no lambda. Now, you can do the opposite cross. What happens when you cross an HFr, lysogen, with and F-, non-lysogen, also gal-, okay? That happened. Well this is what we're going to see on this slide. When you do this you get a burst of phage. This is an HFr lisogen crossed we use an F-, non lysogen and you get for h. You don't get the lambda lysogen as a marker, you get the birth of h as a marker. And it takes 27 minutes for this marker to enter the cell. Takes about 30 minutes for Gal and takes about 27 minutes for lambda. Lambda and Gal are very close. And the same as what we've seen before. If we do an uninterrupted mating, we get the lines starting from the beginning. And if we do the interrupted mating we get the lag time of entry and the burst. Why do we get a burst? This is erotic because these bacteria have sex, an induction of lambda. Because the DNA lambda, when it enters a cell has two choices, it can become lytic, like T4, or lysogen. Usually if the cells are happy, the phage becomes lytic. If the cells are unhappy, then what lambda will do, is it will hide as a prophage waiting for better occasion to make phage. It was expensive to so the ratio of these two will vary, but under healthy condition for bacterium, land that will most likely go through this. So when you do a cross the DNA of lambda is brought in the cell and it behaves like a normal infection. Most of the cell will light. A few percent will survive. Most of the cell will light. This is the phenomena of zygic induction. Now I will close this with two electron micro graph to show you the physical reality of matings. This on the left is a mating between an HFr of e.coli and an F- cells. This is the HFr, and this is the F-, and the cells have been chosen to have marker like the F- has all these hairy beard around itself. And the HFr is shaved. So you can recognize that. And this here, this here is the junction between the two cells. It's not a zygote even though we talk about the guarding induction, it's not a zygote because you don't make a full cell, like when the spermatozoa and ovum fuse, you get a full cell. Here, you get a bridge between the cells. And here, this is shown again. This is the HFr and this is E.coli C. And you can do a cross between k12 HFr and E.coli C because this guy has the f and this guy has no restriction. So it can accept any dna any foreign DNA. It's a promiscuous E.coli. And it's recognized under the microscope by its shape, which is more short and fat than long and thin. And here again, the junction between these two cells is this thin part. Now what you're going to discuss in the next week, on the next week program, is the use of the cross between the HFr and the F- to understand the regulation of the lactose operand and although the paper is. The basis forms the basis of the operating model of gene expression. You will see that a very large fraction of the paper is devoted to the analysis of this structure this round structure. What goes through the bridge between the two cells? This is going to be an important phenomenon to discuss about the phenomenon of zygotic induction and recombination. So recombination was a highlight, because with recombination, you could, first of all, bacteria became respectable beings. Because they had sex like anybody else. Second bacteria bacterial chromosome could be mapped. [INAUDIBLE] Quite nice. Third you could discover things that happen when DNA from one cell goes into another cell. All of this would have been very much more difficult without the discovery of the F factor enough recombination mediated by the HFr. So Lederberg and Tatum, Hayes, Wollman and Jacob, paved the way for an incredible adventure in E.coli genetics.
