So transduction is the process of using a bacteriophage to transfer foreign DNA into a bacterium. This was discovered in 1951 by Lederberg and Zender. What they did is they had 2 mutant E. coli strains, these are 2 mutant bacterial strains, they mixed together. And when they plated them on conditions where neither would grow. They have strain A and strain B, and they mixed them and plated them onto a plate where they shouldn't grow. No growth. That's what they were expecting. They thought they were going to die. But, essentially, what happened is that some of them survived. About 1 in every \(10^5\) E. coli cells did grow, meaning that some kind of DNA transfer had to be occurring. And so what they thought, they were like, "Okay, well, conjugation, we just figured this out, so, it's likely conjugation." And so what they did to prove this, is they put a filter in the growing area. And what the filter did is it would stop conjugation. It was so small that the pili that connect the two bacterial cells together couldn't form, so they completely inhibited conjugation. They did the experiment again, but what they saw is that the cells still grew. And they were like, "Okay. Well, this is definitely not conjugation, but what's going on?" Well, they looked at the filter, and they said that the only thing that could fit through this filter is a bacteriophage. And so they knew that some kind of virus had to be transferring DNA between the two cells to allow them to survive in this area, where they shouldn't be able to survive. So here they are. They plated 2 bacteria, plated where neither should grow, but they both grew, meaning that some kind of DNA transfer had to happen here, and they used the filter to not only prove it wasn't conjugation, but also to say, "Okay, that only a virus is this small to fit through this filter, so it has to be that." That allows for this type of DNA transfer.
There are 2 main types of transduction. The first is generalized, meaning that they can transfer any part of a bacterial chromosome. They can transfer any kind of bacterial DNA. Usually what happens is that some type of bacteria has been lysed, usually by a virus, right, and that releases some cut-up DNA into the environment. The bacteriophage will take it up and then transfer it into another cell during the infection process. This is general transduction. It's just sort of taking up whatever's been released and put into the environment. There's a different form called specialized transduction, and this is the ability to transfer only specific parts of a bacterial chromosome. So how this works is there's a molecule called a transducer, and this transducer actually inserts itself only in one place in the bacterial chromosome. It has a very specific sequence and it says, when that virus is in there, it's going to put itself right in that sequence. When that bacteria is, and that transducer is stimulated to leave, either by a chemical or a virus or, you know, it can be stimulated in lots of different ways, but something activates it to leave, it picks up the nearby genes. So it's only taking those genes in that one specific sequence that are nearby where that transducer inserted itself into the cell. That then goes into the phage and then can be transduced into other bacterial cells. So here's generalized transduction. You have chopped up DNA here. Part of it gets into a phage and that goes into a bacteria. Specialized transduction is different. Here, you have a phage with a transducer. The transducer, upon infection, gets into the bacteria. It inserts itself in this one area here, and that will then go into other phages and be transferred into bacteria. So this is always going to be the same, sort of, genes that are transferred here, and this can be any kind of sequence. It doesn't even have to be a gene, it's just any kind of broken-up sequence.
Now, generalized transduction, this part that kind of takes up anything, can also be used to map genes and study gene linkage. Because the closer the two genes are, the more likely it's going to be that they're transduced together. And this is called cotransduction, when a single bacteriophage carries more than one gene in it. Now before when we've been using mapping, we've been using recombination frequencies, looking at either the offspring themselves or the bacteria in case of bacteriophage recombination frequencies. But in this case, we call this cotransduction frequencies. It measures how often two genes are cotransduced. The closer two genes are, the more likely they're going to be cotransduced together, and the farther they are, the less likely. So you can use this cotransduction frequency to measure how close the genes are together. So here we have an example, chopped DNA. You can see here that the genes are close together and here that they're farther apart, and that means when the DNA was chopped, these two stayed together and these two were separated. And so when these get into the phage and are transduced into the bacteria, then you can see that these two stay together, and this is called cotransduction. Now the frequency of cotransduction can be measured in a lot of different experimental ways, but, essentially, that cotransduction frequency allows you to be able to identify how close the genes are together, and how far apart they are. So, that's transduction, and with that, let's now move on.
