There are two main types of transduction. And I went back to your textbook from OpenStax, and I looked to see how much detail they went into on transduction, because this can be a fairly complicated topic. And I'm going to talk about it at just the level they do, so that we're consistent between the book and then these lecture slides.
So hopefully between those two resources, you'll be able to follow along and get the information you need. Transduction just means that a virus is transferring DNA from one bacterial cell to another bacterial cell. And when we say it's transferring DNA, we mean it's transferring bacterial DNA.
So you know that a virus will inject its own DNA into the bacteria. But once in a while, the virus makes a mistake, and it ends up with some of the host's DNA, and it transfers that to another cell. So that's what we're talking about with transduction. In generalized transduction, which is the easier of the two to understand, Basically what happens is just random fragments of the host DNA are accidentally picked up when the phage is being assembled. And then when that phage goes and infects another bacterium, instead of giving it virus DNA, it's giving it DNA from the prior host that it had infected.
In specialized transduction, we have to understand the difference between a lysogenic and lytic cycle in a phage. So this is... Because of how this works, it ends up not being a random piece of host DNA, but a very specific piece of host DNA.
So let's talk about generalized transduction first. Now, this picture is not a generalized transduction. This picture is just a basic lytic cycle of a virus, but it's easy enough to figure out how this works. So here's our bacteriophage here, and here's our bacterial cell.
bacterial cell is about to get infected and we know it's not going to survive, right? The infection is going to kill it. So the virus injects its DNA. And here we have our host DNA here. And when this happens, the host DNA, as a part of the process of just killing this cell, the host DNA often breaks up into little pieces.
And I'm going to get my little notepad here because I want to be able to draw. Let's see if I can do this. So it turns out that sometimes when this happens, one of these little pieces ends up being a piece of this original host DNA.
So do you see this here? Let's see if I can change the color. I'm not sure I can. Let's see.
It doesn't let me change the color that I can see. So anyway, pretend this is pink, like this host DNA. So we have all of these phage, and we've got all this nice viral DNA that the phage is going to be. we're going to package this inside the phage head. But once in a while, one of the packages gets host DNA instead of viral DNA.
So here we go. I'm going to draw here. So this particular phage, instead of having bacterial viral DNA, it has some of the bacterial DNA. And now when this phage leaves, here we go again, all right, this particular phage has bacterial DNA. And when it goes and infects another bacteria, That DNA, it's not virus DNA anymore.
It's not going to be able to cause a new infection, but it is going to transfer some of this DNA into another cell, the next host that it lands onto. So that's generalized transduction. And for all the times this happens, it probably happens quite a bit. Most of the time, it doesn't have any effect on the cell. Sometimes that DNA just gets degraded by the new host cell.
But once in a while, that ends up being a useful gene, like, for example, an antibiotic resistance gene. And so while this is a rare event, when you think about how frequently, for example, just how many bacteria grow in culture and how many phage there are, even rare events end up happening fairly often because of the sheer number of potential interactions that occur between these organisms. All right, so that's generalized transduction.
Let me go back to my other pointer. Let's talk about specialized transduction. I haven't done a very good job spot. There it is.
Okay. specialized transduction. Before we do that, I want to talk about the difference between the lytic cycle and the lysogenic cycle. I realize I need to clear all my drawings there. There we go.
Okay, so lytic cycle, let's go back for a second. This is the lytic cycle here. This is the normal cycle where the phage infects the bacteria.
The phage takes over the cell. It causes the cell to make all sorts of new phage DNA. The phage DNA gets packaged and then the cell breaks open. In some ways, it kind of just blows up. The cell is usually so full of phage that it kind of explodes and the phage are released when they go infect other cells.
That's called a lytic cycle. Now, there's also something called a lysogenic cycle. And the lysogenic cycle is a little different. So these are called temperate phages.
And what they do is they will put their DNA into the cell. But instead of infecting the cell right away, or they're infected, but instead of causing the cell to break up right away, causing a big major infection, what they do is they sneakily incorporate themselves into the host DNA. Do you see that? So it's nothing, the cell is sort of innocent.
It doesn't think anything bad has happened. But now we have a little piece of the phage DNA inside the cell. And this is called a prophage. So this is actually really important because there are a bunch of virulence factors in some common diseases that are the result of a prophage being incorporated into a previously harmless bacteria.
So this prophage DNA, as the cell divides, it's going to divide, it's going to copy the prophage DNA. So the... This is kind of sneaky, right?
The virus is here, it's getting copied, but it hasn't done anything to the cell yet. It's actually allowing the cell just to copy its DNA every time the cell divides. Now, under certain conditions, for example, under stressful conditions, this piece of viral DNA can pop back out and form a virus again.
And when this happens, it usually transitions from the lysogenic cycle to a lytic cycle. So we're going back to a lytic cycle. Now we're going to go ahead and...
we're going to process this just like we would have in a lytic cycle. We're going to destroy the cell. We're going to make lots of copies of the phage DNA.
We're going to package them, and the cell is going to burst open with phage, and they're going to go and travel and infect other cells. So this is a more complicated life cycle than the lytic cycle. All right, so how does this relate to specialized transduction? Well, in the case of specialized transduction, it involves one of these temperate phage that... result in a lysogenic cycle.
So the phage DNA gets incorporated into the cell, it becomes a prophage. And what happens is, is that when the phage excises itself to go ahead and make new phage, it doesn't do a perfect job. So it ends up with some of the host DNA as well.
When these prophage are inserted, they typically insert in one specific location. And because of that, it's not random what this piece is. It's usually the piece that's always right next to where the insertion occurred.
And so here you have this viral DNA and it's going to get copied, but it's also copying a little piece of the host DNA. And when it gets packaged and it's going to, they're showing a big step here. So the DNA has gotten packaged and now it's going and infecting a new cell.
Do you see that it's got a little piece of the host DNA from the prior cell? is also coming in along with phage DNA. And so now we have phage DNA, old host DNA, and then our new host DNA.
So again, this allows for genetic variability in bacteria because you're actually using the virus to transfer DNA from one host cell, one bacterial host to another bacterial host. So here's just a picture of a prophage. I thought this might be helpful. It's just the genetic material from a virus incorporated into the host genome.
And sometimes these prophage genes are able to make virulence factors. So the toxins that the organism Carinibacterium diphtheriae causes diphtheria, botulinum toxin, cholera toxin, and some of the virulence factors in Streptococcus pyogenes. These are all fairly nasty pathogens.
They can all be traced to prophage DNA that resulted in these particular organisms becoming virulent the way they have. So that's why we've gone ahead and made the effort to understand this, because it does affect many bacteria. And, you know, Streptococcus pyogenes is an interesting example. We talked about the fact that it causes necrotizing fasciitis and how terrible that is. But one of the things we've actually seen is a greater incidence.
necrotizing fasciitis and that's because this organism is affected by lateral gene transfer in variety of ways but in particular it can get these these prophase genes that result in it accumulating more and more virulence as it begins to gain the ability to do a better job making enzymes that break down tissues and allow it to become more invasive so this is definitely something that's been documented in the last 10 years or so Okay, so I hope that was useful, and if you have questions on transduction and conjugation and transformation and mutation, definitely email me through your Canvas inbox so I can help you with some of these challenging concepts. Thank you.