Bacteriophage can actually use two different types of cycles to replicate themselves and here, we're going to talk about both. Now, the first one we're going to talk about is the lytic cycle and this is phage replication that ultimately results in the death of the host. Lytic, that term you see there, think of that as lice, right? When a cell bursts open, this cycle is going to result in tons of viral replication that will basically fill up the cell to the point of bursting and it's going to lyse, burst open with all those viruses, releasing them to the environment so that they can infect other cells. So, the lytic cycle, scroll down a little bit, actually begins with the injection of the viral genome as you see right here. So I'll just label this number 1. Now, the bacteriophage injects its viral genome into the cell and then it will incorporate into the host cell's genome like you see right here. Here we have the viral and here is the bacterial. I'm just going to abbreviate that back. So that's the bacterial portion that was originally in the cell. That's the bacteria's genes. And the virus is actually going to cause its own genes to be replicated and it's actually going to degrade the host genome so that only its genes are being produced. And you can see that happening over here in this third image in the sequence. Now, from its genome, it's going to lead to the production of capsomeres, right? Those components necessary to build new viruses. So here we can see some capsomeres being produced. And eventually, all the components will be produced like we see here in image number 5. And from there, they will spontaneously assemble into phages and those phages will lyse from the cell like we see here in image number 6. That is the basic. Those are the basics of the lytic cycle. A couple of things to note: a virulent phage is a phage that replicates by the lytic cycle only. Now, there are many phages which can enter either cycle and can actually switch between them, but there are some that only use the lytic cycle and we call those virulent phages. Now it's also worth noting that over years, bacteria actually have defensive mechanisms against these bacteriophages. And those come in the form of restriction enzymes that actually degrade the viral DNA that enters the host cell. Now the other cycle is the lysogenic cycle, and this involves replication of the viral genome without actually killing the host cell. As a result, we refer to phages that are capable of replicating through both lytic and lysogenic cycles as temperate phages, right? They're more tempered. They have a more even temper, let's say, because they can kill off the host cell when they want to replicate a bunch of viruses but they can also coexist with the cell without actually really harming it too much. So, let's say those temperate phages are more even tempered. Now, how does the lysogenic cycle actually go down? Well, again, first, the phage inserts its DNA into the host just like we saw before. So this is still our first image of the lysogenic cycle and again, the second phase is when the viral genome inserts itself into the bacterial genome. But now, instead of actually degrading the bacterial genome and replicating its own genome, the viral genome more or less lays dormant. And it allows the bacterial cell to replicate its own genome along with the viral genome which is what we see happening here. You see how the bacterial genome is replicating normally. But because the viral genome has been incorporated into it, the viral genome is getting replicated kind of like it's getting a free ride. And of course, the bacteria will replicate its genome and eventually divide in a process that we know as binary fission which we can see right here. We have 2 daughter bacterial cells. Now, each has that viral genome incorporated into its own genome. Eventually, what will happen is, at some point, the virus will say, well, not going to lay dormant forever and it will divide into 2 like we see here. And from there, it will follow the lytic cycle that we traced before. A couple of points of terminology, prophage: a prophage is when the viral DNA has been integrated into the bacterial chromosome. And you can kind of think of this as the term prophage kind of being like prephage, right? That prefix pro means before kind of. So this is like a protophage in a sense. It's the precursor to the actual phage. So these two cycles can feed into each other. As we saw, the lysogenic cycle will feed into the lytic cycle at some point. However, the lysogenic cycle can repeat itself many times so that, from one single infected bacterium, there could be hundreds of bacteria with prophage inside laying dormant, waiting to enter the lytic cycle, replicate all the necessary capsomeres, self-assemble a bunch of new bacteriophages and lyse the cell to infect more host cells. Alright, let's turn the page.
Table of contents
- 1. Introduction to Biology2h 40m
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- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
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- Introduction to Ecology20m
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- Introduction to Aquatic Biomes27m
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- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
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17. Viruses
Viruses
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