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The immune system
- [Voiceover] So, let's talk about viral replication. Since we know that viruses are made of only proteins and one type of nucleic acid, which means they have no organelles to make copies of themselves with, they have to get inside other cells in order to use their ATP and their organelles to make copies of themselves. So first step, get inside that cell. And I'm just going to mention a few things for review. So remember, there are three ways of getting into a cell. One of the ways of getting in is that bacterial phages, they literally just inject their genetic material inside of the cell. Whereas every other type of virus uses one of two ways. If they're non-enveloped they trick the cell's receptors into letting them in. Or if they're enveloped they can either use that receptor trick, or they directly fuse with the membrane. And once that genetic material is inside of the cell, the virus has hacked in, and it's ready to do a takeover. But here's where it makes a choice. It's kind of either impatient and ready to get going to copy itself, or it sits back and says, "You know what? "I'd rather sit and wait until I'm ready "to take over." So let's just draw these two possibilities out to see what happens. And with the first choice, the impatient virus goes ahead and takes over the cell's machinery. Their ATP, ribosomes, nucleic acids, amino acids, to start making copies of the virus's genetic material, which again, can be RNA or DNA. And also, the virus's proteins to make their protein coat. And these will self-assemble. That means they just come together on their own to form fully functional viruses. And because it keeps making more and more and more of these, it will eventually force the cell to lyse, or break open, and once it breaks open, all of these viruses are released into the environment. And if there are other cells nearby, then this army can start marching out to infect, to hack into those nearby cells, and create more armies. So this makes a lot of sense if you have a lot of hosts around, and your goal is to just create the biggest army in the fastest way possible. And so that's the impatient virus. So what about the other option? The other option is where the virus decides it's just going to sneak in and hitch a ride. It thinks that the bacteria seems to be doing fine on its own, maybe there aren't other hosts nearby, so there's no reason to kill off the host, because lysing it would kill the host, so we don't want that. We're just gonna keep the host alive. And in order to sneak in and let the bacteria do its thing while it's waiting, it's going to combine with the host's genetic information, so that the host really can't tell that it's there. It's basically quietly sitting there because it's repressed. There are repressor genes on this virus. So it's not expressed, it's not transcribed. So it's not doing anything. And this is called a provirus, or you might also hear the word prophage. And again, because it's not doing anything, this is called a dormant or latent phase. So the bacteria just does its own thing, it continues replicating, and of course the virus is already there. So it will continue to replicate. So it will replicate when the bacteria does, because it's part of its genome. But about one in 10,000 times that this happens, or if something happens like the bacteria is exposed to UV light or something. Well, in any case, there's something that weakens that repressor gene that we talked about on that virus. So it's no longer quiet. And the host's genome, like most other genomes, will start wanting to repair itself. That means it will cut out to try to repair its genome, it will excise out part of its DNA that just happens to be the virus. And now, the virus is active. It's ready to make copies of itself, lyse the cell, and get its army out into infecting other cells. So the official terms for the impatient method is the lytic cycle. And the hitch a ride method is called the lysogenic cycle. So now, you should have a good idea of what the lytic and lysogenic cycles are in viral replication.
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