Viruses

Hi in this video we're gonna be talking about viruses V. Roids and prions. So first I just want to point out that I personally think that this is the most interesting topic of all of cell biology. I love these viruses. So what are viruses? Were viruses are small parasitic particles that rely on other organisms for their life cycle. So generally they're very tiny composed of only a few proteins. So what I put here is a range of 4 to 200 proteins. Um So very small. But they do have very similar structure. So all viruses have a caps it which is just a protein coat that encloses its genetic material which is D. N. A. Or RNA. Now some viruses have an envelope which is just the phosphor lipid bi layer very similar to that of the plasma membrane and sells um that surrounds the viral caps. Ids and the viral capsules then surround the D. N. A. And I'm gonna show you a picture of this in just a second. Now capsule approach Come in two main shapes. Helical which is exactly what it sounds like. It's a helical shape helix or I cosa hedren. That's kind of a fancy geometry term. But essentially what it means is that there are 20 identical faces on the surface of the capsule protein. So these are the two main shapes. Now viruses are really simple. They don't contain any organelles. Uh They don't have any cytoplasm and like I said before, they only have a few proteins. Now we know viruses because they're typically named after diseases that they cause and they can cause diseases and plants, they can cause diseases in animals and in bacteria. Um and that's how they typically get their name. Now, viruses are unique in the fact that they are not living organisms because they cannot reproduce independently. They have to have their hostal machinery in order to reproduce. They don't have metabolize and they don't have all of these properties that we've talked about previously that describe life and cells. So they're not living. So to survive, viruses have to infect host cells and use that host cell machinery in order to reproduce itself. Um And then another unique fact is that viruses are typically very specific and the fact that they only infect certain hosts. So for instance, one common one that you'll see a lot in your textbook is a bacteria page and this is just a type of virus that affects only bacteria. Now there are viruses that infect only humans. There are viruses that infect only mammals but generally they're very specific as to what organism they infect. So if we look at just the basic structure of a virus particle here, you can see some of the most all of the um structures that I mentioned above. So one you have this envelope. Um here that is surrounding the entire virus inside the envelope. Like I said, this is only present in some not all viruses. Now you also have inside of this membrane at caps it. And this is actually in the eye. So Cosa Khidr in shape. You can see there are these repeating faces. And if you were to go all the way around there should be 20 of them. Um So I'll put that here. I I co Sahi drawn. Um Sorry if I butchered that spelling and then inside of the capsule the protein caps it you can see the D. N. A. And RNA genes which are right in here. So now that we talked about this, let's move on to the virus life cycle. We flip the page. So viruses have two main life cycles. These are called the little cycle and the life cycle. And we'll go through each one of them individually. So the lyrics cycle is responsible for creating new viral particles. So there are all of these steps which I've listed here. Now you may not necessarily need to know all of these steps. You'll have to just sort of check with your lecture and your professor of whether you need to know these exact names. But most of you probably won't need to know the exact names. Just sort of understand the concept that this is how new viral particles are made. So how this happens is that the virus binds on the host cell usually through some type of cell protein called a receptor and then they are internalized into the cell. Then the penetrations that viruses cross the plasma and bringing penetrate into the cell. The replication step is the viral genes are replicated exactly what it sounds like assembly. Is that the capsule genomes form into a virus particle and then you have finally released in which viral particles are released and this disrupts destroys the cell. You can also use the word lice to say destroys the cell. And an envelope viruses bud which is a specific term used to describe viral particles leaving the cell. They bud from the cell. Now this is different from the lice a genic cycle. And this happens when the virus integrates its genetic material into the host genome. So this isn't a process where it's creating new viruses but instead just making sure that its genetic material is internalized into the host cell. So what happens in this step is the virus usually infects through the little cycle. But instead of going through a process of creating new viruses it decides it's just going to sort of hang around in the cell for a little while um by maintaining its D. N. A. So what happens is or a term for this is called a pro fage. You may see this term as a pro virus but they're essentially the same thing And it's the term for the actual the D. N. A. That's integrated into the host genome. So the profane ages the viral DNA. That's integrated now. How so what after this viral D. N. A. Is integrated into the host genome eventually the virus is going to want to start replicating and producing new viruses and enter back into politic cycle. And so usually how this happens in bacteria. Some sort of DNA damage occurs. Um and this causes this viral DNA to be replicated and then go back to politic cycle. Now I know this is confusing. Give me a second. I'm gonna show you a really clear image on um what I'm talking about and how this happens but we'll get to that in just one second. So um whenever it switches back to the lyrics cycle variations which are new viral particles are created by the pro virus which again is the integrated D. N. A. Um in the eukaryotic cells now um this can be really damaging because you can imagine that if a virus integrates its D. N. A. Somewhere in say a human cell well it can integrate into really anywhere it wants and that can cause some serious problems for your genetic material. And so a lot of times these integrations can actually cause cancer and other types of disease by um you know causing the cell to lose control over its cell division growth and DNA replication. So let's show you I've gone over a lot of verbal explanations of these cycles but I think looking at the picture is going to make everything a lot clearer So remember we have the the analytics cycle which is the new particles. So let's actually start here. So what you can see is that here is a virus this is actually a bacterial fage. And I know this because of its unique bacteriophages look like this and no other viruses do. You don't necessarily need to know that. Um But I just think it's neat so this bacteria fage is um putting its D. N. A. Into this cell. This is a sell this little green rectangle here. And what happens you can see is that this D. N. A. Is then integrated integrated into the host cell that it infected genome now in the light cycle the cycle. What happens is that this section is then um some type of damage occurs or something that allows it to be removed from this area or it's just transcribed depending on the cell type and you don't necessarily need to know in what cases that happens. But eventually this section of the viral genome actually goes through some processing steps and is transcribed and made into various proteins. So that new viral particles can be constructed. So these steps here new viral particles are made and then eventually you can see this this is called licensing which destroys the cell and the viral particles are released free to do more infecting into other cells now for the lice a genic cycle this is a little different because it only deals with the integration part. So what happens is we're gonna start in the same place right here with the bacteria page and injects its D. N. A. And it gets integrated very the same way. But instead of making new viral particles it just stays there and it continues to copy and continues to divide as the cell divides. And now you have two cells here and you have two copies and this can keep going on and on this division where you get all of these cells containing the viral copy. And then eventually it'll go back here and go into the latex cycle politic cycle. And so these are the two life cycles of viruses. So hopefully that's clear. Um If not just review the video and just remember that the lighting cycle is about the virus infecting and producing new particles. Whereas the lice A. Genic cycle is about viral uh D. N. A. Or RNA. A. Integration. So now let's move on to the next concept.

Okay so in this video we're gonna be talking about how scientists use viruses for research. And so viruses are such a great tool that we use as scientists to study cell biology and especially cancer. And so before we can actually study viruses we need to be able to count them. And because we can't see viruses with our eye and because we even can't see them with traditional light microscopes that you may be familiar with. We have to use specialized essays in order to be able to measure how much virus we're working with. And so um the first thing uh really a main way that this is done is through a plaque assay and this is used to calculate the amount of virus in the sample. And so um how this is done is cells are infected with viruses and those viruses then integrate their D. N. A. Into the cell. And this causes the sea cells to just start growing. I mean just insanely fast and when they do they actually start growing on top of each other. And so when you stain them which you can see here with this blue stain. The cells that are growing really fast produce these things called plaques which you actually can't see that color because it's blue. But here are some plaques. So there's two plaque. Here's one plaque two plaques. And so by counting these and using the amount of virus so the volume so one micro leader to micro leaders one meal, you can do some math. Our scientists can do math and then determine how much virus is in their sample. So it's a big asset that's used in cell biology to measure the amount of virus. Now another way that this is done is through retroviruses. Retroviruses are extremely important and scientific research. So retroviral, what is a retrovirus? It is an enveloped RNA virus that incorporates its genome into the host cell chromosome. So this is really important because that means that scientists can actually manipulate what genes get put into what sells through using retroviruses. So how this happens is that because it's an RNA virus, the virus the genetic material is an R. N. A. So in order for it to be put into the host cell genome, it actually has to be turned into D. N. A. And so this process is called reverse transcription. So this reverse transcription is R. N. A. To D. N. A. Which is very which is different than transcription which is the opposite of DNA to RNA. And so this process actually creates two identical DNA strands and then the D. N. A. Can actually integrate itself into the host chromosome which if you'll remember um it's called a pro virus or pro fage. And then the gene the viral gene has been, it has then can be expressed when the cell replicates and transcribes that region of the host cell genome. So scientists use this because they can take out sort of remove the viral genome and replace it with jeans, whatever gene they want and so they can express whatever gene they want in whatever cell type they want using retroviruses. I'm sorry. I just realized they wrote that so that they remove viral genomes and replace it with the jeans that they would like to put in there. So it's really a cool way that viruses are really useful for, um, to study cell biology. So now let's move on to the next concept.

Okay. So we've been talking a lot about viruses but I do want to briefly mention two other types of infectious particles and that's uh prions and very Roids. Soviet Roids are small, circular infectious R. N. A. Molecules found mainly in plant cells so they can be transferred between damaged plant cells but generally how they work is that the RNA doesn't necessarily have any function itself but instead it just binds to proteins and blocks um their function. So um and they like I said are infectious so they can be transferred between plant cells. Now prions are not RNA and instead they're infectious particles made from abnormally folded cellular proteins. And so we're familiar with prions more than you might think they cause some diseases that you've probably heard of including mad cow disease. Um And they're unique in the fact that they're actually not destroyed by cooking which a lot of infectious particles and diseases can be prevented just by cooking food properly. But for prions they're still after the meat has been cooked, can still be passed. Um And that makes these diseases especially dangerous, especially mad cow disease. So let's just very briefly look at the basic very wide structure which you can see here we move out of the way. Um But it's just simply a circular structure very bland. Um You can imagine it is RNA. So there's base pairs binding all throughout here. Um And even maybe a few in here that bind and this is just what it basically looks like when it binds to a protein. So um let's move on now