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When flu viruses attack!

Learn how flu viruses get into and out of your cells using Hemagglutinin and Neuraminidase proteins on their surface. Rishi is a pediatric infectious disease physician and works at Khan Academy. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. Created by Rishi Desai.

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  • leafers sapling style avatar for user Peter Collingridge
    Why do human cells have sialic acid on the surface if it helps viruses get inside?
    (35 votes)
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    • starky tree style avatar for user thmemmott
      Sialic acid binds proteins that allow cells to bind to other cells. This binding is extremely important. When cells are damaged, the proteins bound to sialic acid act as a homing beacon for leukocytes. These leukocytes can then initiate the tissue repair process.
      (30 votes)
  • piceratops ultimate style avatar for user NotMyRealUsername
    What's the difference between DNA and RNA? Also, why do some viruses look like robot spider or like a lantern with legs?
    (7 votes)
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    • leaf red style avatar for user Alex
      The sugar in DNA is called deoxyribose. The sugar in RNA is called ribose.
      RNA is a single strand of nucleotides. DNA is made of two strands of nucleotides.
      DNA is a double helix with hydrogen bonds linking the nitrogen bases. RNA is a linear strand with no hydrogen bonds.
      The bases of DNA are: Adenine, Thymine, Cytosine, and Guanine. The bases of RNA are: Adenine, Uracil, Cytosine, and Guanine.
      RNA does not have Thymine, and DNA does not have Uracil.
      In DNA, Adenine pairs with Thymine while Cytosine pairs with Guanine. In RNA,
      Adenine pairs with Uracil instead, while Cytosine pairs with Guanine.
      DNA is found only in the nucleus. RNA can be found in the nucleus or cytoplasm of a cell.
      DNA's main function is to control cell activities, like telling each organelle what to make and what to do. RNA's main function is to make proteins.
      and the reason they look different is the type of proteins, layers, and the way they get inside, get out, and more
      (8 votes)
  • male robot donald style avatar for user Sun Do Cho
    Can't our immune system become invincible and indestructible even with the help of drugs?
    (3 votes)
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    • aqualine ultimate style avatar for user Udheya Kumare
      interesting question indeed!
      no,unfortunately.because the're is ALWAYS some bacterium left to still do damage to it.drugs just helps to make the disease or bacterium "weak".and yes,sometimes it kills it off,that's true.
      but don't worry,there is always another chance:plenty more to come and continue the "mission impossible"-destroy and weaken your immune system.

      If you have a healthy lifestyle,that's the biggest weapon you can get to fight diseases off,it works better than drugs.As the saying goes....
      :prevention is better than cure!
      hope this helps you^^!
      (5 votes)
  • orange juice squid orange style avatar for user Abraham George
    How big is the virus compared to the human cell?
    (2 votes)
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  • aqualine ultimate style avatar for user madison1207
    I know this was not part of the video but it is very closely related. I am confused about the order of the actions that get a virus to the nucleus. If a virus gets into a cell by the process of endocytosis, then it is in a vacuole, right? Ya, so if in order to get out of the vacuole the PH has to drop which occurs when it nears the nucleus, does the virus hijack the cell's motor proteins to get to the nucleus before or after it escapes the vacuole? And if it hijacks one after it escapes the vacuole, then how does it near the nucleus by its self in order for the PH to drop?
    (4 votes)
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  • leaf green style avatar for user Andrew
    Dose the RNA always go strait to the nucleus or dose is some times go strait to the ribosomes? Also do the ribosomes make the virus with the RNA, hemagglutinin, protein capsid and everything else or do they just make the RNA, hemagglutinin, neuramididase and the virus gets the protein capsid at the surface of the cell?
    (1 vote)
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  • starky ultimate style avatar for user Prathit Sanjay
    What are the things that distinguish DNA and RNA.
    (1 vote)
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    • leaf green style avatar for user Fraser
      RNA, is single stranded, that coils on itself, whilst DNA forms a double stranded helix.
      RNA has a ribose sugar in its nucelotide bases, whilst DNA has a deoxyribose sugar, one of the carbons on the sugar has a hydrogen bonded rather than an -OH group.
      RNA has a uracil base that pairs with adenine, whereas Thymine is the base pair for adenine in DNA
      (1 vote)
  • orange juice squid orange style avatar for user Riky M
    Is cellular damage also another cause of people having body aches?
    (1 vote)
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  • leafers tree style avatar for user Adam JW
    How does the influenza viral RNA work to instruct the cell to replicate the virus? Is there an enzyme similar to the reverse transcriptase found in HIV?
    (1 vote)
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  • duskpin seedling style avatar for user Diana Brown
    Why do bactietria have rna cells instead of dna cells?? :|
    (1 vote)
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Video transcript

So let's talk about exactly how flu causes so much damage to ourselves and why it makes us feel so lousy whenever we get the flu. I'm going to start out by drawing the flu virus here. This is our influenza virus. And we have on influenza a couple of features we have to remember. So on the outside there's this little envelope, and what's on the inside of this envelope are eight bits of RNA. Eight pieces of RNA. And so this RNA is important to remember, because in the human cell, in our cells-- I'm going to draw one of our cells right here-- we have, instead of RNA, we have DNA. Remember. And so this is our nucleus, and on the inside of our nucleus is our DNA. So this is our DNA over here. So the virus has RNA, and we have DNA. And the outside of the human cell-- actually let me label this over here. This is human cell. The outside of the human cell has something called sialic acid. They're these little strands over here that are coming off. I'm drawing them far larger than they are in real life. They're not nearly this big, but they're these little tiny little things called sialic acid. And this sialic acid becomes very important in understanding how the influenza virus gets into and out of our cells. So on the outside, remember, of the influenza virus, there were a couple of proteins. And I'm going to draw one of these proteins here, and I'm going to make it look like a hand. So this is a little hand, and this protein is called hemagglutinin. In fact, previously I had called it the H protein, and you can call it that if you want. But the full name is hemagglutinin. And what hemagglutinin does is that it actually holds onto sialic acid. In fact, that's an easy way to remember it, right? Because H and H go together. It holds sialic acid. And that becomes very important, because that allows it the first step towards getting into the cell. Now there's another protein on the outside here-- I'm going to make it look like a pair of scissors, because that will kind of remind us what this one does. And this is called neuraminidase. And I'm going to-- neuraminidase. And I'm going to pass on explaining what it does, just for the moment. I'll tell you in a little bit what it does. So then the first step to get into the cell is for hemagglutinin to hold on to sialic acid. And then there are a few other small molecular steps that happen, important ones. But I'm going to suffice to say it gets inside. And once the influenza virus gets inside, these RNA segments, they are let loose. So these segments are going to start making their way towards the nucleus. And so once they get into the nucleus, they're in that same kind of area that the DNA is, and what they do is remarkable. They basically, they take over. These little RNA start making many copies of themselves. And what they want to do is make our human cell into a factory. They want to make a factory. And this factory is going to make little proteins, viral proteins, and it's going to make viral RNA. And what it's not going to do, the one thing that the cell is no longer going to be very good at doing, is its normal job. So the human cell, of course, had some job to do. And it's not going to have the resources or the time to do it, because it's basically being taken over by this viral RNA. So what happens then is that the viral RNA is basically turned it into a factory. And what it wants to do is make more and more copies of itself. So let me actually just show you what that would look like. Here's a daughter cell. Let's say it goes over here. And I'm going to clean this up a little bit, just to make sure that we're looking at a nice, neat picture. Let's say something like this. So this is our daughter cell in the other side, and these cells are going to try to make their way out of the human cell, right? Because now they're packaged, they're ready to go, and where do you suppose they want to go next? Where they're going to want to find their own human cell to invade, because they want to continue this process. So we've got more human cells over here down below. And we've got maybe, let's say, one human cell up here. So we've got new targets for this virus, and this virus is going to seek out these targets and try to make its way inside again using its hemagglutinin. But before it can do that, it's got to break loose, right? Because it's still attached to that sialic acid. And so here's where neuraminidase comes in. The neuraminidase, it basically, it nicks, and there's where the N is helpful for remembering it. It nicks, or cuts, sialic acid. And so if it can nick or cut that sialic acid, it can break free. And so I remember the two proteins as hemagglutinin hold sialic acid to enter the cell. That's on entry. And the neuraminidase is going to nick the sialic acid, and that's important for exiting the cell. But we still haven't answered the question, how does all this cause our symptoms? Well, what happens is that as the cells get turned into factories, they start dying, or getting damaged. And all their contents start leaking out. So all these contents from the cell start leaking out. And as they do, they create inflammation. Let me bring up a little bit of canvas. If you have inflammation, let's say that inflammation is happening in your nose. Well, you might say, well, I have a runny nose or a stuffy nose. Or if that inflammation is happening in your throat, you might say, well, I have a sore throat. It might hurt. And if it's happening in your lungs, you might have a cough. So a lot of those respiratory symptoms-- remember we had two categories-- a lot of those respiratory symptoms-- I'm going to shorten as "resp."-- those are going to be explained by inflammation, or at least in part, by inflammation. And remember there are also constitutional symptoms, right? With constitutional symptoms, those are things like having a fever, or having fatigue. And the reason for that is that your immune system is going wild and crazy. When you have influenza, it's going to be attracted to all of those chemicals. We call these cytokines that are being released. And it's going to be attracted to the fact that you've got actual virus particles in the area that's being infected. So that strong immune system is going to create some of your symptoms. It's actually going to rev up your temperature, and you'll start having a fever or chills. And because all your energy is being spent on this attack, you're going to be fatigued. You're going to be fighting off the virus. Since you're going to be fatigued you might have body aches, so a lot of these kinds of symptoms, you get as a result of a strong immune response.