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Genetic shift in flu | Infectious diseases | Health & Medicine | Khan Academy

November 12, 2019

So we’ve talked about how
influenza viruses attack cells. But every once in awhile, it’s
really, really unfortunate, but you’ll actually
get two viruses that affect the same cell. So imagine how bad you’d
feel in that situation. Not only did you get
sick from one virus, you actually picked up two. So this can happen in humans. It can happen in other
animals, like pigs, as well. And it’s really just dumb
luck when this happens. It’s not like it’s a
coordinated attack. This is just, by random
chance, two viruses are going to want to
attack simultaneously. So in the top one, let’s
say we’ve got some RNA. I’m going to put eight
strands of RNA in red. And it’s going to code
for little H proteins. This is– I’m going to name
it H– I don’t know– H3. And I’ll do one more
hemagglutinin protein here. And remember, it
looks like a hand because it holds
onto sialic acid. And you’ve got some
neuraminidase over here. Neuraminidase is going
to nick that sialic acid. It’s going to help
that virus exit. And let’s say this is N2. So we’ve got H3N2
as the top virus. And in the bottom virus,
I’m going to do, let’s say, 5, 6, 7, 8. I’m going to do a
slightly different color. This is also an H protein, but
this is going to be H1, OK? So this is H1, and
I’m using the color to try to help you identify
them as being different. And we, of course,
have some neuraminidase on this virus as well. And this neuraminidase is green. And let’s assume
that that is N1. So we’ve got H1N1 on the bottom. And we’ve got H3N2 on the top. And these two viruses
are ready to go. They’re ready to
attack this cell. And this cell, of course, before
these viruses are coming along, it’s doing its normal,
routine things. It has DNA. But once these
viruses get in there, they basically take over. So the virus RNA gets inside. We’ve got some of
the red RNA there. We’ve got some of
the blue RNA here. And so that RNA
starts taking over and commands the cell to
make more copies of itself. So all of a sudden, the
cell becomes a factory for the blue virus,
the blue RNA virus. That’s the H1N1. It also becomes a factory
for the top virus, the H3N2. They just basically
both take over together. And so this cell
is torn two ways, doing a job for the
two different viruses. And this red RNA is actually
coding for things like H3. This is a protein
down here it’s making. And it’s also coding for N2. So you’ve got N2 here,
more N2 protein over here. And on the other
side, on the top side, you’ve got some H1 being made. So two different types of
hemagglutinin and proteins are being made at the
same time in this cell. And of course, you’ve
got two different types of neuraminidase. You’ve got some N1 being
made over here as well. So this is basically
what we see. We see the cell making a
lot of protein and RNA. And eventually,
this cell is going to start sending
out little viruses. And that’s what I have
on the right side. We have the viruses that
are exiting the cell. And what do they look like? I guess that’s the question. Well, some of them
might actually look exactly like the parents. Like If it actually gets
all the same red RNA or the same blue RNA,
then it might actually look identical to the parents. It might even pick up
the exact same proteins. It might have these purple H3’s. It might have some purple H3’s. It might have some N2 over here. So, actually, this one
looks basically the same as its parent. H3N2 is what we would call
this one if we were to name it. And this bottom
one, the blue one, actually would look
potentially the same as well. This might have
some H1 over there, maybe some H1 on this side. And it might also
have some N1, right? It might have some neuraminidase
over there and there. So if I was to name
this bottom one, I would actually name
it the same thing. I would name it H1N1. But the interesting thing
is that, actually, once in a while, you get some mixing. You might get some
mixing happening. Maybe you get a couple of
blue strands over here, maybe three blue strands. Maybe you get two blue
strands over there. And when you get
mixing of the RNA, you might also get some
mixing of the protein. You might get some
protein over here. That’s actually H3 and
maybe an H3 over here. And maybe on the
bottom one, you’ll actually get
something different. Maybe on this one, you get
an H1, maybe an H over here. So you get some
mixing of the H’s. And maybe on this side you
get an N2, maybe an N2. And of course, that wasn’t what
the parents looked like, right? And up here maybe you get an N1. So, all of a sudden, things
are looking a little different. And If I was to name these
viruses that are coming out, these progeny
viruses, this top one, this would be an H3 because
it’s got a purple-looking looking hemagglutinin. And it’s a green-looking
pair of scissors, and that was the
N1 protein, right. So that would be an H3N1. And this bottom
one over here, this would be an H1, because that’s
the yellow-colored hand. And the orange pair of
scissors, we said that was N2. So, these are actually
quite different. So these are different
than the parents, right? And the other ones are the same. So when you have different
viruses coming out, what does that mean? Well, if the virus
is successful– let’s say this one is really
successful at getting people sick, and it transmits from
one person to another person really effectively. In fact, maybe the first
person gets a few people sick because it’s so, so contagious. Maybe a lot of
people can get sick from just one person being sick. And of course, that’s going
to spread in all directions, right? Well, that’s going to cause
many, many, many people to get sick. And we would say, wow, in
this population of people, in this community, H3N1 seems
to be the dominant new virus. And that process is
called genetic shift. So this process of a new virus
emerging after shuffling up its genes is called
genetic shift. So that’s what
people talk about. And this, of course,
happens with type A viruses. So, remember, we have
type A, B, and C viruses. And this is a process
that really affects type A viruses only. Now, sometimes you might
get the other possibility. Maybe this will come out,
and it becomes kind of a dud. It doesn’t really
affect too many people. It’s not very good
at causing disease. And if that’s the case, it
would soon be forgotten. So this mixing, this genetic
shifting, can happen in people. You might have a person that’s
affected by two viruses. And that person would then
turn around and perhaps affect other people. Or it can actually
happen in animals. You might actually see
this happening in pigs. So there might be a little
pig here that gets sick. And this pig then would
transmit the disease to maybe the farmer. And sometimes you might hear
the term “mixing vessel.” And there, the
mixing vessel refers to whether it was a human,
which was case number one, or a pig, which was case
number two, in which the actual mixing of the
two type A viruses happened.


  • Reply rhoadess January 12, 2013 at 5:20 pm

    Scary! It seems that if you get two viruses initially, you really get four in the end. I suppose the immune system would also be able to fight these simultaneous attacks just as fast as if you had one? If that is true, it might be better to have them all at once than to have them consecutively.

  • Reply khanacademymedicine January 14, 2013 at 4:28 am

    Two viruses infecting a single cell is a rare event (fortunately), but if it does occur then there is the possibility of mixing to occur. The resulting viruses could potentially be more effective than either parent virus at spreading and causing disease, and this could lead to major problems for the community. This has actually happened a few times in the past century!

  • Reply JUX SOUNDZ July 21, 2014 at 9:33 pm

    Try making the audio a little less harsh its like the mid-range frequencies in your voice were compressed to shit so it hurts my ears

  • Reply siloPIRATE May 11, 2015 at 3:18 pm

    0:16 Spoiler alert
    When there are multiple influenzae viruses in your body, something called reassortment can occur, which leads to the creation of new genotype. The 8 segments of RNA can reassort with each at random causing the new genotype. This new genotype causes  a new phenotype to be created. Once you have a new phenotype the influenza epitope is then changed. Because the epitope has changed, the antibodies in your body have a harder time finding it, because the adaptive immune system has not gone through clonal expansion to produce memory and killer cells for the new antigenic epitope. This is called antigenic drift and is why you can be reinfected by influenza. It also makes the creation of new vaccines difficult

  • Reply Ekhlas Hasan January 4, 2016 at 1:07 pm

    Thank you so much!

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