If you remember from a previous module, we talked
briefly how influenza viruses are categorized and
named according to differences in their genetic material.
So what are the processes by which these influenza
genomes becomes so different? It happens
in two ways known antigenic shift and antigenic drift.
Let's imagine an experiment where we look at
genetic change over time. My x axis I have time.
On my y axis I'm looking at genetic change. In this experiment
we take an influenza virus, and we're going to freeze
it so we're not allowing it replicate at all.
As a result we have no genetic change over time.
Let's take a second influenza virus and this time
we will allow it to infect a cell and replicate.
As it replicates it's going to copy its genome
which for influenza viruses is made out of RNA.
As it's genome is copied over and over again
it's going to make these small mutations.
We see an accumulation of genetic change over time.
This is especially true for RNA viruses like influenza,
because RNA doesn't have a proof reading mechanism
like we have in our DNA genome which prevents
these mutations from occurring.
This process is called antigenic drift. This can
happen for all kinds of viruses not just influenza.
What is unique to influenza A viruses is the
ability to undergo huge amounts of genetic change
in a short amount of time. Let's take an influenza
virus in yellow, and same as before, we let it
infect a cell with some other influenza A strains,
and go replicating and all of a sudden we see
a huge jump in genetic change. Then it might go on
replicating as same as before it's accumulating
these mutations, because it's undergoing
antigenic drift just like we saw in the other virus.
Then maybe again we see a huge change in genetic
material. These huge changes are much bigger
than just a bunch of point mutations here and there.
This is an entire section of the genome that's
changing, right. This is what we call antigenic shift.
This is really a special characteristic of
influenza type A viruses.
As I mentioned before, influenza A viruses can
undergo antigenic drift as well as antigenic shift.
Influenza B viruses on the other hands can only
undergo antigenic drift same as any other virus can.
Let me explain in greater detail what's happening
with antigenic shift. Let's take 2 influenza A viruses.
If you remember from a previous module we talked
about how influenza A viruses are named according
to the difference surface proteins that they have.
We're going to call this one H1N1. We're going
to call this one H5N2. It's got H5 surface proteins,
and N2 surface proteins. It's makes influenza
viruses so unique is the fact that their genome
is segmented into 8 pieces.
I'm drawing 8 pieces of RNA in each of these influenza A viruses.
Let's say that these 2 different strains of influenza A
infect the same cell.
When they infect the same cell they have the
opportunity to shuffle their genetic material around.
All of the RNAP pieces from both influenza A viruses
are now in the same cell and a new virus is produced.
This new virus can have genetic material from
each of the parent viruses.
In this situation this new virus, we're going
to say it has the H5 surface proteins, and the
N1 surface proteins. This is an H5N1 virus.
It got the H5 surface protein from this parent
virus, and it got the N1 surface protein from that
parent virus. This is a brand new influenza virus
with new surface proteins that our immune system
has never seen before. When antigen shift occurs
in a population, this is when we have huge pandemics.
The human population isn't able to defend appropriately
against this new virus.
That's the story with antigenic shift and drift .
It's definitely an important one, because it shows
how influenza viruses can potential be so dangerous.