00:00
Let's go to the minus polarity single-stranded
RNAs. There are a number of important human
viruses here. The Paramyxoviruses including
measles virus, and mumps virus, important
childhood infections, completely controllable
by vaccination. Rhabdoviridae, these contain
rabies virus, the infection transmitted by
rabid dogs. The Filoviruses, everyone should
know about the filoviruses now, because it
was just a big outbreak of Ebola virus infection
in Western Africa. The filoviruses are unusual
filamentous particles; they include not only
the Ebola viruses but Marburg viruses first
discovered in Germany at a primate facility.
00:46
The Orthomyxoviridae includes the very important
human pathogen, influenza virus, which causes
annual outbreaks of respiratory disease in
many, many millions of people. And finally
the Arenaviridae, which Lassa virus is the
best-known. This is a virus that also causes
a hemorrhagic fever, largely in Africa. These
viruses have a negative sense RNA genome,
and from what I've told you so far, you should
be able to predict exactly what is going to
happen in this flow of genetic information.
Minus strand RNA cannot be translated, and
the cell cannot make a copy of it to make
messenger RNA so you can predict the answer
to the question: Do negative stranded RNA
viruses carry an RNA polymerase in the particle?
The answer of course is yes, because it has
to come in with the RNA and then copy it to
make plus stranded RNA. If there were no polymerase
in the particle, that would be the end of
the infection, because the negative RNA can't
be translated, it can't be copied by any cell
enzyme; it would be the end of the infection.
So these genomes have evolved to include the
RNA polymerase in the particle. So plus stranded
RNA can be made, messenger RNA can be translated
into proteins, and also plus stranded RNAs
can be made to make more genomes, to make
negative RNA genomes, which get incorporated
into the particles.
02:15
There are two kinds of viruses in general
with negative stranded RNA genomes. We have
some like the influenza viruses, where the
genome is segmented; it's in pieces like the
double-stranded RNA genomes of the real viruses.
So you can see here there are eight negative strand
RNA segments in the influenza viruses. Then
there is another kind of negative strand RNA
virus where the genome is not segmented. It
exists in the virus particle as a long RNA
for the Paramyxoviruses, measles and mumps
15 to 16 kb, for the rabies like viruses 13
to 16 kb. Again these are all negative strand
viruses, two general ways of assembling them
in the virus particle.
03:02
Having a segmented genome, the influenza virus,
the reoviruses, and many others is very important
for genetic variability, because of this example.
When you infect a cell with one of these viruses,
the RNA is replicated in the cell and you
have many, many copies of the RNA which are
then packaged into new virions. If you would
by chance infect a cell with two different
viruses, here we’re showing two different
influenza virus labeled L and M, infecting
the same cell, if they both infect the cell,
their RNAs are all going to be replicated
at the same time. They're all mixed together,
and the new viruses that are produced are
not only going to be L and M viruses, but
recombinants that have RNA segments from either
parent, and one of the recombinants is shown
here. It's called R3 and you can see that
second RNA segment is red, it derives from
the red parent and the rest of them are from
the blue parent. In fact you could get almost
any reassortant possible out of this co-infection
and this is partly the reason why influenza
viruses are problematic. They vary extensively
and this is one of the reasons. This kind
of variation by reassortment, is why we generate
new influenza viruses on a daily basis and
why every 30 to 40 years, we have a brand-new
strain that causes a global pandemic, because
it's a reassortant caused by this process.