00:00
Alright, so that completes our discussion
of the DNA viruses. Now let's turn to viruses
with RNA genomes, and a few points about RNA,
important to make here because this is a very
different molecule. Only viruses have genomes
of RNA. They can have DNA obviously, we’ve
just talked about them, but no other thing
on the planet has an RNA genome, except the
virus. Cells have no way to copy an RNA molecule
into another RNA, at least long RNA molecules.
00:32
They have no RNA-dependent RNA polymerase
(RdRp), that’s the enzyme that would copy
the RNA genome. Therefore that enzyme is always
encoded in the RNA virus genome. So virus
genomes always encode in RNA dependent RNA
polymerase and that RNA polymerase copies
the RNA genome and it also makes mRNA from
the genome as well. So there is no cellular
involvement in this process. The virus has
to encode the RNA polymerase. Now in many
of the slides we’re going to look at, I
draw the RNA as a squiggly green line. Of
course in reality, it's not that at all. Not
only is it not green, but it has much more
extensive structure than we ever show. And on
the right of this slide is probably what most
RNA molecules look like. They have extensive
secondary structure. This is base pairing
within the RNA sequence which you can see
here as stem loops, but they also have tertiary
structure, where the loop regions for example,
can interact over long distances and you can
see that happening here by these dotted lines
between distal sequences. So RNA molecules
are really highly folded up structures that
absolutely do not resemble a line on a piece
of paper.
01:50
Alright let's start with the viruses that
have double-stranded RNA genomes. Just like
the DNA viruses we can have double or single
stranded. Viruses with a double-stranded RNA
genome, interestingly they contain both the
plus and the minus strand. They are double-stranded,
and that plus strand would be the messenger RNA, but
ribosomes cannot get at that plus RNA to translate
it. So you have to make mRNA from these double-stranded
RNA genomes. So the mRNA synthesis that is
shown here in this slide, as the single green
line with the shaded background, that reaction
has to be done by a viral enzyme. Again the
cell cannot make RNA from RNA, and because
this reaction has to be the first thing that
takes place in an infected cell, the enzyme
that carries it out has to be in the virus
particle. So this is going to be a theme we’re
going to get back to over and over with these
RNA viruses, some of them the RNA polymerase
has to be in the particle, because the first
step is the making of mRNA. For other viruses
we don't have to have the polymerase in the
particle and we will see why in a moment.
03:04
So back to these double-stranded RNA viruses,
we’re making a messenger RNA that's then
translated into protein by the cell, of course,
and the proteins go on to produce new virus
particles. And to make new genomes, some of
those mRNAs are simply copied to make a minus
strand by the viral polymerase. It’s a relatively
simple replication strategy; you just have
to remember that double-stranded RNA cannot
be translated, even though it has a plus strand
in it.
03:33
So the well-known double-stranded RNA containing
viruses are the real viridae. They have about
10 double-stranded RNA segments. So these
genomes are in pieces as you can see here,
it's not one long DNA, although there are
viruses with single double-stranded RNA genomes,
and a very important human pathogen in this
group are the Rotaviruses that cause human
gastroenteritis.