We have viruses with DNA genomes but they
have a different configuration, so this is
a separate Baltimore class. These are gapped
double-stranded DNA genomes, and the configuration
is shown in this slide. You can see most of
the DNA is double-stranded but a part of it
is single-stranded, and that's the negative
strand that we see, we call that a gap. In
addition, there is a little piece of RNA attached
to the plus strand. You can see that in green.
And there is also a protein attached to the
three prime end of the negative strand. Now
this genome is present in the virus particle,
it happens to be the genome of hepatitis B
virus, an important human pathogen that causes
liver infections. This genome when it enters
the cell, cannot be made into mRNA. mRNA can
only be made from double-stranded DNA among
the DNA viruses, it can't be made from gapped
DNA, it can't be made from single-stranded DNA.
And so that's a great fact to know, because now
we have three classes of DNA genomes and you
can immediately tell how they get to mRNA.
That is the beauty of the Baltimore scheme.
So this genome, as it gets in the cell, cannot
be copied to mRNA, so what has to happen?
It has to be repaired. So if we start with
a gapped double-stranded DNA at the top of
this slide, you could see the line going to
double-stranded DNA. And that reaction happens
in the cell nucleus, the viral DNA goes all
the way to the nucleus, and cell enzymes repair
it, they make it double-stranded, they take
off the protein and they take off that little
piece of RNA. Now it can be copied to form
mRNA, which is the green molecule shaded to
the right. That mRNA is then translated to
protein and eventually those proteins will
enable the production of new virus particles.
Of course the other aspect of this replication
cycle is we have to make new virus genomes.
And that's done starting from an mRNA, a plus
stranded RNA which you can see on the left-hand
flow of this slide. But curiously, that plus
RNA is then converted to a minus strand of
DNA by an enzyme called Reverse Transcriptase.
That minus strand DNA is then made double-stranded
and in that process the gaps and the protein
and the small piece of RNA, they are all introduced
into that molecule and that is what actually
gets packaged into new virus particle.
It's really a unique and maybe baroque replication
cycle, and more importantly this enzyme reverse
transcriptase, you'll see in another virus
family as well. It's called reverse because
we used to think the flow of genetic information
went from DNA to RNA to protein. But the discovery
of this enzyme by others including David Baltimore,
who has made this Baltimore scheme, showed
that the flow could be reversed from RNA to
DNA. That´s why we call it reverse transcriptase.
Finally, the last of the DNA viruses, these
are viruses with single-stranded DNA genomes,