00:00
Finally, the last of the DNA viruses, these
are viruses with single-stranded DNA genomes,
and these viruses, curiously, can package
either strand, either the minus or the plus
strand of DNA. So when these viruses infect
a cell, this single-stranded DNA, can be circular
or linear, gets into the cell, but again remember,
the only molecule that can make mRNA is a
double-stranded DNA molecule, among the DNA
genomes. So you can immediately predict that
this single-strand has to be repaired to make
it double-stranded, and in fact that's what
happens, it becomes double-stranded and now
mRNA can be made from this double-stranded
DNA. That mRNA is shown in green with the
shaded background. It gives rise to proteins,
which go on to build new virus particles.
At the same time of course, the viral genome
has to be replicated and that double-stranded
DNA is replicated to single-stranded products,
either plus or minus, which both get packaged
into the particles. And I want to tell you
about two different virus families within
this category of single-stranded DNA genomes,
the one on the left of the slide called the
Circoviridae. These are very interesting viruses,
well they are all interesting, but this is
particularly interesting. Circo viruses, first
of all the genome is a circle of single-stranded
DNA. They are very small, between 1.7 and
2.2 kb in length, they don't code for many
proteins at all, but they infect everyone
on the planet. We all have these viruses in
our blood, at most times in our lives, in
fact the blood supply is full of these circo
viruses. But apparently they are of no consequence,
at least not negative consequence. They may
help us but we don't know yet. Alright, those
are the circular single-stranded genomes of the
circle viruses, and then we have the parvoviruses.
02:03
They have linear single-stranded DNA genomes,
these cause an important human disease called
fifth disease, if you happen to have a cat
or a dog, you know that you need to immunize
them against parvovirus infection, because
infection with those viruses can kill them.