Let's take a look at some of the extremes
of viruses, the biggest and the smallest known
virus genomes, just to give you an idea of
how many nucleotides and proteins are in these.
The biggest virus genome identified to date
is from a virus called Pandoravirus salinus,
it's over 2.4 million base pairs of DNA long.
It encodes 2,541 proteins. This is absolutely
huge, the average virus is far, far smaller
than this. And this goes down a list of genomes
to what is a relatively small genome of 610,000
for pithovirus sibericum encoding 467 proteins.
So these are the top 10 or 12 genomes, according
to size, ranging from two and a half million down to
600,000 base pairs. These are really the giants
in the virus world. At the other extreme are
the small virus genomes, let's take a look
at some of those. The smallest known viral
genome is from an agent called a viroid. It's
120 nucleotides of circular RNA and it encodes
no proteins. These are typically plant pathogens
that enter plant cells and cause disease without
encoding any proteins, really, really remarkable
and again, slightly larger genome, 4,000 bases
in length, encoding seven proteins and in
between these extremes you can see there's
some viruses that encode just one or two proteins.
So truly the virus world is remarkable, there
is a huge range, it is quite different from
say, Homosapiens, where our genomes are more
or less the same length.