The genomes of viruses are just like every other genome, they contain
all the genes necessary to replicate a virus. However, viruses are very
lazy, and just use most of the host cell's machinery to make what they
need to replicate. Kind of like that time you let that carnie into your
house, then when you came back your dryer was gone…and there were twelve
carnies and a ring toss game in its place.
Viruses encode a
minimum of two proteins, while other proteins they encode aid in the
process of virus survival. Those two proteins are
- Replicase – an enzyme that replicates the genome
- Capsid – a protein that protects the genome.
Other types of genes that a virus would encode include (in order of common genes to more rare):
- Protease – enzyme that processes viral proteins and allows assembly or maturation to infect other virus cells.
– (enveloped viruses only) allows virus to enter a cell, targets
specific cell types for virus, and aids in virus assembly.
- Host shutoff proteins – virus proteins that shut off host activities so only virus genes get made.
defense proteins – this varies in number of different genes, depending
on the virus. These proteins prevent the host defense mechanism from
stopping virus replication.
These genes are encoded on either RNA or DNA polymers, and are either single stranded or double stranded
Figure 2: Types of virus genomes
that was a lot, right? Well, it gets worse. While most genomes are
non-segmented (the genome is all on one piece of RNA or DNA), some
genomes are segmented, meaning there are several fragments of DNA/RNA
that make a complete virus genome. Also, some genomes are linear,
meaning that there is a beginning and end to the genome, while other
genomes are circular (no beginning and no end, Kind of like that
friendship song, except by "friends" we mean "piles of microscopic
That seems like a lot of information overload, and we
sympathize. For something so small and seemingly simple (like "I can't
believe it's not butter"), viruses are very complex (like the
ingredients of "I can't believe it's not butter"). The important thing
is that if you can tell the difference between RNA and DNA virus, and if
it is linear or circular, and if it is segmented or non-segmented, you
can immediately tell a lot about a virus. So, we attached a table to
serve as a cheat sheet for virus genomes.
|Virus Type||Genome||Example||What It Tells Us|
|I||dsDNA||Adenoviruses, Herpesviruses, and Poxviruses||Virus can use host machinery to make mRNA, and even replicate virus|
|II||ssDNA (+)||Parvoviruses||Need DNA polymerase to make negative strand, can use host machinery to make mRNA|
|III||dsRNA||Reoviruses||Needs RNA polymerase to replicate itself as well as make mRNA|
|IV||ssRNA (+)||Poliovirus and Ebola Virus||Needs RNA polymerase to replicate itself. Genome also serves as mRNA.|
|V||ssRNA (-)||Influenza virus, Hantavirus||Needs RNA polymerase to replicate itself as well as make mRNA|
|-||ssRNA (+)||Retroviruses, HIV||Makes cDNA that integrates into host genome, which makes mRNA and proteins|
|-||Segmented||Influenza||When making viruses, can mix and match segments to allow virus to more easily evolve|
|-||Non-segmented||HIV, Poxvirus||Evolution is more difficult. Requires more error-prone polymerase|
|-||Linear||Needs a primer to start replication from one end of genome.|
|-||Circular||Poxvirus, Herpesvirus||Replication starts anywhere on the genome (but often has a specific origin of replication)|