The first step to a thorough understanding of astrovirology is a thorough understanding of terrestrial virology. Here are some of the basics of the study of viruses.
A virus is a small, intracellular particle. It is a parasite that hijacks a permissive host cell’s biological machinery for the sole purpose of reproduction of more viral particles. Viruses are not alive. They do not think, act or have any motive whatsoever. The genome of a virus is either RNA or DNA and will direct the synthesis of more viral particles within a cell. RNA viruses are mutagenic and comprise some of the more notorious human diseases including ebola, SARS, and influenza.
When the virus is outside a host cell it is referred to as the “virion”. The virion is the molecular complex that delivers a viral genome into a cell. It has an outer protein shell known as a capsid, which is made of repeating protein subunits. The virion must be stable enough to protect the genome outside the cell, but also must disassemble to interact with the host cell when necessary. Virion capsid structures are typically either helical or icosahedral.
The Viral Genome
Nucleic acids form the genome of a virus. There are four types of nucleic acids that are predominantly represented in the Baltimore classification system.
double-stranded DNA viruses (dsDNA)
Double-stranded DNA viruses are the most prolific type of DNA virus.
examples: baculovirus, adenovirus
single-stranded DNA viruses (ssDNA)
examples: gemini plant virus, parvovirus
double-stranded RNA viruses (dsRNA)
examples: picobirnavirus, cystovirus
single-stranded RNA viruses (ssRNA)
Single-stranded RNA viruses are the most prolific type of RNA virus.
examples: SARS coronavirus, dengue fever, influenza
Genome Replication of DNA Viruses
small DNA viruses
Small DNA viruses will typically either have a circular genome (bidirectional replication; example: SV40) or a linear genome (strand displacement; example: adenovirus). They will use the host cell’s replication machinery and polymerase.
large DNA viruses
Large DNA viruses use a rolling circle mechanism and encode their own replication machinery. Example: Herpes simplex.
The genome of single-stranded RNA viruses is classified as plus-sense (+), minus-sense (-), or sometimes ambisense (both polarities).
Plus-sense ssRNA viruses always have an infectious genome. Their mRNA can be directly translated into proteins.
Minus-sense ssRNA viruses genomes are not infectious. Their RNA must be transcribed by virion polymerase to make mRNA.