Viruses are microscopic infectious agents that infect cells. When outside of cells, the consist of
- Genetic material (DNA or RNA)
- A protein capsid surrounding the nucleic acid
- Sometimes, a lipid bilayer around that - an "envelope"
Classification
By nucleic material type
- Double-stranded DNA - e.g. Herpesviruses, poxviruses
- SIngle-stranded DNA (almost universally +, i.e. sense) - e.g. Parvovirus \(\to\) causes 5th disease
- Double-stranded RNA e.g. retroviruses including HIV
- Positive single-stranded RNA (sense, can be used as a template for protein synthesis) e.g. hepatitis C
- Negative single-stranded RNA (antisense; needs to synthesize a positive-sense antigenome in cell prior to translation) e.g. influenza
By mechanism of replication
- Large DNA \(\to\) migrate to nucleus \(\to\) encodes own replicative enzymes (except poxviruses)
- Small DNA \(\to\) migrates to nucleus \(\to\) uses host cell's replicative enzymes
- RNA \(\to\) remains in cytosol (mostly; influenza migrates to the nucleus to make mRNA; retroviruses undergo reverse transcription \(\to\) DNA in nucleus)
Other features
- Is the capsid shape helical or icosohedral?
- Is there a lipid envelope?
Antiviral drugs
- Guanosine mimetics (e.g. aciclovir, ganciclovir): These are phosphorylated into monophosphate form by viral enzymes produced by herpes viruses (i.e. phosphorylation only happens in viral cells), then phosphorylated by host enzymes to triphosphate form, and then are used in place of guanosine in nucleic acids; but do not have a binding site for the next nucleic acid \(\to\) terminates the polymer \(\to\) prevents viral replication
- Neuraminidase inhibitors (e.g. osteltamivir): Inhibit influenza neuraminidase, which cleaves sialic acid that would otherwise stick new virions together / to the host cell membrane when they bud \(\to\) inhibits egress of new virions