What is a Virus? What are Viruses made up of ?
1. What is a Virus?
Viruses are the smallest infectious agents, ranging from 20 nm to 400 nm in diameter. By comparison, the average size of a bacterial cell is 1 µm, hence 2-50 times bigger. Viruses are not cells: they are not capable of self-replication and are not considered “alive”. Viruses do not have the ability to replicate their own genes, to synthesise all their proteins or to replicate on their own; thus, they need to parasitise the cells of other life-forms to do so. Viruses invade cells then hijack the cell’s machinery to promote their replication. Newly formed viruses are then released from the host cell to invade more cells.
Viruses are simple, and are made up of up to three constitutive elements:
- A genome, made up of nucleic acids that can be DNA (like humans) or RNA. RNA is very similar to DNA; both are made up of of chains of nucleotides (ACGT/U) that make up genes that are translated in proteins.
- The genome is protected by a protein coat, called a capsid. The capsid proteins (capsomeres) assemble around the viral genome in different arrangements, termed either helical (e.g. influenza or coronavirus) or icosahedral;a more complex 20 sided shape (e.g. herpes or norovirus).
- Some viruses possess an outer lipid envelope, which is derived from the host cell and can take many forms. Coronaviruses, like SARS-CoV-2, are spherical, Ebola virus is long and filament shaped. Enveloped viruses are more susceptible to destruction of the lipid through the use of soap or alcoholic hand-gels, than viruses without lipid envelopes.