Mutation of Virus

Mutation, an alteration in the genetic material (the genome) of a cell of a living organism or of a virus that is more or less permanent and that can be transmitted to the cell’s or the virus’s descendants.
Like all life, viruses carry a genetic code in the form of nucleic acids — either DNA or RNA.
When cells multiply, the DNA within them replicates as well, to make copies for the new cells.
During replication, random errors are introduced into the new DNA, much like spelling errors when we write.
While the errors in DNA virus genomes can be corrected by the error-correcting function of cells in which they replicate, there are no enzymes in cells to correct RNA errors.
Therefore, RNA viruses accumulate more genetic changes (mutations) than DNA viruses.

Evolution requires not just mutations, but also selection.
While most mutations are deleterious to the virus, if some allow a selective advantage — say better infectivity, transmission, or escape from immunity — then the new viruses out-compete the older ones in a population.
The mutations can be synonymous (silent) or non-synonymous (non-silent); the latter also changes an amino acid (protein building block) at that position in the coded protein.

Viruses with mutations within the receptor-binding domain (RBD) of the Spike protein have the most potential to evade antibodies that develop as a result of natural infection or vaccination.
The RBD binds the cellular receptor allowing the virus to infect cells, and anti-RBD antibodies neutralize the virus.
Such mutations were recently found in variant viruses that emerged in the UK, South Africa and Brazil.

Indirect tests are done in laboratories to assess if an emerging variant might escape antibodies developed after natural infection or vaccination.
Serum (the blood components that contain antibodies) from recovered patients or vaccinated people, and antibodies are known to neutralize the original virus, are tested.
Serial dilutions of the serum or antibodies are separately mixed with a fixed amount of the original and variant viruses, and the mixture is added to cells in culture.
After a period of incubation, cells are washed and stained. Cells infected and killed by viruses multiplying within them appear as clear zones (plaques) on a dark background.
The effectiveness of serum or antibody is expressed as an inhibitory concentration (IC) or plaque reduction neutralisation titer (PRNT) value.