of the severe acute respiratory syndrome coronavirus 2 RNA sequence enabled tests to be manufactured quickly. There are several proven methods for cloning viruses and their components. Small pieces of DNA called cloning vectors are often used and the most common ones are laboratory modified plasmids (small circular molecules of DNA produced by bacteria). The viral nucleic acid, or a part of it, is inserted in the plasmid, which is the copied many times over by bacteria. This recombinant DNA can then be used to produce viral components without the need for native viruses. Phage virology The viruses that reproduce in bacteria, archaea and fungi are informally called "phages", and the ones that infect bacteria – bacteriophages – in particular are useful in virology and biology in general. Bacteriophages were some of the first viruses to be discovered, early in the twentieth century, and because they are relatively easy to grow quickly in laboratories, much of our understanding of viruses originated by studying them. Bacteriophages, long known for their positive effects in the environment, are used in phage display techniques for screening proteins DNA sequences. They are a powerful tool in molecular biology. Genetics All viruses have genes which are studied using genetics. All the techniques used in molecular biology, such as cloning, creating mutations RNA silencing are used in viral genetics. Reassortment Reassortment is the switching of genes from different parents and it is particularly useful when studying the genetics of viruses that have segmented genomes (fragmented into two or more nucleic acid molecules) such as influenza viruses and rotaviruses. The genes that encode properties such as serotype can be identified in this way. Recombination Often confused with reassortment, recombination is also the mixing of genes but the mechanism differs in that stretches of DNA or RNA molecules, as opposed to the full molecules, are joined during the RNA or DNA replication cycle. Recombination is not as common as reassortment in nature but it is a powerful tool in laboratories for studying the structure and functions of viral genes. Reverse genetics Reverse genetics is a powerful research method in virology. In this procedure complimentary DNA (cDNA) copies of virus genomes called "infectious clones" are used to produce genetically modified viruses that can be then tested for changes in say, virulence or transmissibility. Virus classification A major branch of virology is virus classification. It is artificial in that it is