Classification of Viruses

Many classification schemes have been proposed for viruses, but the one in general use is David Baltimore's 1971 scheme, which sorts viruses into seven classes purely by molecular criteria: the nature of the genome (DNA or RNA), whether that genome is single-stranded (ss) or double-stranded (ds), whether a single-stranded RNA genome is of the same sense as mRNA ('sense'/positive) or the opposite sense ('antisense'/negative), and whether the virus relies on the enzyme reverse transcriptase (RT) to convert between DNA and RNA during its life cycle. Each of the seven resulting classes has a well-known representative, from dsDNA adenoviruses through to the RT-dependent retroviruses and hepadnaviruses. Separately, viruses can also be classed by the chemical identity of their genome alone: those with DNA are called 'Deoxyviruses' and those with RNA are called 'Riboviruses'. Most animal and bacterial viruses are DNA viruses (HIV being a notable RNA exception among animal viruses), while most plant viruses carry RNA (Cauliflower Mosaic Virus being a notable DNA exception). On this basis viruses fall into four categories: ssDNA (e.g. Parvoviruses), dsDNA (e.g. Bacteriophages), ssRNA (e.g. TMV) and dsRNA (e.g. wound tumour v …