Baltimore classification

The Baltimore classification, developed by David Baltimore, is a virus classification system that groups viruses into families, depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds), etc..) and their method of replication.

Classifying viruses according to their genome means that those in a given category will all behave in much the same way, which offers some indication of how to proceed with further research. In short:

These types of viruses must enter the host nucleus before they are able to replicate. Furthermore, these viruses require host cell polymerases to replicate the viral genome and, hence, are highly dependent on the cell cycle. Proper infection and production of progeny requires that the cell be in replication, as it is during replication that the cell's polymerases are active. The virus may induce the cell to forcefully undergo cell division, which may lead to transformation of the cell and, ultimately, cancer. Examples include Herpesviridae, Adenoviridae, and Papovaviridae.

There is only one well-studied example in which a class 1 virus is not replicating within the nucleus: the Poxvirus family, a highly pathogenic virus that infects vertebrates and includes the smallpox virus.

The mRNA is transcribed in the normal way from viral DNA using the host transcriptase enzymes, into two types of mRNA's: 1) early mRNA, transcribed prior to the synthesis of viral DNA, and 2) late mRNA, transcribed from progeny DNA.

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