FIG 2.

The amended scheme of the seven Baltimore classes of viruses. Shown is the transfer of genetic information between genomic nucleic acids encapsidated into virions (genomes, for short) and mRNA. This transfer is enabled by enzymatic reactions; in the case of the RNA viruses, with the exception of hepatitis delta and related viruses, these reactions are catalyzed by the virus-encoded RNA-dependent RNA polymerases, which are encapsidated into virions of viruses of BCIII and BCV, but not BCIV; reverse-transcription of RNA to DNA is catalyzed by virus-encoded reverse transcriptases, which may or may not be encapsidated into virions; replication of DNA genomes is catalyzed by virus-encoded or host-encoded DNA polymerases, which are not encapsidated. The roman numerals denote the BCs. BCVII was added to the scheme. Adapted from reference 1 with permission.