Classification of Viruses

Viruses are classified into families, genera, and species. Based on the organisms they infect, the first broad division of viruses is into bacterial, plant, and animal viruses. Within these classes, other criteria for subdivision are used. Among these are general morphology envelope or the lack of it; nature of the genome (DNA or RNA); structure of the genome (single- or double-stranded, linear or circular, fragmented or non-fragmented); mechanisms of gene expression and virus replication (positive- or negative-strand RNA); serological relationship; host and tissue susceptibility; pathology (symptoms, type of disease).

The animal DNA viruses are divided into five families: Poxviridae, Herpesviridae, Adeno­viridae, Papovaviridae, and Parvoviridae. RNA animal viruses may be either single-stranded or double-stranded. The single-stranded are further subdivided into positive-strand and negative-strand RNA viruses, depending on whether the RNA contains the messenger RNA (mRNA) nucleotide sequence or its complement, respectively. Further, the RNA genes may be located on one or several RNA molecules (nonfragmented or fragmented genomes, respectively). The positive-strand RNA animal viruses contain six families: Picornaviridae, Calciviridae, Coronaviridae, Togaviridae, Retroviridae, and Nodamuraviridae. The nucleocapsid of negative-strand RNA animal viruses contains an RNA-dependent RNA polymerase required for the transcription of the negative strand into the positive mRNAs. Virion RNA is neither capped nor polyadenylated. The group is divided into five families: Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Rhabdoviridae, and Bunyaviridae. The double-stranded RNA animal viruses contain only one group, the Reoviridae.

This following picture is the Baltimore Classification

I: Double-stranded DNA (Adenoviruses; Herpesviruses; Poxviruses, etc)

Some replicate in the nucleus e.g adenoviruses using cellular proteins. Poxviruses replicate in the cytoplasm and make their own enzymes for nucleic acid replication.

II: Single-stranded (+)sense DNA (Parvoviruses)

Replication occurs in the nucleus, involving the formation of a (-)sense strand, which serves as a template for (+)strand RNA and DNA synthesis.

III: Double-stranded RNA (Reoviruses; Birnaviruses)

These viruses have segmented genomes. Each genome segment is transcribed separately to produce monocistronic mRNAs.

IV: Single-stranded (+)sense RNA (Picornaviruses; Togaviruses, etc)

a) Polycistronic mRNA e.g. Picornaviruses; Hepatitis A. Genome RNA = mRNA. Means naked RNA is infectious, no virion particle associated polymerase. Translation results in the formation of a polyprotein product, which is subsequently cleaved to form the mature proteins.

b) Complex Transcription e.g. Togaviruses. Two or more rounds of translation are necessary to produce the genomic RNA.

V: Single-stranded (-)sense RNA (Orthomyxoviruses, Rhabdoviruses, etc)

Must have a virion particle RNA directed RNA polymerase.a) Segmented e.g. Orthomyxoviruses. First step in replication is transcription of the (-)sense RNA genome by the virion RNA-dependent RNA polymerase to produce monocistronic mRNAs, which also serve as the template for genome replication.b) Non-segmented e.g. Rhabdoviruses. Replication occurs as above and monocistronic mRNAs are produced.

VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (Retroviruses)

Genome is (+)sense but unique among viruses in that it is DIPLOID, and does not serve as mRNA, but as a template for reverse transcription.

VII: Double-stranded DNA with RNA intermediate (Hepadnaviruses)

This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is repair of the gapped genome, followed by transcription.