Virus Classification

At present about 4000 virus species are known with more than 30,000 strains. How does one actually classify viruses? The International Committee on Taxonomy of Viruses devised a system, based on the genetic material of the virus and the way mRNA is produced (multiplication strategy).

Further subdivisions can be based on items such as (1) genetic material present as one piece or multiple pieces, (2) size of the genome, (3) envelop present or not, (4) morphology.

The decision as to what is species is arbitrary because a species usually contains several strains that may differ substantially (10% or more) in nucleotide sequence. Virus species that exhibit close relationship are grouped into a genus. Genera are grouped into families. Higher taxonomical classifications are rarely used.

The genetic material of viruses can consist of DNA (single or double stranded) or RNA (single or double stranded). The proteins necessary for the construction of complete virions are always made via the information coded in mRNA. By definition, mRNA has positive polarity. There are several different ways in which viral mRNA can be made.

1. Double stranded DNA, from which mRNA is generated.

• Adenoviridae (e.g. adenovirus)

• Poxviridae (e.g. smallpox, vaccinia)

• Herpesviridae (e.g. herpes implex, varicella, Epstein-Barr, CMV)

• Polyomaviridae (e.g. JC virus)

• Papillomaviridae (e.g. warts)

1. Double stranded DNA, from which mRNA is generated. Viral reverse transcriptase produces a negative DNA strand from positive mRNA. The negative DNA strand serves as a template to produce double stranded DNA. Because of this RNA-to-DNA step, they are also known as pararetroviruses.

• Hepadnaviridae (e.g. hepatitis B)

1. Negative single strand DNA, used as a template to produce mRNA.

• Parvoviridae (e.g. parvovirus B19)

1. Double stranded RNA, one strand functions as mRNA.

• Reoviridae (e.g. rotavirus)

1. Negative single strand RNA, used as a template to produce mRNA.

• Rhabdoviridae (e.g. rabies)

• Filoviridae (e.g. Marburg)

• Arenaviridae (e.g. Lassa)

• Bunyaviridae (e.g. Hantavirus)

• Bornaviridae (e.g. bornavirus)

• Orthomyxoviridae (e.g. influenza)

Paramyxoviridae (e.g. mumps and measles)

1. Positive single strand RNA, used to make negative single strand RNA, from which mRNA is produced.

   Flaviviridae (e.g. yellow fever, hepatitis C)

   Picornaviridae (e.g. poliovirus, rhinovirus)

   Togaviridae (e.g. rubella)

   Coronaviridae (e.g. SARS)

   Astro- and Caliciviridae

2. Positive single strand RNA, used as a template to produce negative single strand DNA. This ssDNA is converted to double strand DNA, which then will be used as a template to produce mRNA.

• Retroviridae (e.g. HTLV-1, HIV)

Category: Microbiology Notes