Baltimore classification
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2008-06-14T23:31:11Z
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/* Class II: Single stranded DNA viruses */
[[Image:Baltimore Classification.png|right|thumb|300px| The Baltimore Classification of viruses, created by the American biologist<ref name="pmid4329869">{{cite journal
|author=Baltimore D
|title=Expression of animal virus genomes
|journal=Bacteriol Rev
|volume=35
|issue=3
|pages=235–41
|year=1971
|pmid=4329869
|doi=
|url=http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=4329869
}}</ref> [[David Baltimore]], is based on the method of viral [[mRNA]] synthesis]]
The '''Baltimore classification''' is a [[virus classification]] [[Scientific classification|system]] which groups [[virus]]es into families depending on their type of [[genome]] ([[DNA]], [[RNA]], single-stranded (ss), double-stranded (ds) etc.) and their method of [[DNA replication|replication]].
==Classifications==
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:
{{Baltimore groups}}
=== Class I: Double stranded DNA viruses ===
{{Main|dsDNA virus}}
This type of virus usually must enter the host [[nucleus]] before it is able to replicate. Furthermore, these viruses require host cell [[DNA polymerase|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 that is when the cell's polymerases are active. The virus may induce the cell to forcefully undergo [[cell division]], and chronically, this may lead to [[cell transformation|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, that is the [[Poxvirus]] family, a highly pathogenic virus that infects [[vertebrate|vertebrates]] and includes the [[smallpox]] virus.
=== Class II: Single stranded DNA viruses ===
{{Main|ssDNA virus}}
Viruses that fall under this category includes ones that are not as well studied, but still do pertain highly to vertebrates. Two examples include the [[Circoviridae]] and [[Parvoviridae]]. They replicate within the nucleus, and form a double stranded DNA intermediate during replication. A prevalent but [[asymptomatic]] human Circovirus called [[Transfusion Transmitted Virus]] (TTV) is included within this classification.
=== Class III: Double stranded RNA viruses ===
{{Main|dsRNA virus}}
As with most [[RNA]] viruses, this class replicates in the [[cytoplasm]], not having to use the host replication polymerases to as much a degree as [[DNA]] viruses. This family is also not as well studied as the rest and includes 2 major families, the [[Reoviridae]] and [[Birnaviridae]]. Replication is [[monocistronic]] and includes individual, segmented genomes, meaning that each of the genes code for only one protein, unlike other viruses which exhibit more complex translation.
=== Class IV & V: Single stranded RNA viruses ===
These viruses consist of two types, however both share the fact that replication is primarily in the cytoplasm, and that replication is not as dependent on the cell cycle as other DNA viruses. This class of viruses are also one of the best well studied, alongside the double stranded DNA viruses.
==== Class IV: Single stranded RNA viruses - Positive (+) sense ====
{{Main|positive-sense ssRNA virus}}
The positive sense RNA viruses and indeed all genes defined as '''[[positive sense]]''' can be directly accessed by host polymerases to immediately form proteins. These can be divided into two groups, both of which reproduce in the cytoplasm:
* Viruses with [[polycistronic]] [[mRNA]] where the genome RNA forms the mRNA and is translated into a [[polyprotein]] product that is subsequently cleaved to form the mature proteins. This means that the gene can utilize a few methods in which to produce proteins from the same strand of RNA, all in the sake of reducing the size of its gene.
* Viruses with complex transcription, for which [[subgenomic]] mRNAs, [[ribosomal frameshifting]] and [[proteolytic]] processing of polyproteins may be used. All of which are different mechanisms with which to produce proteins from the same strand of RNA.
Examples of this class include the families [[Astroviridae]], [[Caliciviridae]], [[Coronaviridae]], [[Flaviviridae]], [[Picornaviridae]], [[Arteriviridae]] and [[Togaviridae]].
==== Class V: Single stranded RNA viruses - Negative (-) sense ====
{{Main|negative-sense ssRNA virus}}
The negative sense RNA viruses and indeed all genes defined as '''[[negative sense]]''' cannot be directly accessed by host polymerases to immediately form proteins. Instead, they must be [[transcription|transcripted]] by viral polymerases into a "readable" form, which is the positive sense reciprocal. These can also be divided into two groups:
* Viruses containing non segmented [[genomes]] for which the first step in replication is transcription from the (-) stranded genome by the viral RNA-dependent RNA polymerase to yield monocistronic mRNAs that code for the various viral proteins. A (+) sense genome copy is then produced that serves as template for production of the (-) strand genome. Replication is within the cytoplasm.
* Viruses with segmented genomes for which replication occurs in the [[nucleus]] and for which the viral RNA-dependent RNA polymerase produces monocistronic mRNAs from each genome segment. The largest difference between the two is the location of replication.
Examples in this class include the families [[Arenaviridae]], [[Orthomyxoviridae]], [[Paramyxoviridae]], [[Bunyaviridae]], [[Filoviridae]] and [[Rhabdoviridae]] (the latter which includes [[rabies]]).
=== Class VI: Positive (+) sense single stranded RNA viruses that replicate through a DNA intermediate ===
{{Main|ssRNA-RT virus}}
A well studied family of this class of viruses include the [[Retroviridae|retroviruses]]. One defining feature is the use of [[reverse transcriptase]] to convert the positive sense RNA into DNA. Instead of using the RNA for templates of proteins, they use DNA to create the templates, which is spliced into the host genome using [[integrase]]. Replication can then commence with the help of the host cell's polymerases. A well studied example includes [[HIV]].
=== Class VII: Double stranded DNA viruses that replicate though a single stranded RNA intermediate ===
{{Main|dsDNA-RT virus}}
This small group of viruses, exemplified by the [[Hepatitis B]] virus (which is in the [[Hepadnaviridae]] family), have a double-stranded, gapped genome that is subsequently filled in to form a covalently closed circle ([[ccc DNA]]) that serves as a template for production of viral [[mRNA]]s and a [[subgenomic]] RNA. The pregenome RNA serves as template for the viral reverse transcriptase and for production of the DNA genome.
==See also==
{{Baltimore classification}}
==References==
[http://athena.bioc.uvic.ca/bioDoc/cuptonclass/virus-taxonomy "Virus Taxonomy Portal."] (Website.) ''Viral Bioinformatics Resource Center & Viral Bioinformatics - Canada''. Retrieved on [[2007]]-[[09-27]].
{{reflist}}
==External links==
*[http://www.virology.net/Big_Virology/BVFamilyGroup.html Family Groups - The Baltimore Method]
* [http://www.ncbi.nlm.nih.gov/ICTVdb/ The Universal Virus Database of the International Committee on Taxonomy of Viruses]
* [http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?name=Viruses The taxonomy portal of the Genbank database]
[[Category:Viruses]]
[[Category:Classification systems]]
[[ro:Clasificare Baltimore]]