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 CLASSIFICATIONS
I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses) CLASSIFICATIONS Genome organization within this group varies considerably. Some have circular genomes (Baculoviridae, Papovaviridae and Polydnaviridae) while others have linear genomes (Adenoviridae, Herpesviridae and some phages). Some families have circularly permuted linear genomes (phage T4 and some Iridoviridae). Others have linear genomes with covalently closed ends (Poxviridae and Phycodnaviridae).
I: dsDNA viruses CLASSIFICATIONS Most have head-tail morphologies and linear double- stranded DNA genomes. Other morphologies have also been described: spindle shaped, rod shaped, filamentous, icosahedral and spherical. Additional morphological types may exist.
II: ssDNA viruses CLASSIFICATIONS All viruses in this group require formation of a replicative form – a double stranded DNA intermediate – for genome replication. This is normally created from the viral DNA with the assistance of the host's own DNA polymerase.
II: ssDNA viruses CLASSIFICATIONS A division of the circular single stranded viruses into four types has been proposed. This division seems likely reflects their phylogenetic relationships. Type I genomes are characterized by a small circular DNA genome (approximately 2-kb), with the Rep protein and the major open reading frame (ORF) in opposite orientations. This type is characteristic of the circoviruses, geminiviruses and nanoviruses.
II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses) CLASSIFICATIONS Type III genomes contain two major ORFs in the same orientation. This arrangement is typical of the anelloviruses. Type II genomes have the unique feature of two separate Rep ORFs. Type IV genomes have the largest genomes of nearly 4-kb, with up to eight ORFs. This type of genome is found in the Inoviridae and the Microviridae. Given the variety of single stranded viruses that have been described this scheme – if it is accepted by the ICTV – will need to be extended.
III: dsRNA viruses CLASSIFICATIONS Double-stranded (ds) RNA viruses are a diverse group of viruses that vary widely in host range (humans, animals, plants, fungi, and bacteria), genome segment number (one to twelve) and virion organization (T-number, capsid layers or turrets).
III: dsRNA viruses CLASSIFICATIONS Members of this group include the rotaviruses, known globally as a common cause of gastroenteritis in young children, and bluetongue virus, an economically important pathogen of cattle and sheep.
IV: (+)ssRNA viruses CLASSIFICATIONS An RNA virus is a virus that has RNA (ribonucleic acid) as its genetic material. This nucleic acid is usually single-stranded RNA (ssRNA), but may be double- stranded RNA (dsRNA). Notable human diseases caused by RNA viruses include SARS, influenza, hepatitis C, West Nile fever, polio and measles.
IV: (+)ssRNA viruses CLASSIFICATIONS The proposed classification of positive strand RNA viruses is based on the RNA dependent RNA polymerase. Three groups have been recognised: I. The picorna like group (Picornavirata) II. The flavi like group (Flavivirata) III. The alpha like group (Rubivirata)
V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses) CLASSIFICATIONS Negative-sense ssRNA viruses must have their genome copied by an RNA-dependent RNA polymerase to form positive-sense RNA. This means that the virus must bring along with it the RNA replicase enzyme. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host ribosomes. The resultant protein goes on to direct the synthesis of new virions, such as capsid proteins and RNA replicase, which is used to produce new negative-sense RNA molecules.
VI: ssRNA-RT viruses CLASSIFICATIONS A retrovirus is an RNA virus that replicates in a host cell. First it uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, reverse of the usual pattern, thus retro (backwards). This new DNA is then incorporated into the host's genome by an integrase enzyme. The cell then treats the viral DNA as part of its own instructions, which it follows blindly, making the proteins required to assemble new copies of the virus.
VII: dsDNA-RT viruses CLASSIFICATIONS DsDNA-RT viruses are not considered as DNA viruses (class I of Baltimore classification), but rather reverse transcribing viruses because they replicate through an RNA intermediate. It includes Hepadnaviridae and Caulimoviridae. The term "pararetrovirus" is also used for this group.
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