Baltimore classification Totally Explained

Everything about The Baltimore Classification totally explained

The Baltimore classification is a virus classification system which groups viruses into families depending on their type of genome (DNA, RNA, single-stranded (ss), double-stranded (ds) etc.) and their method of 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:
Class I: Double stranded DNA viruses This type of virus usually must enter the host nucleus before it's able to replicate. Furthermore, these viruses require host cell 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's when the cell's polymerases are active. The virus may induce the cell to forcefully undergo cell division, and chronically, this may lead to 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 isn't replicating within the nucleus, that's the Poxvirus family, a highly pathogenic virus that infects vertebrates and includes the smallpox virus.

Class II: Single stranded DNA viruses

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 human Circovirus called TTV is included within this classification and is found in almost all humans, infecting them asymptomatically in nearly every major organ.

Class III: Double stranded RNA viruses

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 isn't 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

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's 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

The negative sense RNA viruses and indeed all genes defined as negative sense can't be directly accessed by host polymerases to immediately form proteins. Instead, they must be 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

A well studied family of this class of viruses include the 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

This small group of viruses, exemplified by the Hepatitis B virus (which is in the Hepadnaviridae family), have a double-stranded, gapped genome that's subsequently filled in to form a covalently closed circle (ccc DNA) that serves as a template for production of viral mRNAs and a subgenomic RNA. The pregenome RNA serves as template for the viral reverse transcriptase and for production of the DNA genome.

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