Baltimore

The baltimore classification is a virus classification system which groups viruses into families depending on their types of genome ( DNA, RNA , single-stranded , double stranded ) and the ways the viruses replicates.

Class 1 : Double stranded DNA viruses

Double stranded DNA viruses usually enter the host nucleus before it is able to replicate.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 progent requires that the cell be in a replication which is when the cell's polymerases is active thus the virus can then undergo cell division leading to tranformation of the cell and finally cancer. Example : poxvirus.

Class II : single stranded DNA viruses

These viruses replicate within the nucleus and form a double stranded DNA intermediate during replication.
This class of viruses are not very well studied.

Class III: Double stranded RNA viruses

This viruses replicates in the cytoplasm and does not require the hot replication polymerases as much as DNA viruses. Replication is monocistronic and includes individual , segmented genomes which means that each of the genes code for only one protein which is unlike other viruses which exhibit more complex translation.

Class IV and V : single stranded RNA viruses

There are two types of these virus, but they share the fact that replication is primarily in the cyotplasm, and that replication is not as dependent on the cell cycle as other DNA viruses.

Class IV: Single stranded RNA viruses - Positive (+) sense

Positive sense RNA viruses can be directly accessed by host polymerases to immediately form proteins. which are divided into 2 groups both reproduced in the cytoplasm.
(1)
Viruses with polycistronic mRNA where the genome RNA and is translated into a polyprotein product that is eventually cleaved to form the mature proteins.
(2)
Viruses with complex transcription , subgenomic mRNAs . ribosomal frameshifting and proteolytic processing of polyproteins might be used.and proteins produced from the same strand of RNA.


Class V: Single stranded RNA viruses - Negative (-) sense

these viruses cannot directly be accessed by host polymerases to immediately form proteins unlike positive. these viruses must be transcripted by viral polymerases into a form which is positive sense reciprocal. there are 2 groups.
(1)
contains non segmented genomes where first step in replication is transcription from negative stranded genome by the viral RNA-dependent RNA polymerase to yield monocistronic mRNAs which code for various viral proteins.replication occurs within the cytoplasm.
(2)
Contains segmented genomes which replication occurs in the nucleus and where the viral RNA-dependent RNA polymerase produces monocistronic mRNAs from each genome segment. the difference of both is the location os replication.

Class VI: positive sense single stranded RNA viruses that replicate through a DNA intermediate

Feature of this virus is the use of reverse transcriptase to convert the positive sense RNA into DNA. these viruses use DNA to create the templates of proteins instead of the RNA, which is spliced into the host genome using integrase,replication can then start with the aid of the host cell's polymerases. example : HIV

Class VII: Double stranded DNA viruses that replicate though a single stranded RNA intermediate

these viruses which are a small group have a double-stranded ,gapped genome that is subsequently filled in to form a covalently closed circle that serves as a template for production of viral mRNAs and a subgenomic RNA. the pregenome RNA servres as template for the viral reverse transcriptase and for production of the DNA genome.