Baltimore classification system of viruses (mrNA synthesis & replication strategy)

The most commonly used system of virus classification was developed by Nobel Prize-winning biologist David Baltimore in the early 1970s. In addition to the differences in morphology and genetics mentioned above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by transcription in much the same way as with cellular DNA.

Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur.

Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.

Group IV viruses have ssRNA as their genome with a positive polarity. Positive polarity means that the genomic RNA can serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates, are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity (complimentary to the positive-stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.

Group V viruses contain ssRNA genomes with a negative polarity, meaning that their sequence is complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.

Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse transcriptase, to dsDNA; the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome.

Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication. The characteristics of each group in the Baltimore classification are summarized in Table 3 with examples of each group.

It's advantages are:-

· Classification between positive strand RNA viruses that do & do not undergo Reverse Transcription. (Class 4 are positive sense singlestranded RNA like Picornaviruses is the latter one and Class 6 are positive sense singlestranded RNA viruses like Retroviruses is the former one)

· It can also classify between double stranded DNA viruses that do & do not undergo Reverse Transcription. (Class 1- Adenoviruses, Herpes viruses is the latter & Class 7- Hepadna viridae is the former one).

· It is very helpful in understanding the genome replication of viruses. .


Although no major books have mentioned it(as per my knowledge) but I think these can be it's disadvantages:-

· No common ancestors of viruses have been shown. No phylogentic tree. Also, from where viruses came it still debatable.

· It only tells us about the genomic material. No structural or morphological details can be inferred from this type of classification.

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