Baltimore classification of viruses presentation

a brief presentation of the baltimore classification of virus under introduction to vriology. the presentation is by Theophilus A. Apraku

1. BALTIMORE CLASSIFICATION OF VIRUS.

2. BMS THEOPHILUS A. APRAKU. DEPT. OF BIOMEDICAL LAB. SCIENCES . UNIVERSITY OF GHANA SCHOOL OF BIOMEDICAL AND ALLIED HEALTH SCIENCES Presented by:

3. BALTIMORE CLASSIFICATION OF VIRUSES. This kind of viral classification was created by an American Biologist named David Baltimore. It is basically based on the method of viral synthesis . It groups viruses into families according to their type of genome. As such there are seven groups of classification; Groups I - VII

4. CLASSIFICATION GROUP I They are double- stranded DNA viruses . These viruses enter the host’s nucleus before they can replicate. ( EXCEPTION : Poxvirus family ) Again, they require the host’s polymerases to replicate the viral genome. The virus may induce the cell to undergo cell division which may lead to transformation of the cell.(leading to cancer). Examples include H erpesviridae, Adenoviridae and Papoviridae.

5. CLASSIFICATION CT’D. GROUP II They are Single-Stranded DNA viruses . Most of these viruses have circular genomes . ( exception is the Parvoviruses ). Eukaryotic-infecting viruses replicate in the nucleus via rolling circle mechanism , forming a double-stranded DNA intermediate. Examples include; Anelloviridae , Circoviridae , and parvoviridae (vertebrates); Geminiviridae and Nanoviridae (plants), Microviridae (prokaryotes).

6. CLASSIFICATION CT’D GROUP III They are Double-Stranded RNA viruses. They replicate in the core capsid in the cytoplasm. They do not have to use the host’s replication polymerases. Replication is monocistronic and includes individual segmented genome (each gene codes for one protein). Examples include the Reoviridae and Birnaviridae .

7. CLASSIFICATION CT’D GROUP IV They are Single-Stranded RNA viruses-Positive sense Positive sense RNA viruses can be accessed directly by host ribosome to form proteins. They are grouped into two. Viruses With Polycistronic mRNA: genome RNA form mRNA which is translated into polyproteins ; then cleaved into proteins. Viruses with complex transcription . ( subgenomic mRNA, ribosomal frameshifting , and proteolytic processing of polyproteins used). Examples of this class include the families Astrovirdae , Caliciviridae,Coronovirdae , Flaviviridae,Picornaviridae , Arteriviridae , and Togaviridae .

8. CLASSIFICATION CT’D GROUP V They are Single-Stranded RNA viruses – Negative sense. Negative sense RNA viruses cannot be accessed directly by Host’s ribosomes. Instead they must be transcribed into readable forms. They are grouped into two. Viruses with non segmented genome : Negative stranded genome is transcribed into monocistronic mRNA. Replication occurs in the cytoplasm. Viruses with segmented genome : each genome segment produces monocistronic mRNA. Replication occurs in the nucleus. Examples include Arenaviridae , Orthomyxoviridae , Paramyxoviridae , Bunyaviridae , Filoviridae .

9. CLASSIFICATION CT’D GROUP VI They are Postive -Sense Single Stranded RNA viruses . They replicate through DNA intermediates . These viruses use reverse transcriptase to convert positive sense RNA to DNA. The DNA are spliced into host’s genome using integrase and then replication commences. Example include Retroviruses.

10. CLASSIFICATION CT’D GROUP VII They are Double Stranded DNA viruses . They replicate through Single Stranded RNA intermediates . DNA is filled in to form covalently closed circles. These serve as templates for production of viral mRNA and subgenomic RNA. The pregenome RNA serves as a template for viral reverse transcriptase which produces DNA genome. Examples include Hepadnaviridae .

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