CLASSIFICATION OF VIRUS
Viruses classified based on various factors viz., Host, disease in human, target tissues, mode of transmission, vector, Structure and Nucleic acids.
• Viral genome’s nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication determine its class.
• Other classifications are determined by the disease caused by the virus or its morphology.
• Viruses can be placed in one of the seven groups.
Classification Based on Structure of Virus
• Structurally Virus are
1. Helical Symmetry:
▶ The protein and nucleic acid are arranged in a spiral.
▶ Examples: Tobacco Mosaic Virus (TMV), Rabies, Influenza.
▶ Most helical viruses are enveloped and RNA viruses.
2. Icosahedral Symmetry:
▶ The protein subunits form a symmetrical shell around the nucleic acid core.
▶ An icosahedron has 12 vertices, 20 faces, and 30 edges.
▶ Examples: Poliovirus, Adenovirus, Hepatitis virus.
▶ Icosahedral capsids are stable and common in human pathogenic viruses.
3. Complex Symmetry:
▶ Includes viruses with more intricate structures.
▶ Examples: Poxvirus, Bacteriophages.
Classification based on the nucleic acid of virus/Baltimore system of Classification
▪ The most commonly used system of virus classification was developed by Nobel Prize-winning biologist David Baltimore in the early 1970s.
▪ The Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.
▪ Viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA).
▪ Viruses can contain
- double-stranded DNA (dsDNA),
- single-stranded DNA (ssDNA),
- double-stranded RNA (dsRNA),
- single-stranded RNA with a positive polarity (ssRNA),
- ssRNA with a negative polarity,
- diploid (two copies) ssRNA,
- partial dsDNA genomes.
Positive polarity means that the genomic RNA can serve directly as mRNA and a negative polarity means that their sequence is complementary to the mRNA.
• 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.
• e.g. Adenoviruses, Herpesviruses, Poxviruses
• 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.
• e.g. Parvoviruses
• 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.
• e.g. Reoviruses
• 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.
• e.g. Coronaviridae, Flaviviridae, Astroviridae, and Picornaviridae
• Group V viruses contain ssRNA genomes with a negative polarity
• dsRNA intermediates are used to make copies of the genome and produce mRNA.
• Full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.
• e.g. Orthomyxoviruses, Paramyxoviridae, Rhabdoviruses
• 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.
e.g. Retroviruses
• 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.
e.g. Hepadnaviruses
Comments
Post a Comment