VIRUS CLASSIFICATION

Dr.Jitender Kumar

Department of Biotechnology

HMV,Jalandhar

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Definition

• They are aceelular,ultramicroscopic,obligatory parasites made up of nucleoprotein and have the ability to replicate within living host.

Characters of Viruses

• They are non-cellular organisms, which is enclosed in a protective envelope.
• The presence of spikes helps in attaching the viruses to the host cell.
• These viruses do not grow, neither respire nor metabolize, but they reproduce.
• They are surrounded by a protein coat – capsid and have a nucleic acid core comprising DNA or RNA.
• They are considered both as living and non-living things. These viruses are inactive when they are present outside of host cells, but become active within host cells. These viruses cause several infections and reproduce within the host cell by using the enzymes and raw materials

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Classification

• Viruses can store their genetic information in six different types of nucleic acid which are named based on how that nucleic acid eventually becomes transcribed to the viral mRNA capable of binding to host cell ribosomes and being translated into viral proteins.

Classification based on the presence of nucleic acid�

• The virus, having DNA as its genetic material. There are two different types of DNA virus
• Single-stranded (ss) DNA virus: e.g. Picornaviruses, Parvovirus,  etc.
• Double-stranded (ds) DNA virus: e.g. Adenovirus, Herpes virus, etc.

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RNA Virus�

• The virus, having RNA as its genetic material. There are two different types of RNA virus
• Double-stranded (ds) RNA virus: e.g. Reovirus, etc.
• Single-stranded (ss) RNA virus.
• It is further classified into two Positive sense RNA (+RNA) and negative sense RNA (-RNA).
• Poliovirus, Hepatitis A, Rabies virus, Influenza virus are examples of single-stranded RNA virus.

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Genetical material

• Both (+) and (-) represent complementary strands of nucleic acid.
• Copying of a (+) strand by complementary base pairing forms a (-) strand. Only a (+) viral mRNA strand can be translated into viral protein.

Enzymes

• Regarding the enzymes involved in nucleic acid replication, the "dependent" part of the name refers what type of nucleic acid is being copied.
• The "polymerase" part of the name refers what type of nucleic acid is being synthesized, e.g., DNA-dependent RNA-polymerase would synthesize a strand of RNA complementary to a strand of DNA

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Double-stranded DNA (+/-)

•  To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy both the (+) and (-) DNA strands producing dsDNA viral genomes.

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Double-stranded DNA (+/-)

• To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA.
• The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include most bacteriophages, Papovaviruses, Adenoviruses, and Herpesviruses.

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Single-stranded DNA (+)�

•  To replicate the viral genome, DNA-dependent DNA polymerase enzymes copy the (+) DNA strand of the genome producing a dsDNA intermediate.
• DNA-dependent DNA polymerase enzymes then copy the (-) DNA strand into ss (+) DNA genomes.
• To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA.
• The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include Phage M13 and Parvo viruses.

Double-stranded RNA (+/-)

•  To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy both the (+) RNA and (-) RNA strands of the genome producing a dsRNA genomes.
• To produce viral mRNA molecules, RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA.
• The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Reoviruses are an example

RNA (-)

• To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (-) RNA genome producing ss (+) RNA.
• RNA-dependent RNA polymerase enzymes then copy the (+) RNA strands producing ss (-) RNA viral genome.
• The (+) mRNA strands also function as viral mRNA and can then be translated into viral proteins by host cell ribosomes.
• Examples include Orthomyxoviruses, Paramyxoviruses, Rhabdoviruses.

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RNA (+)

•  To replicate the viral genome, RNA-dependent RNA polymerase enzymes copy the (+) RNA genome producing ss (-) RNA.
• RNA-dependent RNA polymerase enzymes then copy the (-) RNA strands producing ss (+) RNA viral genome.
• To produce viral mRNA molecules. RNA-dependent RNA polymerase enzymes copy the (-) RNA strand into (+) viral mRNA.
• The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes. Examples include Picornaviruses, Togaviruses, and Coronaviruses.

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(+) RNA Retroviruses

• To replicate the viral genome, reverse transcriptase enzymes (RNA-dependent DNA polymerases) copy the (+) RNA genome producing ss (-) DNA strands.
• DNA-dependent DNA polymerase enzymes then copy the (-) DNA strands to produce a dsDNA intermediate.
• DNA-dependent RNA polymerase enzymes then copy the (-) DNA strands to produce ss (+) RNA genomes.
• To produce viral mRNA molecules, DNA-dependent RNA polymerase enzymes copy the (-) DNA strand into (+) viral mRNA.
• The (+) viral mRNA can then be translated into viral proteins by host cell ribosomes.
• Retroviruses, such as HIV-1, HIV-2, and HTLV-1 are examples.

References

• General Microbiology by R.P.SINGH
• Biotechnology expanding horizon by B.D.Singh

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• Thanks