Classification of Viruses

    The XE variant of Covid-19 has started spreading among people and it is a highly transmissible virus!!! Yes guys, it is Covid-19 pandemic period. Let us see about more viruses. Keep reading and keep sharing!

    What comes to our thought  when we hear the word "virus"? Yes, of course it is Covid-19! Since we have been living in Covid-19 pandemic period, for most of us virus means Covid-19. But there are many other viruses too. Generally, they are known as submicroscopic organisms, means they are not visible through microscope. They are present everywhere, and they are very tiny organisms with DNA or RNA as genetic material enclosed by a protein coat called capsid. Some of them will have envelope which is derived from the host cell. They are generally considered as non-living organisms due to the fact that viruses need a host cell and its components for its replication. Let us have a look on the types of viruses.

Viruses can be classified based on their,

1. Morphology (Based on shape and size)
2. Chemical composition and mode of replication (Based on nucleic acid type)

Morphology based classification

1. Helical Viruses - Tobacco Mosaic Virus
2. Polyhedral Viruses - Adenovirus
3. Spherical Viruses - Coronavirus
4. Complex Viruses - Bacteriophage

Chemical composition and Mode of replication

1. RNA Viruses - Coronavirus, Retrovirus
2. DNA Viruses - Adenovirus, Herpesvirus

    Viruses that have RNA as genetic material is called RNA viruses. They have elevated mutation rate due to the error rate of enzymes involved in RNA replication. RNA virus genome may be of single stranded (ss) or double stranded (ds) RNA. In the case of ssRNA, the RNA can be either sense strand which can function as mRNA or an antisense strand which cannot function as mRNA.

    Viruses that have DNA as genetic material is called DNA viruses. The subtypes are linear dsDNA, circular dsDNA, linear ssDNA and circular ssDNA. The examples of DNA and RNA viruses are mentioned in the below table.

RNA Viruses
Type of Nucleic acid	Examples
ssRNA (sense strand)	Picornavirus, Coronavirus
ssRNA (antisense strand)	Arenavirus, Orthomyxovirus
dsRNA	Reovirus
DNA Viruses
Linear dsDNA	Adenovirus, Poxvirus
Circular dsDNA	Papovavirus
Linear ssDNA	Parvovirus
Circular ssDNA	Circovirus

What is Baltimore System of Classification?

    Viruses do not have complete system for protein production. They are dependent on host cell translational machinery for the protein synthesis. So, the Baltimore system classifies the viruses into 7 groups based on the replication method and type of genetic material present. About 30 years ago, this classification was devised by David Baltimore, a virologist. In this, he assigned the mRNA at the core and the pathways to mRNA from DNA or RNA denotes different classes of viruses. The 7 types of viruses are summarized in the below table.

Baltimore Classification of Viruses
Group	Type of viruses	Examples	mRNA synthesis
Group I	dsDNA viruses	Herpes virus, Adenovirus	mRNA transcription directly from DNA template
Group II	ssDNA viruses	Parvovirus, Circovirus	ssDNA is converted to dsDNA. Then, mRNA transcription
Group III	dsRNA viruses	Reovirus, Birnavirus	mRNA is transcribed from RNA genome
Group IV	ssRNA viruses (positive-sense RNA)	Coronavirus, Picornavirus	Genetic material function as mRNA
Group V	ssRNA viruses (negative-sense RNA)	Paramyxovirus, Orthomyxovirus	mRNA is transcribed from RNA genome
Group VI	Positive-sense ssRNA reverse transcriptase viruses	HIV, Metavirus	Reverse transcription of RNA to DNA, DNA is incorporated into host cell, then subsequent mRNA transcription
Group VII	dsDNA reverse transcriptase viruses	Hepatitis B virus	Viral DNA replication through RNA intermediate, this RNA may serve as template to produce mRNA or may serve as mRNA

References :

• Shaffer, C., PhD. (2019, February 26). The Baltimore Classification System. News-Medical.Net. https://www.news-medical.net/life-sciences/The-Baltimore-Classification-System.aspx.
• Lumen Learning. (n.d.). Virus Classification | Biology for Majors II. https://courses.lumenlearning.com/wm-biology2/chapter/virus-classification.
• Gelderblom HR. Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 41. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8174/.
• Virus. (n.d.). Genome.Gov. https://www.genome.gov/genetics-glossary/Virus.
• A World of Viruses. (n.d.). Harvard Museums of Science & Culture. https://hmsc.harvard.edu/world-viruses.

               

               Thank you!

t