Classification of Virus, based on Nucleic acid, structure and host

Classification of virus, based on Nucleic acid (either DNA or RNA), Structure and Host for Replication. - By : Dhaneesh Ram M V

1. TAXONOMY OF VIRUS Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification system used for cellular organisms. Viruses are classified by phenotypic characteristics, such as nucleic acid, structure, host and morphology. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system.

2. Viruses are classified on the basis of morphology, chemical composition, and mode of replication. Th . e viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria. The following properties have been used as a basis for the classification of viruses . Virion morphology, including size, shape, type of symmetry , presence or absence of peplomers , and presence . or absence of membranes .

3. BALTIMORE CLASSIFICATION Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their  nucleic acid (DNA  or  RNA), strandedness (single-stranded or double-stranded),  sense, and method of  replication. Named after  David Baltimore, a  Nobel prize-winning biologist, these groups are designated by Roman numerals . Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups :

4. I:  dsDNA viruses (e.g. Adenoviruses,  Herpesviruses , Poxviruses) II:  ssDNA viruses (+ strand or "sense") DNA (e.g.  Parvoviruses ) III:  dsRNA viruses (e.g .  Reoviruses ) IV:(+) ssRNA viruses (+ strand or sense) RNA (e.g.  Coronaviruses ,  Picornaviruses ,  Togaviruses ) V: (−) ssRNA viruses (− strand or antisense) RNA (e.g.  Orthomyxoviruses ,  Rhabdoviruses ) VI:  ssRNA -RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g.  Retroviruses) VII:  dsDNA -RT viruses DNA with RNA intermediate in life-cycle (e.g.  Hepadnaviruses )

5. BASED ON STRUCTURE Cubical virus:  They are also known as icosahedral symmetry virus. Eg . Reo virus, Picorna virus. Spiral virus:  They are also known as helical symmetry virus. Eg . Paramyxovirus , orthomyxovirus . Radial symmetry virus:  eg . Bacteriophage . Complex virus:  eg. Pox virus.

6. BASED ON HOST The virus can be classified on the basis of the type of host. They are : Animal viruses Plant viruses Bacteriophage

7. ANIMAL VIRUS : The viruses which infect and live inside the animal cell including man are called animal viruses. Eg ; influenza virus, rabies virus, mumps virus, poliovirus etc. Their genetic material is RNA or DNA. PLANT VIRUS : The viruses that infect plants are called plant viruses. Their genetic material is RNA which remains enclosed in the protein coat. Some plant viruses are tobacco mosaic virus, potato virus, beet yellow virus and turnip yellow virus etc .

8. BACTERIOPHAGES : Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as genetic material. There are many varieties of bacteriophages . Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria .