Systematic & Classification of Viruses PDF

Systematic position of viruses According to Burg's manual 1957: Viruses was added to plant kingdom in a special division with blue green algae,bacteria and rickettsia. According to Barrtha and Atlas 1981: Viruses was added to Protests other with microorganisms and considered as Acellular Organism...

Systematic position of viruses According to Burg's manual 1957: Viruses was added to plant kingdom in a special division with blue green algae,bacteria and rickettsia. According to Barrtha and Atlas 1981: Viruses was added to Protests other with microorganisms and considered as Acellular Organism 1  Classification of viruses The classification of viruses poses many problems, and several different approaches have been used. One method is to classify viruses on the basic of their natural host. Hence, there are bacterial viruses plant viruses, and animal viruses. This method is not always satisfactory; for example, a few viruses infect both plants and animal. Another method is based primarily on the type of their nucleic acid (DNA or RNA), and single or double strandedness of the nucleic acid. Structural symmetry also provides additional ciritertion for classification. In 1962, Andre Lwoff, Robert Home and Paul Tournier proposed a system of classification of viruses which is based on certain physical and chemical properties and on the range of hosts that can be infected. These chemical properties are as follows: i. Type of nucleic acid (DNA or RNA). 2  ii. Single or double strandedness of nucleic acid. iii. Capsid morphology (size, symmetry, and number of capsomeres). iv. Presence or absence of envelope. v. Host range. A broad outline of this system (LHT system) of classification is given below: Phylum Vira Sub phylum DEOXYVIRA Sub phylum RIBOVIRA (DNA viruses) (RNA viruses) 1. Sub phylum DEOXYVIRA (DNA viruses) A. Class Deoxyhelica (helical symmetry) Order Chitoviral (enveloped) Family Poxviridae B. Class Deoxycubica (Cubical symmetry) Order Haplovirales (no envelope) Family Microviridae ‫ ـــــــ‬12 Capsomeres Family Parvoviridae ‫ ـــــــ‬32 Capsomeres Family Papiloviridae ‫ ــــــ‬72 Capsomeres Family Adenoviridae ‫ ـــــ‬252Capsomeres Family Iridoviridae ‫ ــــــــ‬812 Capsomeres Family Inoviridae C. Class Deoxybinala (viruses with head and tail) 3  Order Urovirales Family Phagoviridae (bacteriophages) 2. Sub Phylum RIBOVIRA (RNA viruses) A. Class Ribohelica (helical symmetry) Order Rhabdovirales Sub-order Rigidovirales (plant viruses) Family Delichoviridae (12 – 13 nm) Family Protoviridae (15 nm) Family Pachyviridae (20 nm) Sub-order Flexiviridales (plant viruses) Family Leptoviridae (10 – 11 nm) Family Mesoviridae (12 – 13 nm) Family Advoviridae (15 nm) Order Sagovirales Family Myxoviridae (9 nm) Family Paramyxoviridae (18 nm) Family Stomatoviridae B. Class Ribocubica (cuboidal symmetry) Order Gymnovirales ‫ ــــــــ‬32 Capsomeres Family Napoviridae ‫ ـــــــــــ‬32 Capsomeres Family Reoviridae ‫ ــــــــــــــ‬92 Capsomeres Order Togavirales Family Arboviridae 4  Cryptogram: Four pairs to describe viruses First pair: type of nucleic acid / number of strand Second pair: M.W. of nucleic acid / % of M.W. to its virion Third pair: outline of virion / outline of nucleocapsid Four pair: host /method of transmission Tobacco mosaic virus (TMV) according to cryptogram: R/1 2/5 E/E S/O 5