Classification of Viruses
No evolutionary or phylogenetic exist between viruses. A nature system of classification cannot, therefore, be devised for viruses.
Holmes (1948) proposed order vials for the viruses and divided it into three groups on basis of the category of hosts-
1. Zoophaginae - Animal viruses
2. Phytophaginae - Plant viruses and
3. Phaginae - Viruses of bacteria blue green algae and fungi.
In 1962 Lwoff Horne and Tournier decised a system of classifying viruses mainly on bases of nature of genome, capsid symmetry, number of structural units capsomeres in the capsid size of virions, etc. This system was largely adopted by the international committee on taxonomy of viruses (ICTV).
Under the LHT system, viruses are placed under a separate phylum, vira vira is division into two subphyla and each subphylum is further divided into classes as follows-
These include bacteriophages, cyanophages and mycophages. These are shaped like tadpoles or vertebrate sperms with a head and a tail. The genome, contained in the head, is usually double stranded DNA .A few spherical or filamentous phaages, possessing ss DNA or ds RNA ssRNA have also been discovered. Tailed phage's use their tails as hypodermic syringes for injecting their genome into their host cells through the rigid host cell wall.
In respect of host cell phage relationship there are two types of bacteria phage - virulent or tlytic and temperate or lysogenic , Lyrics bacteriophages multiply in host bacterium which then undergoes complete lyses' to release the resulting daughter phage's, In case of glycogenic bacteriophages, the viral genomes gets intergraded into bacterial genome and is, now called lysogeny, and the host bacterium is called lysogenic. The prophase codes for new proteins which impart some new properties to the bacterium .This is called lyogenic or phage conversions. At times of binary fission, or any other stress condition, the prophases may separate from bacterial chromosome and become virulent. Lysogenic bacteriophages may, therefore , act as carriers of genes from one bacterium to others. This is called transduction.
About 600 or so known animals virus have been segregated into about one and a half dozen families. Of these, the following families include the viruses of important human diseases.