The virus can be classified on the basis of the type of host and genetic material. They are of three types. They are:
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 isRNA or DNA.
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.
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.
On the basis of nucleic acids, viruses are classified into following three types;
The viruses that possess DNA as genetic material is called DNA viruses. Bacteriophages, cyanophages and most of the animal viruses contain DNA as genetic material.
The virus that possesses RNA as genetic material is called RNA viruses. Eg; plant virus
The RNA tumor viruses called Leukoviruses and Rous's viruses contain both DNA and RNA as genetic material.
On the basis of presence of number of strands, four type of nucleic acid have been in viruses
Double stranded DNA;
It is found in pox viruses, the bacteriophages T2, T4, T6, T3, T7 and Lamda, herpes viruses, adenoviruses etc.
It is found in bacteriophagesφ, X, 74 bacteriophages.
It has been found within viral capsid in the reoviruses of animals and in the wound tumour virus and rice dwarf viruses of plants.
Viroid: Small viruses without capsid.
It is found in most of the RNA viruses eg: tobacco mosaic virus, influenza virus, poliomyelitis, bacteriophage MS-2, Avian leukemia virus.
The viral genome is very small containing just a few genes that enable viruses to infect their respective host cells and code for the synthesis of few proteins for the capsid. Viruses are host biosynthetic machinery ( ribosomes, RNA, and enzymes ) and raw materials ( nucleotides and amino acids ), and synthesis copies of their own nucleic acid and protein.
Three stages of bacteriophages as suggested by Lwoff are extracellular virons, vegetative phage and prophage.