In this article we will discus about the classification of animal viruses.
Baltimore (2008) classified the animal viruses in the following seven groups according to the relationships between virion, nucleic acid and mRNA transcription Table (17.1).
The RNA within the virion is known as plus (+) or sense strand because it acts as mRNA, whereas the newly synthesized RNA which is complementary in base-sequence to the original infectious strand is called minus (-) or antisense strand. It acts as template to produce additional (+) strand which may act as mRNA.
1 kb = 1,000 base pairs
Class 1. dsDNA viruses:
The mRNA is synthesized on a dsDNA genome template (± dsDNA → (+) mRNA) which usually occurs in a cell.
Following are the example of some viruses:
Papova-viruses: Polyomavirus, SV40
Poxviruses: Vaccinia virus
Adenoviruses: Human adenovirus
Herpes-viruses: Herpes simplex virus type I and type II, Epstein-Barr virus.
Class 2. (+) ssDNA viruses:
In such viruses an intermediate DNA is synthesized before the synthesis of mRNA transcript (+ ssDNA → + mRNA). The mRNA has the same polarity as the DNA.
Parvoviruses: Adeno-associated viruses, mouse minute virus.
Class 3. (+) ssRNA viruses:
The RNA has similar polarity as the mRNA.
Viruses of this class have been grouped into the following two classes:
Subclass 3a: Individual mRNA encodes a polyprotein which is broken later on to form viral proteins.
Picornaviruses: e.g. polio virus.
Subclass 3b: From (+) ss RNA two types of mRNA molecules are transcribed, one is of same length as virion RNA and the other is a fragement of virion RNA.
Togaviruses: Alpha viruses (group A), sindbis virus, semliki forest virus, Haviviruses (group B) e.g. dengue virus, yellow fever, St. Louis encephalitis virus are the important examples.
Class 4. (-) ssRNA viruses:
The virion RNA is complementary to mRNA.
Following two types of viruses are found in this class:
Subclass 4a: The ssRNA genome encodes a series of monocistronic mRNA. Rhabdoviruses: e.g. Mumps virus, measles virus, sendai virus.
Subclass 4b: Each segment molecule of the genome acts as template for the synthesis of mRNA which are monocistronic or encodes polyprotein.
Orthomyxo-viruses: e.g. Human influenza virus
Bunya viruses: e.g. Bunyawera virus
Arena-viruses: e.g. Lassa virus
Class 5. dsRNA viruses:
All the viruses of this class have segmented genome. Each chromosome encodes a single polypeptide. The dsRNA acts as template and asymmetrically synthesize (+) mRNA. Reoviruses: e.g. reovirus of humans.
Class 6. (+) ssRNA-RT viruses:
In these viruses (+) ssRNA directs the synthesis of (-) DNA which in turn acts as template for the transcription of mRNA (RNA→ (-) DNA→ + RNA). Virion RNA and mRNA are of the same polarity.
Retroviruses: e.g. Rous sarcoma virus, mouse leukemia virus.
Class 7. dsDNA-RT viruses:
This group consists of DNA containing hepatitis B Viruses.