CLASSIFICATION OF VIRUSES
Different types of plant, animal and bacterial viruses are classified on the basis of morphology, cover and nucleic acid properties.
The genetic material may be DNA or RNA
The genetic material may be naked, enveloped or complex.
On the basis of morphology viruses are classified into rod shaped (T.M.V), spherical (poliovirus), tadpole like (bacteriophage) and many more figure.
RNA tumor viruses are widely distributed in nature. These produces tumor in the fowls, rodents, cats and many other animals. The most familiar virus is the human immunodeficiency virus (HIV) which causes acquired immune deficiency syndrome (AIDS).
They are single stranded RNA viruses which are spherical in shape and about 100 nm in diameter. They are enveloped by host plasma membrane.
Retroviruses have following characteristics:
A few retroviruses are non-specific and can infect any cell. Most of them infect host cells that have required receptors.
In the case of AIDS virus, the host cell has a receptor that allows viral adsorption and penetration types of leukocytes (white blood cells) and tissue cells.
The retroviruses have a special enzyme called reversetranscriptase. It can convert a single stranded RNA genome into double stranded viral DNA.
Mode of Action:
The DNA of the retrovirus infects the host cells. They also unite with host genome as a provirus that can be passed to progeny cells. In this way some of the retroviruses can convert normal cells into cancer cells.