All viruses are belonged this phylum. It has two sub-phyla:
- Sub-Phylum: Deoxyhelica
The sub-phylum includes viruses with DNA core. It has three classes.
(i) Class Deoxyhelica: This class includes DNA viruses. They have helical symmetry. This class has only one order: Chaetovirales. It has a single family: Poxviridae.
(ii) Class Deoxycubica: This class contains DNA viruses. But they have cubical symmetry. It has two orders:
(a) Haplovirales: They are without envelope. It has six families.
(b) Peplovirles: They have envelope. It has only one family: Herpesvirdae.
(iii) Class Deoxybinala: The class possesses DNA containing binal viruses. They have single order Urovirates. It has only one family Phagoviriclae.
- Sub-Phylum: Ribovira
These viruses contain RNA core. It is further divided into two classes.
(i) Class Ribohelica: It includes RNA viruses. They have helical
symmetry. The class includes two orders: •
- Rhabdovirales: They are without envelopes. It has six families.
- Sagorvirales: They have envelope. It has three families.
(ii) Class: Ribocubica: The class contains RNA viruses. They have cubical symmetry, It consists of two orders:
- Gymnovirales: They are without envelope It has two families.
- Togovirales: They have envelopes. It consists of single family.
The RNA viruses which can convert their single stranded RNA into double stranded DNA are called retroviruses. These viruses have special enzyme called reverse transcriptase. This enzyme is responsible for the conversion of RNA into DNA. This DNA infects the host cells. It can also incorporate into host genome (DNA) as
Provirus. This provirus can pass to daughter cells. Therefore, some of the retroviruses can convert normal cells into cancer cells. Most of the retroviruses are specific. They infect only those host cells which possess required receptors. For example, in case of AIDS, the host cells like leukocytes (white blood cells) and tissue cells possess receptors. These receptors allow the adsorption and penetration of these viruses. Two types of retroviruses are most common:
(a)RNA tumor viruses: These are single stranded RNA
retroviruses or oncoviruses. Their diameter is about 100nm. Their shape is spherical. They are enveloped by plasma membrane of the host. These viruses have been known for many years. These viruses are widely distributed in nature. They produce tumor in a number of animals like fowl, rodents and cats.
(b)HIV Virus: The most familiar virus is the Human immunodeficiency virus (HIV). It causes acquired Immune deficiency syndrome (AIDS).