Virus Classification on the basis of morphology and replication
Viruses are a specific group of microorganisms that can replicate only inside of a cell. These are extremely small organisms and visible only under an electron microscope. Viral infections can occur in all types of life forms including plants, animals and other microorganisms (bacteria, archaea). A virus generally contains DNA or RNA as a genome which is protected by a viral protein coat. Some enzymes are often attached to the genome of the virus. A complete viral particle is known as a virion. At the time of infection, viruses insert their genome inside of a host cell, which is then incorporated in the host genome. As a result, the host genome codes the viral proteins important for viral replication.
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These viruses are classified under several groups on the basis of morphology and mode of replication which is further described in the following paragraph.
There are several viruses found with a helical morphology. These viruses consist of identical protein subunits or protomers which assembled in a helical structure around the genome. This type of protein subunits generally forms a rigid nucleocapsid. Moreover, the helical structure provides flexibility to the filaments. The morphology of this type of viruses can be analyzed by an electron microscope. The helical nucleocapsid is also characterized by length, width, number of promoters per helical turn and pitch of the helix. The most common example of a helical virus is the tobacco mosaic virus. Apart from that Sendai virus is also reported to have a helical structure.
An icosahedron structure refers to a type of polyhedron with 20 equilateral triangular faces and 12 vertices. The rigid structure provides protection to the genome. The common examples of viruses reported to have an icosahedral structure are papovavirus, picornavirus, adenovirus, toga virus, etc.
These groups of viruses do not come under the above-motioned groups. These viruses consist of complex structural components which made it different from the other two groups. A common example of this group of the virus is the pox virus.
More of Replication
RNA virus genome
Around 70% of viruses contain RNA as their genome. Several studies reported that RNA viruses show higher mutation rates than DNA viruses due to the errors of the enzyme involved in RNA replication. Inside the capsid, the genome can be either single-stranded RNA (ss RNA) OR double-stranded RNA (ds RNA). Apart from that, the strands can be intact or fragmented. In the case of ss RNA, the RNA strand can function either as a sense strand or as an antisense strand. The sense strand can function as mRNA, therefore, can involve directly in the production of viral proteins. While the antisense strand is complementary to its mRNA, therefore, has no translational function. Several members of the reovirus contain fragmented ds RNA and each fragment is attached with its complementary strand. The retrovirus genome, on the other hand, contains two identical sense ssRNA, along with that reverse-transcriptase is also attached to the RNA strands.
DNA virus genome
Linear double-strand DNA (ds DNA) is mostly found as a genome in DNA viruses. However, there are exceptional cases such as papovaviruses which contain circular DNA genome. Apart from that, single-strand DNA viruses (ss DNA) are also found in members of the parvovirus family. Circular single-stranded DNA (ss DNA) is found in members of the circovirus family.
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