Virus: types, classification and reproduction

virus types

Virus classification and reproduction

There is no nomenclature system for virus classification since they are not considered as truly living. As we all know that living organisms have a definite cellular structure. That raises an interesting question of what actually a virus is?

They are named according to the type of host they are using like plant viruses, animal viruses or bacteriophage etc to carry on their lifecycle or the disease caused such as poxvirus or some coded system like T1, T2 phages. Based on these criteria virus classification is broadly done into three classes.

Below describes the virus classification in short with their mode of reproduction, host type and simple diagrams of each.

1. Bacterial viruses or bacteriophage

Bacteriophage, as the name suggests, are those viruses which infect the bacteria. The word bacteriophage is derived from two words namely bacteria and phage “to eat”.

Some examples of common bacteriophages are as follows

a. T4 Bacteriophages

T4 Bacteriphage

These are generally tadpole-shaped viruses. Mainly comprises of capsids with icosahedral head, short neck with collar and a long helical tail. The tail has 6 long fibers with a hollow midpiece and a hexagonal or dome-shaped base plate or endplate. The tail tube of T4 Bacteriophages is made up of gp19 polymerized protein. It is capped by tail tube terminator protein gp3 hexamer. And the contractile sheath is made up of protein gp18 (1). The tail fiber, collar, and whisker are made up of gp34, gp35, gp36, and gp37 proteins.

The T4 genome or chromosome is a single DNA molecule which 60 micrometers long and is packed tightly within the head. The T4 DNA codes for at least 30 different types of enzymes to ensure the rapid replication of the phage chromosomes.

b. M13 bacteriophage

It is a filamentous bacteriophage with circular DNA and infects the E. coli bacteria. This bacteriophage is used in genetic recombination experiments as a cloning vector. The single-stranded DNA molecules are packed inside rod-shaped protein capsids.

2. Plant viruses

Plant viruses are viruses that affect the plants or depends upon the plants to complete their life cycles. It disturbs the plant metabolism and causes serious damage. Some of the important plant viruses are tobacco mosaic virus (TMV), tobacco rattle virus (TRV), potato virus, southern bean mosaic virus (SBMV), turnip yellow virus (TYV). Notable plant diseases caused by viruses are leaf roll of tomato, black-ring spot of cabbage and a little leaf of brinjal. Some examples of plant viruses are

a. TMV virus

This is known as the tobacco mosaic virus one of the first discovered viruses having RNA as nucleoprotein. It is a rod-shaped helical symmetry virus. TMV infects the leaves of tobacco. The infection is caused by some host or vector or mechanical means like rubbing or handling. TMV has a cigarette-like shape with a length of 3000 Angstrom.

The reproduction cycle of TMV

TMV once when gets inside the host it starts synthesizing its own protein and replicates its RNA molecule. After the synthesis of this raw material the assembly started and numerous virus forms and released after the lysis of the host.

b. Potato virus Y

Potato Y Virus

This is an aphid borne virus that causes yield loss and affects tuber quality. Potato virus Y is a filamentous shaped RNA virus with a leant=gth of 400-800 nm. The common symptoms of PVY affected plants are leaf drop, acropetal necrosis.

Reproduction of PVY

Penetrates host cell, unwind its viral genetic material. Use the host cell mechanism and material for its RNA replication and protein synthesis. Assemble and release from the host.

c. Cucumber mosaic virus (CMV)

This is a single-stranded RNA virus that infects the plants of family Cucurbitaceae, tomato, potato, and spinach. Main symptoms of CMV are leaf mottling,  crinkling and curling of edges that occur on cucumber. Petioles and internodes are shortened resulting in the stunted and compact appearance of the plant.

Reproduction of CMV

Replication takes place in the cytoplasm. Host cells recognize the virus and it gets attached to it. The virus then gets un-coats the capsid and releases the genomic RNA into the cytoplasm. RNA1 and RNA2 encode protein la and protein 2a, respectively. New virions are assembled after completion of protein synthesis. Finally, virions are released by lysis of the infected host cell.

3. Animal viruses

This virus infects the animal cells and causes fatal diseases to animals and human beings. The genetic material of animal viruses is either DNA or RNA. The protein coat or capsid of animal viruses are surrounded by an envelope.

Some examples of animal viruses are the common cold, influenza, chickenpox, polio, mumps, rubella.

a. Influenza virus

influenza virus
Influenza Virus

There are generally four main types of influenza viruses namely influenza viruses A, B, C and D. Out of which A is commonly found in animals like ducks, chickens, pigs, whales, and horses. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and the neuraminidase (N). B influenza is common human influenza circulated widely. Influenza type C infections generally cause mild respiratory illness and are not thought to cause epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

b. Poliomyelitis

This is RNA containing animal virus infects humans and spread from one human to another. The virus lives in an infected person’s throat and intestines. It enters the body through the mouth and spreads through contact with feces (poop) of an infected person and, though less common, through droplets from a sneeze or cough. 

This virus has a protein shell in a spherical form with 60 asymmetrical proteins. This shell or capsid has a single RNA molecule with 5200 nucleotides.

c. Herpes virus

It is a DNA containing a human virus with an icosahedral capsid having 162 capsomeres. The DNA of this virus is single and linear douv\ble stranded and codes about 100 average-sized proteins.



Why is virus not a living thing?

Viruses are not classified or do not have a scientific name like other plants, animals and bacteria do. This is because there is still a huge question that are viruses living or nonliving. Some believed that they are not truly living. As viruses do not have a cell structure. They are therefore non-cellular organisms having inert crystalline structure outside the living cells.

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