The word virus is derived from Latin word Venom. Venom means poisonous fluid. Viruses can be defined as non-cellular infectious particle which contains either RNA or DNA. It is normally enclosed in a proteinaceous coat and it reproduces only in living cells”. Viruses use the biosynthetic systems of their hosts for their own synthesis. They are then transferred to other cell.
Virus was called poison during the time of Louis Pasteur (1822-1895) and Robert Koch (1843-1910). The viruses caused disease and death. The present concept of virus has entirely changed. Now viruses are described, as particles of nucleic acid with a protein coat. They replicate in the cell and cause many diseases like influenza, hepatitis, small pox and AlDS. The branch of biology which deals with the study of virus is known as Virology.
Viruses cause many diseases. First vaccine was discovered against virus. Edward Jenner vaccinated an 8 year boy in 1796. Cowpox is a viral disease of cow. The cow develops lesion on its skin. Material (pus) was removed from this lesion on the hand of milkmaid. Jenner used this pus for vaccination of the boy.
After 6 weeks, Jenner removed pus from the small pox patient. He injected this pus into the boy. The boy did not develop disease. So the boy had been successfully vaccinated. Later, Jenner used material from cowpox lesion for vaccination. He successfully vaccinated 23 persons. Cow is called vacca in Latin. As the material for vaccination, was obtained from cow or vacca. So, Louis Pasteur used the term Vaccination.
He was an associate of Pasteur. In 1884, he found that the bacteria cannot pass through porcelain filters. While Virus causing rabies can pass through these filters. Rabies is a human disease. It is transferred to human by the biting of rabid dog, fox, cats, bats and other animals. In those days, people called virus a toxic substance as it caused diseases. They described these unseen diseases causing agent as filterable viruses.
Work of lvanowski
He found in 1892 that the viruses of tobacco mosaic disease are filterable. Lvanowski grounded the infected plant. He took bacteria free filtrate from it. Lvanowski placed this filtrate on the healthy leaves of tobacco. This filtrate produced disease in the healthy plant. Later, it was discovered that similar filterable ultramicroscopic viruses also caused foot and mouth disease (1898) and yellow fever (1901) in animals.
Bacteriophage or phage viruses are viruses that infects bacteria. These were independently discovered by Twort in 1915 and D’Herelle in 1917. Twort observed that sometimes, bacteria undergo lysis. During lysis, bacteria break, dissolve and disappear. This lysis can be transferred from one colony to the other.
Even a small amount of material from this lysed colony can transfer this lysis effect. If this filtrate is heated, the lysis property is destroyed. He concluded that the lytic agent was virus. D’Herelle rediscovered this phenomenon in 1917. He used the word Bacteriophage, for that virus. Bacteriophage means “bacteria eater”
Isolation of Virus by Stanley
Stanley purified and crystallized the filterable tobacco mosaic virus in 1935. He chemically analyzed these particles. The particle contained nucleic acid and protein. It suggests that viruses have simple chemical composition.
Characteristics of viruses
Viruses show following characteristics:
- The viruses have very small size. Their size is less than bacteria. They are not visible under compound microscope and can be seen only by electron microscope.
- The size of virus is from 250 nanometer (1nm = 10-9 meter) of pox viruses (small pox causing virus) to 20 nm of Parvoviruses (chronic anemia causing virus).
- They are 10 to 1000 times smaller than most of bacteria. So they can pass through the filter from which bacteria cannot pass.
- Viruses cannot grow on artificial med. They can reproduce in animals and plant cells or microorganisms.
- They reproduce by replication. Many copies or replicas of a virus are form in this process.
- The viruses are obligate intercellular parasite, a parasite that cannot live without its host.
- The viruses lack metabolic machinery. So they cannot synthesize their own nucleic acid and proteins. They depend on their host cell for carrying out these functions.
- The viruses may cause disease in host cells during reproduction.
- All viruses are resistant to antibiotics like penicillin, streptomycin and others.
- They infect only hosts and specific tissues. Tobacco Mosaic Viruses can infect only tobacco, potato and few other plants, while Polio Virus causes disease in man, monkey and no other animals.
Structure of Virus
1. Central core
The complete viruses are called virion. The virion is composed of central core of nucleic acid, further more this nucleic acid may be DNA or RNA. This nucleic acid is also called genome.
The Capsid forms protein coat around the nucleic acid. So virus is also called nucleocapsid. The Capsid gives definite shape to virion. Capsid is made up of protein subunit called capsomeres. Each virus has characteristic numbers of capsomeres. For example, the Capsid of herpes viruses (causes chicken pox) has 162 capsomeres and Capsid of adenovirus (causes common cold) has 252 capsomeres.
3. The envelope
Some animal viruses are covered by another membrane called the envelope. This envelope is derived from the host cell. The non-enveloped viruses are called naked virion.
4. Viral Tail Fibers
In some viruses, fiber like structures are present at the lower side known as tail fibers. These fibers help in the attachment of virus to the body of host.
Shape and Types of viruses
Shapes of Viruses:
- PoIyhedron: having many sides.
- Helical or spiral.
Types of Viruses:
- Plant Viruses: These viruses attack the plants and cause many disease.
- Animal Viruses: These viruses cause disease in men and other animals.
- Bacteriophages attack bacterial cells. They have two different structural shapes:
- Cubical Phage: They are regular solid or icosahedral (having 20 faces).
- Helical Phage: They have a rod like shape.
Many phages consist of head and tails. The heads of these viruses are polyhedral but tails are rod shaped.
Classification of viruses
The viruses are classified on the basis of morphology and nucleic acid properties. The genetic material may be DNA or RNA. It may be naked or enveloped (by protein). It may be simple or complex. Viruses are classified into following types on basis of their morphology:
- Rod shaped:M.V (Tobacco Mosaic Viruses)
- Tadpole like:
Recent Classification of Viruses
The recent classification of viruses is related to their mode of origin. According to this classification the viruses are of the following types:
- Unenveloped plus-strand RNA virus: These viruses are without enveloping. They act as m-RNA so they are plus-strand, e.g. polio virus and Rhino virus.
- Enveloped plus-strand RNA viruses: They also act as m-RNA. They have lipid rich envelop. They infect Arthropods and vertebrates called capsid, e.g. Hepatitis A, C and Arbo virus.
- Minus-strand RNA viruses: These viruses do not act as m-RNA. They have carry the RNA strand complimentary to the m-RNA envelop called capsid, e.g. Rhabdo virus, Paramyxo virus.
- Retro virus: These viruses causes Aids or Hepatitis-B. In Aids, these are single- stranded RNA viruses while in Hepatitis-B these are double stranded DNA viruses. It replicates using reverse transcriptase enzyme in host cell produce DNA from its RNA genome.
- Double stranded RNA viruses: They have double strand of RNA with capsid, e.g. Reo virus and Colorado tick fever.
- Small genome DNA viruses: These viruses contain small DNA genome. They contain envelop and capsid, e.g. Parvo virus cases Hepatitis warts in man.
- Medium-genome and large genome DNA viruses: These viruses contain medium sized or large sized genome with capsid and envelop, e.g. Herpes virus and Cancer virus.
- Bacteriophage: These viruses attack bacteria, and also they have hexagonal head and tail. In the head DNA is present, covered by capsid.