Viruses are the smallest filterable ultra microscopic particle present in the environment surrounding us. It is an obligatory parasite totally dependent on host cells. Outside host cells, they are inactive.
The word virus means poison and 1st discovered by Ivanovsky (1892); during the study of tobacco leaves infected by tobacco mosaic disease. These microbes are not affected by antibiotics; they can be crystallized and stored in bottles.
W.M Stanley awarded Nobel Prize (1946) for obtaining the tobacco mosaic virus (TMV) in a pure crystalline form.
The truly distinctive features of viruses are their simple structural organization and their mechanism of multiplication. They contain a single type of nucleic acid, either DNA or RNA.
They also contain a protein coat, sometimes itself covered by an envelope of lipids, proteins, and carbohydrates.
Multiply inside living cells by using the synthesizing machinery of the cell.
Viruses have few or no enzymes of their own for metabolism.
The host range of a virus is the spectrum of host cells the virus can infect.
There is the virus that infects invertebrates, plants, protests fungi, and bacteria. Viruses that infect bacteria are called bacteria phages or phages.
The size of viruses varies from 10 mill microns to 350 mill microns (mm). Even the most abundant viruses are smaller than the smallest bacteria.
A virus has a different type of shapes these are
- Rod-shaped or cylindrical (Tobacco mosaic virus )
- Rectangular ( Vaccine virus )
- Polyhedral ( Adana virus )
- Spherical ( Influenza and polio viruses )
- Tadpole shaped (Bacteria phages)
Type of viruses
Based on the kind of host, viruses are of a different kind. These are
(i) Plant virus (pagophagia)
These are parasites on the plant cells. They possess single-stranded RNA.
The common plant viruses are tobacco mosaic virus (TMV), tobacco rattle virus (TRV), Bee yellow virus (BYV), potato virus, turnip yellows virus (TYV), and others.
(ii) Animal viruses
Animal viruses mostly attack animals, including humans. They possess double-stranded DNA. However, polio and influenza virus contains RNA among animal viruses.
The most dangerous viruses are chickenpox, smallpox rabies poliomyelitis encephalitis, mumps, influenza, hepatitis, dengue fever, common cold, trachoma, and most deadly AIDS (Acquired Immune deficiency syndrome).
The virus responsible for AIDS is HTLV-iii or HIV (Human Deficiency virus).
Some leukemia’s and cancers are also caused by viruses these viruses are called Reo-virus and one –virus.
Viruses that parasitize bacteria are called bacteriophages. These were first described by Heeler (1917). These contain DNA as genetic material.
These viruses infect blue-green algae. They contain DNA as genetic material.
These viruses infect yeast. They also contain DNA as genetic material. In-plant, viruses are RNA, while in other viruses, it is DNA.
Structure of the virus
A virus consists of a central core of nucleic acid surrounded by a protein coat.
A virion is a complete, fully developed infectious viral particle composed of nucleic acid and surrounded by a protein coat outside a host cell.
In prokaryotic and eukaryotic cells, the primary genetic material is DNA, and RNA plays a supporting role, but in the case of a virus, either DNA or RNA as the nucleic acid of a virus can be single-stranded or double-stranded.
These include bacterial and most of the animal viruses. Their DNA may be either
(i) Double-stranded linear molecule
(ii) double-stranded circular molecule
(iii) single-stranded linear molecule or
(iv) single-stranded circular molecules.
These include all plant viruses and a few animal viruses.
The RNA may be (I) single strand or (ii) double-stranded.
In some viruses like the influenza virus, the nucleic acid is in several separate segments.
The nucleic acid has genetic information for the synthesis of protein that either form capsid or new virions or can redirect the host cell to synthesize virus protein.
A protein coat protects the nucleic acid of a virus called the capsid. The capsid is composed of a large number of protein subunits (all are the same size). These units are known as capsomers.
The capsomers in the capsid are arranged either in a helical pattern or form a polyhedron. The helical capsids are found in plant and bacterial viruses, whereas polyhedral capsids are present in animal viruses, causing chickenpox, common poxviruses.
In some viruses, the capsid is covering by an envelope, usually consisting of some combination of lipids proteins and carbohydrates.
Bacteria phages are viruses which are obligatory parasites on bacteria. These are tadpole-shaped viruses.
Bacteria phages that infect E.coli are known as coliphage or T4 phages.
The bacteriophage is divisible into five regions. They are head, tail, collar, endplate, and tail fibril.
The head is hexagonal, which consists of a core containing double-stranded DNA and capsid made up of proteins.
The tail consists of an empty core surrounded by a protein sheath. Head and tail jointed by a collar. A hexagonal plate is present at the tip of the tail. It Produces six tail like fibers which are useful for attachment with the bacterial cell during infection.
Reproduction and life cycle of Virus
A free virus is inert because metabolism does not occur in it due to the absence of a metabolic enzyme.
A virus particle capable of infecting a specific host organism is known as a virion. Certain viruses enter the host cell by the process of fusion or phagocytosis, but most of them, in their capsids, possess a lytic enzyme known as neuraminidase.
As a virus particle comes in contact with a live cell, it sticks to the plasma membrane of the cell and dissolves it, at the point of contact with its lyric enzyme. Then, the virus’s nucleic acid enters into the host cell, while its capsid is left outside.
After entering the host cell, the viral nucleic acid assumes full control of the entire cellular metabolism. It employs the cell in its rapid multiplication so that thousands of viral nucleic acid filaments form in a short time. Thus the viral nucleic acid commands the host cell into just like a master-slave relationship. This affords an example of parasitism at the gene level.
A fresh protein coat is soon formed around each nucleic acid filament, ultimately forming thousands of virus particles inside the host cell.
Virus genome codes for the secretion of an enzyme called lysozyme, by the host cell, which dissolves (lysis) the plasma membrane of the host cell.
Consequently, virus particles (viruses) which are liberating may invade new cells it has been recently discovered that viral infected animal cell produces a protein called interferon, which serves to protect neighboring cells from the attack of the same virus.
Transmission of viruses
The viruses are mostly spread by insects such as houseflies, mosquitos, aphids, and hoppers. Certain plant viruses are transmitting through seeds, infected leaves roots, and mechanical robbing. Animal viruses are generally transmitting through cough, sneeze, sputum, and excreta.
Significance of viruses
Viruses are considered to be a biological mystery. They possess characteristics of non- living and living chemical systems.
Therefore nobody can say positively whether these are inert particle or microorganisms.
Non –living characters
- Ability to get crystallized.
- Inability to live independently as a living cell.
- Absence of energy storing system.
- Absence of growth and division.
- Absence of respiration.
- Presence of genetic material
- Ability to reproduce or multiply.
- The occurrence of mutation.
- Variations are inheritable.
- Autoclaving and ultraviolet rays can kill viruses.
- It is formed of organic molecules that occur only in living beings.
Viruses are the smallest filterable ultra microscopic particle. It is an obligatory parasite totally dependent on host cells. It was 1st discovered by Ivanovsky (1892); during the study of tobacco leaves infected by tobacco mosaic disease. They contain a single type of nucleic acid, either DNA or RNA. Multiply inside living cells by using the synthesizing machinery of the cell. The size of viruses varies from 10 mill microns to 350 mill microns (mm). There are five different type of virus viz. plant virus, animal virus, bacteriophages, Cyan bacteria, zymophages. A virus consists of a central core of nucleic acid surrounded by a protein coat. A virion is a complete, fully developed infectious viral particle composed of nucleic acid and surrounded by a protein coat outside a host cell. Bacteria phages are viruses which are obligatory parasites on bacteria. These are tadpole-shaped viruses. A virus particle capable of infecting a specific host organism is known as a virion. The viruses are mostly spread by insects such as houseflies, mosquitoes, aphids, and hoppers.