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In this article we will discuss about the structure of viruses.
1. Viruses are much smaller than bacteria. A virus particle is called virion. The virions vary widely in size. The smallest virus measures about 10 mm in diameter (e.g., foot-and-mouth disease virus). The largest virus, (e.g., poxvirus) measures about 250 nm, i.e., as large as the smallest bacteria or mycoplasma.
2. Being ultramicroscopic, the viruses can be seen only by an electron microscope.
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3. Generally, plant viruses are smaller than animal of bacterial viruses.
4. The shape of the virions is highly variable in different groups of viruses. They may be rod shaped, bullet shaped, brick-shaped, oval, irregular and pleomorphic, or even like a piece of coir rope.
5. In ‘tailed’ or T-bacteriophages the virion is made up of complex head and an attached tail.
6. A virus particle or virion consists of nucleic acid core surrounded by a protein coat or capsid. The capsid with the enclosed nucleic acid is called nucleocapsid. The capsid is made up of many morphological units, called capsomeres. Chemically, the units of capsid are polypeptide molecules, which form an impenetrable shell around the nucleic acid core.
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7. The envelope of the viruses is derived from the host-cell membrane, and is lipoproteinaceous in nature. Recent researches have shown that the lipid of the lipoprotein is of host-cell origin, whereas its protein is of viral origin.
8. The tail consists of a hollow core (Figs. 300, 301) surrounded by a contractile sheath.
9. At the terminal end of the tail is present an end- plate which has attached tail fibres.
10. At the head end the tail connects the head through a thin disc or collar.
11. The nucleic acid in the head of the T-even bacteriophages is double stranded DNA.
12. The end-plate is hexagonal. It has a pin at every corner and remains connected to six very long tail fibres. Bacteriophages remain attached to the host cell through these tail fibres.