Viruses are no cellular, ultramicroscopic structures of proteins and nucleic acids which grow and multiply only in the living cells.
They are obligate parasites. They cause highly infectious diseases to animals including human beings, angiosperms, bacteria and fungi. So far no virus diseases of gymnosperms, ferns, mosses and algae have been reported.
They live outside the living host in inert state for a quite long period of time. But they become activated only when they enter into the living host cell. They are invisible under the light microscopes. Even the highest resolutions of microscopes cannot detect the details of viruses. The viruses, however, can be observed properly with the help of electron microscopes. Chemical
Chemically, viruses are nucleoproteins constituted of two components, proteins and nucleic acids. The nucleic acids are the core of virus particle. Around it lies the protein coat. The nucleic acid proportion varies from 1-40 per cent.
The nucleic acids that occur as the component of viruses are only of one type. It is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Plant viruses normally contain RNA while DNA is the normal nucleic acid component of bacterial and animal viruses. There are, however, a number of cases of deviations in the occurrence of nucleic acids.
The protein coat of the viruses is inert and no genetic. It is only protective in function. The large cow pox viruses contains lipid, carbohydrates, copper and one or two vitamin-like substances.
Thus, viruses are structures which have below the level of cellular organization and do not have structures like cytoplasm, nucleus and chromosomes etc. The only thing they have in common with the living organisms is the occurrence of proteins and nucleic acids, DNA and RNA but never both.
The viruses have definite and stable hereditary mechanism. They therefore, can carry out self replication. The nucleic acids, the infective agents, are called virions.
Shape, Size and Structure:-
The purified extracts of some viruses, when viewed under the electron microscope, show detailed structures. The viruses may be spherical; oval, rod-like, brick- shaped or like a tadpole on the basis of their shapes. The viruses’ chiefly fall under the following four types.
(i) Straight, tubular, long, rigid rods with helical structure – Tobacco mosaic (TMV), Barley stripe mosaic fall under this type.
(ii) Long flexuous thread like rods – The examples of this type is bean mosaic, wheat streak mosaic virus.
(iii) Polyhedral Viruses: Turnip yellow mosaic, cucumber mosaic. The shape of these viruses are described to be spherical
(iv) Tadpole shaped: Bacterial viruses or bacteriophages are shaped like tadpoles with a head and tail.
The plant viruses range in size from 10 nm to more than 300nm. The straight rods of tobacco mosaic virus are about i8nm in diameter and 300 nm in length. The polyhedral plant virus particles vary in range from 16mm to 22nm.
The animal viruses have wider range of size. Polyhedral viruses vary in size from 8micron to about 100 micron diameter. There are larger ones which may be spherical, oval or brick shaped. The small pox virus is about 150 nm length. The spherical virus particles causing poliomyelitis are between 20-25 nm in diameter.
Structure: – As described earlier, the virus particle consists of an outer, inert, no genetic protein coat or shell which surrounds and protects the core of genetic material or nucleic acid. Protein coat is called capsid.
It is made up of numerous small subunits called capsomeres. The capsomeres are closely packed and arranged in a geometrical pattern around the nucleic acid core resulting in helical, cubical or complex symmetry.
The nucleic acid molecules in its protein coat are called nucleocapsid.
Ultrastructurc of Plant Viruses:
Tobacco mosaic virus is a typical plant virus. Its structure has been extensively studied. The tobacco mosaic virus. Capsid of this virus is twisted into a uniform spiral forming a hollow cylinder of 18 micron in diameter and 300 micron in length. Single stranded RNA molecules in the form of a long helix occur within this. RNA extends to the entire length of the virus particle. The protein content is about 94 percent and the remaining 6 percent is RNA.
The virus protein is made up of amino acids joined together by peptide linkages. These virus proteins are unusual in size and are of high molecular weight. Nearly 2200 capsomeres closely packed together to form the capsid of TMV. Each capsomere contains about 158 amino acid residues.
Structure of Bacterial Viruses (Bacteriophages)
Twort discovered phages in 1915 and d’ Herelle made detailed study of the virus. The bacteriophage which infects colon bacterium (Escherichia coli) is called coliphage.
Ultra structure: It has a tadpole like shape. The phage particle has a head and a tail. Besides, there is an attachment portion adapted to stick to the surface the bacterium. This virus is nearly 200-280 micron in length.
The head is hexagonal in outline. It consists of protein coat surrounding a core of genetic material (DXA) molecule. This is a single thread like double stranded DNA packed tightly in the head.
The protein coat is made up of globular proteins. The cylindrical tail consists of protein sheath only surrounding an empty core. The tail sheath can contract longitudinally. The attachment apparatus of the bacterial virus consists of 6 long, slender protein fibers known as tail or caudal fibers.