In this article we will discuss about Viruses:- 1. Meaning of Viruses 2. Sizes and Shapes of Viruses 3. Structure.
Meaning of Viruses:
Virus is a Latin word which means poison. Adolf Mayer described for the first time a disease of tobacco plant. Dimitri Ivanowski, a Russian botanist in 1892 demonstrated experimentally that sap of mosaic tobacco plant was capable of inducing the mosaic disease in healthy tobacco plants after it had been passed through bacteria proof filter.
This indicated that the infective agent was smaller than any known bacteria and so he was the first to give clear cut evidence of virus. Beizerinck, a Dutch microbiologist in the year 1898 confirmed the observation of Ivanowski and named the infectious fluid obtained from the diseased tobacco plant as ‘contagium vivum fluidium’ and referred it as virus.
Stanley (1935) isolated a crystalline protein from the diseased tobacco leaves. Bawden and Pirie (1937) established that viruses are nucleoproteins. Marcham (1949) isolated tobacco mosaic virus (TMV).
Viruses comprise a unique group of infectious agents which are characterised by their small size, simple composition and parasitic mode of life.
Sizes and Shapes of Viruses:
Viruses are much smaller than bacteria and their size is variable. The larger viruses may be about 300 A in diameters, i.e., they may be as large as some of the small bacteria. The majority of the viruses are about 200 Å in diameters.
Viruses occur mainly in the following three shapes:
1. Spherical or Polyhedral, as for example. Polio virus, Adeno viruses and Herpes viruses (Fig. 1.5).
2. Helical or Cylindrical, as for example, Tobacco mosaic virus. Influenza virus, etc., (Fig. 1.6).
3. Complex symmetry, as for example. Vaccinia viruses and some bacteriophages.
Viruses cannot grow and multiply outside the living cell (total parasite). They do not have independent metabolic system of their own and are inactive when they are outside the host cell.
Structure of Viruses:
Viruses have simple morphology. They consist of two distinct parts: a core of nucleic acid and the protein coat. The protein coat is known as capsid. The protein coat or capsid is composed of several closely packed morphological unit called capsomeres (Fig. 1.7).
In this respect viruses differ from typical bacterial cells which are made up of proteins, carbohydrates, lipids, nucleic acids etc. Some viruses, e.g. Myxoviruses have additional membranous envelope containing proteins, lipids and carbohydrates outside the usual protein coat. They do not have plasma membrane.
Viruses lack cytoplasm and thus cell organelles such as mitochondria, golgibodies, ribosomes and lysosomes as well as enzyme systems are absent.
Viruses usually have either DNA or RNA whereas the typical cell contains both DNA and RNA. Certain animal viruses, e.g. Rous sarcoma virus (RSV) have both DNA and RNA.
Thus with respect to nucleic acid, the viruses are of three types:
1. RNA virus,
2. DNA virus, and
3. DNA-RNA virus.
Plant viruses contain only RNA (Fig. 1.8).
Most bacteriophages (viruses infecting bacteria) contain only DNA (some bacteriophages like MS-2, F2 coliphages, R-17 are the exceptions which contain RNA). Animal viruses contain RNA or DNA or rarely both DNA-RNA. The nucleic acid component of viruses may be composed of either single stranded or double stranded RNA or double stranded DNA or single stranded DNA.
Table 1.1 shows the type of nucleic acid and the number of strand(s) in different viruses: