In this article we will discuss about:- 1. Definition of Viruses 2. Size of Viruses 3. Shape 4. Classification 5. Types.
Definition of Viruses:
Virus (L. poison) is a nucleoprotein entity which is able to utilize the synthetic machinery of a living cell of another organism for its multiplication which does not involve growth and division. Even before its discovery a lot of work had been done on virus. Small pox and polio are two viral diseases known since pre-historic times.
Broken Tulip (a symbol of love and nicety) was a product of viral infection which was transferable from one variety to another. Jenner (1796) discovered vaccination against small pox. Pasteur (1880) found rabies to be infectious disease and produced anti-rabies vaccine. He also coined the term virus before its scientific discovery.
Mosaic disease of Tobacco was found to be caused by a filterable agent present in the extract of diseased tobacco plant by Ivanowski (1892). He is credited with the discovery of virus. Beijerinck (1896) called it ‘contagium vivum fluidum’ (living infectious fluid).
Virus was seen under light microscope by Takahashi and Rawlins (1933) and under electron microscope by Stanley (1946). Stanley (1935) crystallized Tobacco Mosaic Virus (TMV) for the first time. Nucleoprotein nature of virus was discovered by Bawden and Pirie (1936). Polio virus was cultured for the first time in human cells by Enders (1949). Virus cannot grow on non-living culture medium.
It requires living cells for its metabolism and multiplication. Hershey and Chase (1952) confirmed that DNA is genetic material in bacteriophages. Franklin Conrat (1956) and later Gierere and Schramm (1956) found RNA to be genetic material in Tobacco Mosaic Virus (TMV). Sinsheimer (1959) observed the presence of single stranded DNA in bacteriophage ф x 174. Retroviruses were discovered by Temin (1970).
Virus is obligate parasite. It is inert outside the host cell. An inert virus is called virion. It can be crystallized and stored indefinitely. A biosynthetic machinery is absent. There is no system to liberate energy. A virus does not grow. It does not divide or reproduce like typical organisms.
Instead it multiplies by independent formation of its parts using host machinery and then assembly of parts to produce virus particles. A virus lacks irritability and motility. It requires a vector for transfer from one host to another. Virus having an arthropod as vector or intermediate host is called arbovirus.
Size of Viruses:
Virus is the smallest entity. Size varies from 10 nm (Foot and Mouth Virus of Cattle), 17 nm (Alfalfa Mosaic Virus), 300 x 17.5 nm (in TMV), 400 nm (Parrot Fever Virus), 1250x 40 nm (Beet Yellow Virus), 1300 x 6 nm (Pseudomonas Pf).
Shape of Viruses:
Three architectural forms are found in viruses— helical (elongate body, e.g., TMV), cuboidal (short broad body with rhombic, rounded, polyhedral shape, e.g., Poliomyelitis virus) and binal (with both cuboidal and helical parts, e.g., many bacteriophages like T2).
Classification of Viruses:
Genetic material is either DNA or RNA.
Accordingly viruses are divided into two groups:
(a) Deoxyvira or DNA viruses. All the three structural forms are known— deoxyhelica, deoxycubica and deoxybinala.
(b) Ribovira or RNA viruses.
They are of two types, ribohelica and ribocubica. Most of the animal viruses are DNA viruses with a few important ones having RNA, e.g., Rabies Virus, Polio Virus, retroviruses including HIV or AIDS virus. Most of the plant viruses are RNA-viruses with a few having DNA (e.g., Cauliflower Mosaic Virus). Bacteriophages have commonly double stranded DNA but all other genome types also occur.
Types of Viruses:
Viruses are host specific. Holmes (1948) has divided viruses into three groups.
(a) Plant Viruses (Phytophagineae):
They cause disease in plants, e.g., Tobacco Mosaic Virus (Fig. 2.2); Potato Mosaic Viruses, Banana Bunchy Top Virus, Tomato Leaf Curl Virus,
(b) Animal Viruses (Zoophagineae, Fig. 2.3).
They parasitize animals including human beings, e.g., Poliomyelitis Virus, Influenza Virus, Small Pox Virus, Hepatitis Virus, Mumps Virus, Rhino Viruses (common cold viruses),
They parasitise lower organisms— bacteriophages (bacterial viruses, e.g., T2, T4, lambda, Fig. 2.4), coliphages (bacteriophages of Escherichia coli), cyanophages (blue-green algal viruses, e.g., LPP-1, SM-1, N-l), phycophages (algal viruses), mycophages (fungal viruses), zymophages (mycophages of yeast).