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PinkMonkey Online Study Guide-Biology

Figure 14.3 Tobacco mosaic virus (TMV)
(plant virus)

Classification of viruses : According to the type of hosts they infect, viruses may be classified as one of the following three types:

(i) Plant viruses: These are pathogenic viruses which infect plants. They are usually rod-shaped, containing nucleic acid in the form of RNA, e.g. tobacco mosaic virus (TMV), beet yellow virus (BYV) etc. The capsomeres of TMV virus are elliptical and arranged helically around the central nucleic acid core (fig. 14.3).

Common plant viral diseases are (a) mosaic disease of tobacco, papaya, apple etc., (b)black ring spot of cabbage, (c) leaf-roll of potato, and (d) spotted wilt of tomato.

(ii) Animal viruses: These are pathogenic viruses infecting animals. They are generally polyhedral or spherical in shape. The capsid in some is surrounded by envelope, and the nucleic acid is either DNA or RNA.

According to the type of tissue which they infect, they are as follows:

(a) Dermatotropic: Viruses infecting the skin, e.g. measles, chicken pox.

(b) Viscerotropic: Viruses infecting viscera, e.g. yellow fever, jaundice, mumps.

(c) Neurotropic: Viruses infecting nervous system, i.e. polio, meningitis.

(iii) Bacterial virus: These are pathogenic viruses infecting bacteria and are called bacteriophages or simply phages. their nucleic acid is DNA, e.g. T2, T4, T6 bacteriophages.


1. Viruses are a kind of biological puzzle to biologists as they are at the threshold of living and non-living, showing the characteristics of both.

2. Viruses are used by humans in eradicating harmful pests like insects and in controlling the population of organisms such as rabbits by inducing viral infection. Thus they are used as a form of biological control.

Figure 14.4. Adenovirus (animal virus)

3. Viruses have gained a prominent position in world because of their value as biological research tools. Due to simplicity of structure and rapid multiplication, they are widely used in research, in the fields of molecular biology, medicine and genetic engineering. Their role in fundamental research to unlock the intricate phenomena of life, can never be over- emphasized.

Figure 14.5 Bacteriophage ( bacterial virus )

4. Viruses have also concerned agriculturists. Apart from causing diseases in crops, bacteriophages attack the nitrogen fixing bacteria of the soil and are responsible for reducing fertility of the soil.

5. In industry, however, viruses are used in preparation of sera and vaccines to be used against diseases like rabies, polio, etc. The multiplication of viruses in bacterial cell is also utilized in the production of antibodies.

Table of Contents

14.0 Introduction
14.1 Kingdom : Monera
14.2 Kingdom : Protista
14.3 Kingdom : Plantae
14.4 Kingdom : Fungi

Chapter 15


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