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PinkMonkey Online Study Guide-Biology


14.0 Introduction

A five-kingdom classification system fails to classify viruses because they are not considered to be living. They neither reproduce independantly nor utilize energy.

Viruses are now defined as ultramicroscopic disease-producing entities living in a host as obligatory intracellular parasites.

Brief history

Viruses were not known to biologists for a long time due to thier ultramicroscopic structure although its presence was apparent by infectious diseases which were not due to bacteria. They attracted the attention of investigators in the 19th century when mosaic disease (TMV) caused severe damage to commercially important tobacco crop.

Figure 14.1 A few different shapes of viruses

Mayer (1886) took the initiative and demonstrated that the disease could be transmitted just by applying the sap of infected leaves to the leaf of healthy plant. Mayer thought that the causative agent was a bacterium. A crucial experiment was performed by Russian biologist Iwanowsky (1892) who demonstrated that the sap of infected leaves, even after passing through the finest porcelain filter (which prevents bacteria to pass through), remained infective. He, therefore, ruled out the presence of bacterium but could not think of any other causative agent. Dutch microbiologist Beijerinck (1898) confirmed Iwanowsky’s findings and suggested that the causative agents were not any submicroscopic particles but the fluid itself which he called "contagium vivum fluidum" which means contagious living fluid. This was later on called viron (meaning poison) and the disease causing agent called virus. Twort (1915) and d’ Herelle (1917) independently discovered viruses attacking bacteria for the first time.

After the discovery of the ultramicroscope, the controversy was finally laid down to rest when W. M. Stanley (1935), the American biochemist, isolated the virus in crystalline form and showed that even in this state it retains infectivity. These findings gave inspiration to other scientists all over the world to study more aspects of viruses. This marked the beginning of a new branch of science called virology.

General characters:

A large number of viruses are now known. They exhibit diversity of form and infect a number of organisms. Despite diversity of form and structure, they show the following important characteristics common to all viruses:

1) They are ultramicroscopic disease-producing entities.

2) They have no cellular organization and also no metabolic machinery of their own.

3) They are simple in structure, basically composed of nucleic acid wrapped up in a protein coat.

4) Nucleic acid is only of one type, either DNA or RNA, but never both.

5) They are obligatory intracellular parasites as they are completely inactive outside the host.

6) They multiply within the host by commandeering the metabolic machinery of the host cell.

7) They are specific in action, i.e. they always infect particular organ or organism.

8) They are incapable of growth and division.

9) They can be crystallized and even in crystalline form, they retain their infectivity.

10) They are unaffected by antimicrobial antibiotics.

11) They may undergo mutations.

Size and shape : Viruses are minute entities, even smaller than the smallest bacterium. They can be seen only under electron microscope as small particles called virons. Being minute, they are measured in millimicrons (1m m = 1/1000 m ). Generally they vary from 10 mm to 300 mmð in size.

Viruses occur in three main shapes, viz.

(I) Polyhedral or spherical, e.g. adeno virus, herpes virus etc.

(II) Helical or rod-like, e.g. tobacco mosaic virus, (TMV), influenza virus, etc.

(III) Complex or irregular, e.g. bacteriophage, vaccinia, etc.

Structure: The virus is composed of two major parts: (I) Capsid and (ii) Nucleic acid.

Figure 14.2 A generalized structure of a virus

The capsid is the outer protective coat mostly made up of specific protein. It protects nucleic acid from inactivation by enzyme nuclease in the environment. It is often composed of many identical subunits called capsomeres. The shape and arrangement of capsomeres determine the shape of the virus. Some highly specialized viruses, for example. influenza virus, mumps virus etc. show an outer covering called an envelope which contains cell membrane obtained during exit of the virus from its host cell.

The nucleic acid is in the central core. Unlike living organisms it contains a single molecule either of DNA or RNA, but never both. Nucleic acid is the only active part of a virus, hence viruses are sometimes called "wandering genes". The infectivity of virus is due to nucleic acid while host specificity is determined by the protein coat.

The capsid in close contact with nucleic acid, is known as nucleocapsid. Viruses may be enveloped or non-enveloped (naked).

Chemical composition: A virus is a nucleoprotein, i.e. mainly consisting of nucleic acid and proteins. Nucleic acid is-- either DNA or RNA, but never both. When only RNA is present, genetic information is solely carried by RNA, which is the unique property of virus. The envelope, if present, contains lipoproteins. The lipid is mostly derived from the host plasma membrane while the protein is virus coded. Viruses normally do not possess any biosynthetic enzymes.

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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