Essay on Viruses

In this essay we will discuss about Viruses. After reading this essay you will learn about: 1. General Characters of Viruses 2. Size of Viruses 3. Shapes 4. Chemistry and Microstructure 5. Symmetry 6. Classification 7. Biological Status or Nature 8. Economic Importance.

Contents:

1. Essay on the General Characters of Viruses
2. Essay on the Size of Viruses
3. Essay on the Shapes of Viruses
4. Essay on the Chemistry and Microstructure of Virus
5. Essay on the Symmetry among Viruses
6. Essay on the Classification of Viruses
7. Essay on the Biological Status or Nature of Viruses
8. Essay on the Economic Importance of Viruses

Essay # 1. General Characters of Viruses:

A large number of viruses are 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 and smaller than bacteria.

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

3. Unit of structure of virus is virion which is a-cellular and lacks protoplasm.

4. Virion is simple in structure, basically composed of nucleic acid wrapped in a protein coat.

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

6. They are highly physiologically specialized i.e., host specific.

7. They are obligatory inter-cellular parasites as they are completely inactive outside the host.

8. They multiply within the host by taking over the metabolic machinery of the host cell.

9. They are transmissible from diseased to healthy hosts.

10. They are incapable of growth and division.

11. They are effective in very small doses.

12. They are resistant to antimicrobial antibiotics and extreme physical conditions.

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

14. They can be inactivated by chemotherapy and thermotherapy.

15. They may undergo mutation.

Essay # 2. Size of Viruses:

Viruses are very small ultra-filterable and ultramicroscopic structures. The average size of viruses ranges between the smaller bacterium and the largest protein molecule. The foot and mouth disease virus is the smallest animal virus approximately 10 nm in size (Fig. 1). The largest animal virus is parrot fever virus, approximately 400 nm in size. The bacterial and animal viruses are larger than the plant viruses. The plant viruses vary in size from 17 nm to 300 nm.

Essay # 3. Shapes of Viruses:

Viruses may be classified in relation to their shape into several groups:

(i) Spherical Form:

Viruses are spherical in shape e.g., cucumber mosaic virus (CMV), viruses of influenza, measles and fever etc. (Fig. 2A).

(ii) Rod Shaped Form:

These are rod shaped forms e.g., Tobacco mosaic virus (Fig. 2B).

(iii) Cuboidal Form:

These are cube like e.g., cow pox (vaccinia) virus, small pox virus (Fig. 2C).

(iv) Spermatozoid or Complex Form:

Characteristic for viruses of the lower plants for example, Bacteriophages (Fig. 2D).

Rabies or Rhodoxin are bullet shaped (Fig. 2E) and pox viruses are brick shaped (Fig. 2F).

Essay # 4. Chemistry and Microstructure of Virus:

Chemistry of TMV was studied for the first time by Bawden and Pirie in 1937. According to them virus particles are micro-proteins of high molecular weight.

Each virus particle (technically called virion) consists of two parts:

(1) Nucleic acid or Nucleoid and

(2) Protein coat or capsid; and enzyme systems with the help of which the virus penetrates into tissue cells.

(i) Nucleoid:

It is present within the protein coat (Fig. 3). Each virus particle has only one type of nucleic acid either RNA or DNA. The viruses containing DNA are called Deoxyviruses and those having RNA are known as Riboviruses.

All the plant viruses had been reported to contain single stranded RNA while animal viruses either single or double stranded DNA. Bacterial viruses contain mostly double stranded DNA. Most of the insect viruses contain RNA and only a few have DNA.

The DNA of several animal and bacterial viruses is circular whereas in others it is like RNA. Shephered et al (1968) reported that cauliflower mosaic plant virus contains double stranded DNA. Double stranded RNA is also reported in Rice dwarf viruses and wound tumor viruses (Miura et al 1968).

The type of nucleic acid and the number of strands in different viruses are given ahead (Table 1):

(ii) Capsid:

It is the protein coat surrounding the internal nucleoid. It is made up of repeating protein subunits called capsomeres. The capsomeres are composed of either one or several types of protein. Host specification of viruses is due to protein of capsid.

The number of capsomeres in the capsid of a given virus is constant (32 in the poliomyelitis virus, 252 in the Adenovirus, 2130 in the Tobacco Mosaic Virus etc.) and they are arranged in a very symmetrical manner and give a specific shape to a particular virus.

Usually they are arranged in two geometric forms:

1. Helical Viruses:

The nucleic acid is coiled like a spring and capsomeres are helically arranged around their strings, e.g., Tobacco mosaic virus.

2. Icosahedral Type:

The nucleic acid is packed in unknown manner within a hollow polyhedral head. The capsid is antigenic and protects nucleic acid from un-favourable extra-cellular environment. It also makes easy the entry of nucleic acid into the host cells. The core within capsid is called the nucleocapsid core.

The Envelope:

Some animal viruses e.g., Herpes virus and pox virus develop a 10-15 nm thick lipoprotein envelope around their protein coat while it is absent in plant viruses. The lipid is derived from the host while the protein is of viral origin. This envelope is also known as mantle or limiting membrane and is covered with projections or spikes. It is made up of several subunits called peplomers.

Enzymes and Other Contents:

In addition to the nucleoprotein, viruses have enzymes, water and carbohydrates. The enzymes are collectively called “transcriptase’s”. These enzymes are essential for infection and growth of viruses. However, the function of carbohydrates is not known.

Essay # 5. Symmetry among Viruses:

Symmetry (the property of bodies to repeat their parts) is an important feature in the structure of virion. The electron microscopic studies have revealed that the arrangement of the capsomeres determines the shape of the virus particles.

There are three different types of symmetry among viruses:

(i) Viruses with Cubic or Icosahedral Symmetry:

This type of symmetry is found in bacteriophage ф (phi) x 174, Turnip yellow mosaic virus, Adenovirus, Tipula iridescent virus etc. In it the viruses are spherical and capsomeres are closely arranged in polyhedral manner.

An icosahedral capsid comprises of two types of capsomeres:

(a) Pentameres. Containing 5 structural units or monomers.

(b) Hexameres. Containing 6 structural units or monomers.

Each monomer is connected with the neighbouring monomers on either side with the bonds. In the same manner the capsomeres are also connected with weaker bonds. The minimum number of capsomeres in an icosahedral virus is 12 or its multiple e.g., Bacteriophage ф (Phi) x 174= 12 capsomeres, Turnip yellow mosaic virus (TYMV) = 32 capsomeres, Polyoma virus and papilloma virus = 72 capsomeres, Reovirus = 92 capsomeres, Herpes virus = 162 capsomeres. Adenovirus = 252 capsomeres, Tipula iridescent virus = 812 capsomeres.

(ii) Viruses with Helical Symmetry:

The common example of this type of symmetry is Tobacco Mosaic virus (TMV), Bacteriophage M 13, Influenza virus. In it the viruses are cylindrical or rod like and the capsomeres are arranged in spiral manner around the long axis.

(iii) Viruses with Complex Symmetry:

The common example of this type of symmetry is Bacteriophages, vaccinia virus, and small pox virus.

In complex symmetry the capsids are of two shapes:

(a) Without identifiable capsids e.g., pox viruses etc.

(b) Tad pole shaped—Combined mixture of both helical and cubical symmetry e.g., Bacteriophage.

Essay # 6. Classification of Viruses:

Earlier, the viruses, on the basis of their hosts were classified into following groups:

(a) Plant viruses. Viruses infecting plants.

(b)Invertebrate viruses. Viruses infecting invertebrates.

(c) Vertebrate viruses. Viruses infecting vertebrates.

(d) Dual host viruses. Viruses parasitic on two different hosts.

(e) Bacterial viruses. Bacteriophage.

Later, in 1923 in Bergey’s Manual of Determinative Bacteriology all the viruses are placed in order Virales of class Schizomycetes, phylum Protophyta.

Holmes (1948) divided order Virales into following 3 sub-orders:

Sub-order 1. Phytophagineae:

Viruses causing plant diseases e.g., Tobacco Mosaic Virus (TMV), Best Yellow Virus (BYV).

Sub-order 2. Phagineae:

Viruses infecting bacteria, e.g., T₂, T₄, T₆ bacteriophages.

Sub-order 3. Zoophagineae:

Viruses causing diseases to men and animals. With the advancement of knowledge of the physiochemical characteristics, a new classification had been proposed by A. Lwoff, R. W. Horne and P. Tournier in 1966. This classification has been accepted by the Provisional Committee of Nomenclature of Viruses (PCNV), especially constituted for this purpose.

This system is based on classical monothetic hierarchical system applied by Linneaus to plants and animals. This is a logical system known as LHT system, in which divisions are made as to relative importance of different properties which are then used to place a taxon in a particular phylum, order, family, genus etc.

This classification is based on the following major criteria of the viruses:

(1) Kind of nucleic acid in the particle-DNA or RNA.

(2) The architecture and symmetry of the capsid-helical-cubical or both.

(3) Presence or absence of the envelope around the capsid.

(4) Diameter of the helical nucleocapsid.

(5) Number of the capsomeres in cubical forms.

In this classification viruses are placed under phylum VIRA as follows:

Phylum Vira:

Divided into the subphylums—

Subphylum 1. Deoxyvira (containing DNA)

Subphylum 2. Ribiovira (containing RNA)

Subphylum Deoxyvira dhided into 3 classes:

Subphylum RIBOVIRA is divided into two 2 classes:

Adansonian Classification:

An alternative system of classification was proposed by Adanson (1763). He considered that taxa were best derived by considering all available characters and equal weight to each. The method is laborious and has not been much used recently. The availability of computer has renewed interest in this kind of classification.

Nomenclature or Cryptogram of Viruses:

Latin binomial system of nomenclature of viruses is inappropriate at present. The main reason for the is that such names attempt to provide both a label and some information about the virus being labeled. In the present stage of knowledge or lack of it for most viruses, such name would be found to require frequent changes in future.

In an attempt to overcome this problem, Gibbs et. al. (1966), Gibbs and Harrison (1968), proposed a new scheme of classification popularly known as cryptogram of viruses. It is also known as Adansonian classification. It is a two part system, in which the first part of virus name is an unchanging label (the current vernacular name) and the other is a codified information store, which can be readily adopted and changed as facts and information accumulate about the virus.

Each cryptogram consists of four pairs of symbols as follows:

I. Pair: Type of nucleic acid/strandedness of nucleic acid.

(Symbol: R=RNA and D=DNA, 1 represents single stranded, 2 for double stranded).

II. Pair: Molecular weight of nucleic acid (in millions)/percentage of nucleic acid present in infect particle.

(Symbol: l-one million and one percent. 2-two million and two percent and so on)

III. Pair: Shape and the outline of virus particle/shape of capsid.

(Symbol: S-spherical, E-elongated with parallel sides, ends not rounded; U-elongated with parallel sides, ends rounded; X-complex or none of above).

IV. Pair: Kinds of hosts infected/kinds of vectors used.

Symbol for kinds of hosts: A-Actinomycete, B-Bacterium, F-fungus, I-Invertebrate, P-Pteridophyte, S-seed plant, V-Vertebrate.

Symbols for Kinds of Vector:

Ac-mites and ticks, Al-white fly, Ap-Aphid, Au-Leafy plant or tree hopper, Cl-Beetle, Di-fly) and mosquito; Fu-Fungus, Ne-Nematode, Si-Flea, Th-Thrips, Ve-Vector known but none of above, O-Spreads without a vector via a contaminated environment.

Here in all cases, Asterisk (*) indicates property of virus is unknown and ‘parenthesis’ ( ) indicates enclosed information is doubtful.

The cryptogram of some important viruses can be represented as:

1. Tobacco mosaic virus: R/1: 2/5: E/E: S/O.

2. Human influenza virus: R/1: (2 – 3)/10: S/E: V/O.

3. Coli phage ф x 174: D/l: 17/25: S/S :B/O.

The cryptogram of Tobacco Mosaic Virus may be explained as:

I. Pair. Single stranded RNA i.e., R/l.

II. Pair. 2 million molecular weight with percentage of nucleic acid in infective particles = 5 i.e., 2/5.

III. Pair. Nucleocapsid is elongated with parallel sides, ends not rounded i.e., E/E.

IV. Pair. Host is seed plant and spreads without a vector via contaminated environment i.e., S/0.

Essay # 7. Biological Status or Nature of Viruses:

With the discovery of viruses, the question regarding the nature of the virus has been a very interesting and controversial topic. Some virologists are of the view that they are animate (living) objects, while in the opinion of other virologists they are inanimate (non-living) objects.

The following points support the different views:

Viruses are Living Particles:

(1) They can grow and reproduce only inside specific hosts.

(2) They show mutation.

(3) They can infect healthy plants.

(4) They contain genetic material either DNA or RNA.

(5) They show sensitivity to heat stimuli and radiation.

(6) They have antigenic properties and stimulate the production of antibodies in the body of vertebrates.

(7) They show physiological specialization.

Viruses are Non-Living Particles:

1. They can be crystallized into fine crystals like a sugar molecule.

2. They do not respire or show any sign of metabolism.

3. They are characterized by high specific gravity.

4. Out-side the host they behave like complex organic molecules.

5. Membrane or cell wall is absent.

6. They can be sedimented like proteins.

Viruses differ from other living organisms:

According to Lwoff and Tournier (1966) the following characteristics separate viruses from other living agents:

(1) One type of nucleic acid either DNA or RNA is present in virion (other agents possess both types nucleic acid).

(2) Viruses are reproduced from their sole nucleic acid either from DNA or RNA whereas other agents reproduce from the integrated sum of their constituents and by division.

(3) Viruses make use of the ribosomes of their host cells.

(4) The genetic information for the synthesis of Lipman system (the system responsible for the production of energy with high potential) is absent in viruses.

(5) Viruses are incapable of growing or undergoing binary fission.

So, we find that supporters of both the views are correct at their own places. So, the question is where to place the viruses? Can we call virus a cell? The cell may be defined as “the smallest unit of life capable of independent existence and should be able to reproduce itself”. Second important difference is that the enzymes, ribosomes and cytoplasmic environment is not possessed by viruses. Because of the absence of ability to grow independently, the viruses cannot be regarded as ‘organism’. They are a-cellular or non-cellular.

Viruses can be crystallized like chemicals. Since they are able to reproduce inside the host cells they can be said as living. They are neither living nor non-living. A. Lwoff (1966), a French scientist, once said, ‘A virus is a virus’. It is neither a living organism nor a non-living chemical but in the transition stage and represents the threshold of life.

Essay # 8. Economic Importance of Viruses:

(i) Viruses are used in preparation of sera and vaccines to be used against diseases like rabies, polio etc.

(ii) Multiplication of viruses in bacterial cell is also utilized in the production of antibiotics.

(iii) Due to simplicity of structure and rapid multiplication, they are widely used in research, in the fields of molecular biology, medicine and genetic engineering.

(iv) Viruses are used by humans in eradicating the harmful pests insects and to control the population of organisms. Thus, they are used as a form of biological control.

(v) Viruses cause dreadful diseases in crop plants, domesticated animals and man.

(vi) Bacteriophages attack the nitrogen fixing bacteria of soil and the responsible for reducing fertility of the soil.