In this essay we will discuss about:- 1. Meaning of Virus 2. Host Specificity of Virus 3. Classification.
Essay on the Meaning of Virus:
In Latin, the word ‘virus’ means poison or venom. It was loosely used to mean any harmful agent until this name was assigned to a specific group of biological entities having, distinct physical and chemical properties. The understanding of the nature of viruses began in the early decades of the twentieth century. However, some of the severe diseases, now known to be caused by viruses, have been known for several thousand years.
Even preventive measures to combat such diseases were discovered without the knowledge of the causal agents. For example, Edward Jenner developed an effective method for preventing small-pox by vaccination as early as 1796 which saved thousands of lives from this dreadful disease and this measure has made it possible by 1979 to completely eradicate this disease. But Jenner never knew the nature of the causal agent — the small-pox virus.
About 100 years later Louis Pasteur, in 1885, invented a vaccine against rabies and another viral disease. It was in Pasteur’s laboratory that the first clue to the nature of the virus causing rabies was obtained when Charles Chamberland discovered that the causal agent passed through a porcelain filter which prevented the bacteria to pass through.
Such a filter was prepared by Chamberland himself and the pore size was smaller than the average breadth of bacterial cells. Chamberland concluded that the rabies agent was filterable. In 1892 Dimitri Iwanowski, using Chamberland filter, claimed that the causal agent of mosaic disease of tobacco plants was also a filterable agent.
Similarly, the agent of foot-and-mouth disease of cattle was found to be filterable by Frosch and Loeffler (1898). Martinus Beijerinck made an important finding that the agent of tobacco mosaic disease was precipitable and the precipitate could reproduce the disease in healthy plants. He concluded that the agent was a ‘contagious living fluid’.
In 1915 Frederick Twort and in 1917 Felix D’Herelle independently discovered that bacteria were also susceptible to attack by such filterable agents. These agents, as observed by D’Herelle, caused clear zones on a bacterial ‘lawn’. In these zones (later called plaques), the bacteria were completely lysed. He named this agent as bacteriophage which meant bacteria-eater.
Thus, by the end of the first two decades of the twentieth century, it became clear that a new kind of biological agents — which were smaller in size than bacteria — could cause diseases of plants, animals and human, or could even destroy bacteria.
A general name of these agents came to be used, the virus. Till then, it was generally believed that the viruses were minute organisms, smaller than bacteria and, therefore, they were invisible under the microscope.
The first clue to the chemical nature of viruses came from the study of W.M. Stanley (1935) when he reported that the tobacco mosaic virus (TMV) was a crystallizable protein. Soon afterwards, Bawden and Pirie found that TMV also contained a small but constant amount of ribonucleic acid (RNA) in addition to protein. Thus, viruses came to be known as nucleoproteins, so far as their chemical composition was concerned.
Regarding the knowledge about the form and structure of the viruses nothing was known till the electron microscope was invented (1932) and refined to a degree to produce sufficiently magnified and clear images of the objects. Electron microscopic observations revealed an outstanding feature that the viruses did not have a cell as a structural unit.
They are acellular or non-cellular. They also have a great variety of form and size and are, in general, much smaller than the smallest bacteria. Whereas bacteria are generally measured in micrometers (μm), viruses have to be measured in nanometer (nm) which is one-thousandth part of a micrometer.
Gradually, other features which distinguish viruses from all living organisms became known. The most significant among these is their mode of reproduction. All viruses are capable of multiplication only in other living cellular organisms.
Not only that, the components from which viruses are made when they multiply are synthesized separately and new viruses are made by assembly of the components. This mode of multiplication is unique, because all other living organisms do not lose their cellular integrity during multiplication.
Another significant characteristic which differentiates viruses from other organisms is that any particular virus does not contain both DNA and RNA, but has only one of the two as its genetic material. All cellular organisms possess both types of nucleic acids of which DNA acts as the genetic material.
As viruses do not have a cell, they naturally do not have cytoplasm and the associated cytoplasmic bodies, ribosomes etc. Although some viruses may carry one or two enzymes, they have no metabolism of their own, nor do they carry genetic information for synthesis of enzymes necessary for carbohydrate metabolism, ATP synthesis etc. But they do carry genetic information for synthesis of viral proteins and nucleic acid. The synthesis of these virus-specific substances takes place in the host cell using the biochemical machinery of the host.
On the basis of these unique features, viruses may be defined as a group of obligately parasitic non-cellular biological entities having either RNA or DNA as genetic material, and which are able to reproduce within host cells, by assembling their components, synthesized using the biochemical machinery of the host cell.
The form in which a virus exists outside the host is called a virion. Virions are formed in the host cells by assembly of the components and are released. In this form they are inactive and do not show any life activity. But they can infect new host cells and manifest life activities. When Stanley crystallized TMV, a question was raised whether viruses should be considered as living or non-living.
As virions, viruses are just complex chemical substance, e.g. TMV is a large macromolecule of nucleoprotein. But when a virion enters an appropriate host cell it shows life activities. In fact, in many cases the virus takes over the control of the cell and uses it for its own benefit. The virion may be considered just as a vehicle for transporting the genetic element of a virus from one host cell to another.
As viruses are obligate parasites, they cannot be cultivated in artificial media. This proved to be a constraint in virus research. A break-through was obtained when it was discovered that some viruses could be cultivated in the fertilized hen egg. Later, animal cell cultures have been used as a suitable medium for growing viruses.
Essay on the Host Specificity of Virus:
Most viruses show a narrow range of host specificity. This means that a particular type of virus can infect only some hosts i.e. the host-virus relationship is specific. Such specificity is determined by several factors. In case of animal viruses, one important factor is the ability of the virion to attach itself to the host cell membrane.
The attachment is often a complex process. In its simplest form, a single protein of viral coat interacts with a single receptor molecule of the host cell membrane. Depending on the host, viruses are broadly divided into animal viruses, plant viruses, bacterial viruses, fungal viruses etc.
Each broad class is then further classified depending on several characters. The bacterial viruses are generally known as bacteriophage or simply phage. Similarly, viruses affecting cyanobacteria and fungi are known as cyanophage and mycophage, respectively.
Essay on Classification of Viruses:
A universal classification of all viruses, growing in animal, plant, bacteria, fungi etc. as host organisms, is a difficult task. An International Committee on Taxonomy of Viruses has endeavored to work out a classification and nomenclature.
It has been generally agreed that the viral families will have the ending viridae and the viral genus will have the ending virus. Confusion still exists in the classification of plant-viruses, because many of them are not so thoroughly studied as the animal and bacteriophages.
The main taxonomic characters which form the basis of classification of viruses are:
(i) The nature of nucleic acid — RNA or DNA.
(ii) The strandedness of the nucleic acid — single-stranded or double-stranded. In case of ss-RNA, whether the virion RNA is (+) or (-).
(iii) The presence or absence of an envelope.
(iv) The symmetry of the nucleocapsid — icosahedral or helical or complex.
(v) Size of the virion i.e. diameter of icosahedral types and length and breadth of helical types.
(vi) Site of capsid assembly in the eukaryotic host cells — nucleus or cytoplasm.
(vii) Nature of the host organism, animals, plants, insects, bacteria, cyanobacteria, fungi etc.
For the sake of convenience, the classification of viruses infecting the three major groups of hosts, viz. animals, plants and bacteria, have been treated separately, although many of the viruses have common molecular features. The primary classification of all the three groups is based on the nature of the nucleic acid forming the genome of the virion.
1. Animal Viruses:
Animal viruses can be primarily classified into DNA viruses and RNA viruses.
An outline classification of the DNA viruses infecting humans and animals including insects and arthropods is shown in Fig. 6.5:
The RNA containing animal viruses are classified in 14 families. In most of these, RNA is single- stranded. The single-stranded RNA can be either (+) or (-). The retroviruses carry reverse transcriptase by which they synthesise DNA during replication and from DNA, viral RNA is produced by transcription.
An outline classification of animal RNA viruses is shown in Fig. 6.7:
[ss-RNA(+) denotes that virion RNA can act directly as m-RNA during replication. ss-RNA(-) strand is first transcribed into a (+) strand by an RNA-dependent RNA polymerase.]
It can be seen from Fig. 6.7 that the majority of animal RNA viruses are enveloped and only a few are naked. Also there are few with an icosahedral capsid and most of these viruses have a helical symmetry. Another important feature of the RNA genome is that in several viruses, the genome is segmented i.e. there are several fragments, each of which carries separate genetic information.
Some retroviruses, like the human immunodeficiency virus (HIV), have two copies of identical RNA genomes. Many of the ss-RNA viruses having a (-) strand carry specific RNA transcriptase for transcribing the (+) strand in their virions.
The salient characteristics of animal RNA viruses are briefly described in Table 6.4:
Diagrammatic sketches of some representative RNA containing animal viruses are given in Fig 6.8:
2. Plant Viruses:
From the structural point of view, the viruses which infect plant hosts do not differ significantly from the animal viruses. Therefore, the same criteria are applicable to their classification. The plant viruses, like-their animal counterparts, possess helical or icosahedral nucleocapsids with or without an envelope.
Their genomes also possess all possible types of variations, like double-stranded DNA or RNA, single-stranded DNA or RNA and single-stranded RNA with a positive or a negative character. However, majority of plant viruses possess single-stranded RNA. Also, in comparison to the animal viruses, there are very few plant viruses having an enveloped virion.
Because the plant cells are protected by a cell wall, the viruses gain entry either through openings caused by mechanical injury of the cell wall, or passively transmitted through insects, like aphids which feed on plants.
Occasionally, plants are infected through nematodes or fungi, or even by parasitic plants, like dodder (Cuscuta). In contrast, the animal viruses attach to host cells at specific sites with the help of special surface proteins, generally present on the envelope. In plant viruses, such mechanism does not operate.
Classification of plant viruses, in general, is not so well-organized as that of animal viruses or bacterial viruses. They are mainly classified into several groups rather than in families. Some plant viruses have been placed in the same families as those of animal viruses.
For example, the families Rhabdoviridae, Reoviridae and Bunyaviridae include both animal and plant viruses. But majority of plant viruses have been assorted into groups without assigning any family names. For example, tobacco mosaic virus and related viruses are known as Tobamovirus group.
A brief outline classification of the plant viruses together with their characteristics and examples is given below:
Outline classification of plant viruses:
I. DNA Plant Viruses:
1. Genome is double-stranded DNA:
Non-enveloped, icosahedral virions; size about 50 nm.
Example: Cauliflower mosaic virus transmitted by aphids.
2. Genome is single-stranded DNA:
Non-enveloped, icosahedral virions in pairs; size 18 nm; one pair of virions contains a single DNA molecule.
Example: Gemini virus causing maize streak.
II. RNA Plant Viruses:
1. Genome is double-stranded RNA:
Non-enveloped, icosahedral virions; size about 65-70 nm.
Non-enveloped, icosahedral virions; size about 35-40 nm.
2. Genome is single-stranded RNA (+):
(i) Icosahedral virion
Non-enveloped; 25 nm.
Example: Bromovirus causing brome mosaic. Cucumovirus — cucumber mosaic virus.
Non-enveloped; 20 nm.
Example: Tombusvirus — tomato bushy stunt disease.
Non-enveloped; size variable;
Example: Tobacco streak virus.
Non-enveloped; about 20-25 nm;
Example: Tymovirus — turnip yellow mosaic virus.
Non-enveloped; approximately 20-25 nm;
Example: Tobacco ring-spot virus.
Non enveloped; approximately 2C 75 nm;
Example: cowpea mosaic virus.
Non-enveloped; about 15-20 nm;
Example: Barley yellow dwarf virus.
(ii) Helical symmetry; capsids long, tubular, rod-shaped or flexuous; diameter variable; genome ss-RNA (+).
Example : Tobacco mosaic virus (18 nm x 280-300 nm). Tobravirus group
Non-enveloped; rod-shaped; length variable.
Example: Tobacco rattle virus. Potexvirus group
Non enveloped; flexuous rod, 500 nm or longer.
Example: Potato X virus
Non-enveloped; long flexuous rod, about 700 nm in length.
Example: Potato Y virus.
Non-enveloped; long flexuous rod; about 600 nm in length.
Example : Carnation latent virus.
Non-enveloped; long flexuous rod; very long more than 1,200 nm.
Example: Beet yellow virus.
3. Genome is single-stranded RNA (-):
(i) Enveloped, bullet-shaped virion with helical capsid. Family Rhabdoviridae.
Example: Lettuce necrotic yellow virus.
(ii) Enveloped, spherical virion with icosahedral capsid containing fragmented RNA. Family Bunyaviridae.
Example: Tospovirus — Tomato-spotted wilt virus.
The structures of some representative plant viruses are diagrammatically drawn in Fig. 6.9:
3. Bacterial Viruses:
The characteristics used for classification of viruses attacking bacterial cells are similar to those used for higher organisms. These include the nature of nucleic acid, strandedness of the nucleic acid, the symmetry of the nucleocapsid, the presence of an envelope and nature of the host. Many bacterial viruses have an additional feature not present in animal or plant viruses.
It is the presence of a tail. In some members, the tail may be covered by a sheath which may be contractile. Furthermore, the tail in some forms has a tail plate and tail fibres as well as spikes. Thus many bacterial viruses have a more complex structure than other viruses. The tailed bacteriophages have a binary symmetry.
The main body of the virus consists of an icosahedral nucleocapsid, called the head and the tail having a helical symmetry. Presence of an envelope is rare among bacterial viruses. There are also some filamentous bacterial viruses.