Viruses:

Meaning and History of Viruses:

Viruses (Latin Venum – poisonous fluid) are simplest forms of life. They are not cells, but their study has provided a great deal of information about cells. Study of viruses is a branch of biology called Virology. Viruses are cellular parasites. They are smaller than bacteria and have a much more simplified organization.

History:

Russian Botanist Iwanowsky (1892) was first to give clear cut evidence of virus. He demonstrated their occurrence in tobacco leaves suffering from mosaic disease. Mayer (1886), Beijerinck (1898) and Loeffler & Frosch (1898) established the existence of viruses by discovering many viral diseases in plants and animals.

Schlesinger (1933) was first to determine the chemical composition of virues. W. M. Stanley, an American microbiologist crystallized tobacco mosaic virus (TMV) after isolating from infected tobacco leaf juice. He thus showed that viruses are not like typical cells. Stanley was awarded Nobel prize for this work.

Nature of Viruses:

Viruses are infective microorganisms.

They show several differences from typical bacterial cells:

1. Size:

On the whole viruses are much smaller than bacteria. Most animal and plant viruses are invisible under the light microscope. Some of smaller viruses are only 200Å in diameter.

2. No independent metabolism:

Viruses cannot multiply outside a living cell. No virus has been cultivated in a cell-free medium. Viruses do not have an independent metabolism. They are metabolically inactive outside the host cell because they do not possess enzyme systems and protein synthesis machinery. Thus viruses are obligatory intracellular parasites.

3. Simple structure:

Viruses have a very simple structure. They consist of a nucleic acid core surrounded by a protein coat. In this respect they differ from typical cells which are made up of proteins, carbohydrates, lipids and nucleic acids. Myxoviruses have a membranous envelope consisting of proteins, carbohydrate and lipid outside the usual protein coat, but this envelope is derived from the host cell.

4. Absence of cellular structure:

Viruses do not have any cytoplasm, and thus cytoplasmic organelles like mitochondria, Golgi complexes, ribosomes, lysosomes etc. are absent. They do not have any limiting cell membrane.

5. Nucleic acids:

Viruses usually have only one nucleic acid, either DNA or RNA. Typical cells have both DNA and RNA. Rous Sarcoma virus (RSV), producing certain cancer, is the only virus having both DNA and RNA.

6. Crystallization:

Many of the smaller viruses can be crystallized, and thus behave like chemicals.

7. No growth and division:

Viruses do not have the power of growth and division. The genetic material of virus reproduces only in a host cell.

Thus viruses do not show all the characteristics of typical living organisms. They, however, possess two fundamental characteristics of living systems. Firstly, they contain nucleic acid as their genetic material. The nucleic acid contains all the instructions for the structure and the function of the virus. Secondly , they can reproduce themselves, even if only by using the host cells’ s synthesis machinery.

Structure of Viruses:

(a) Size:

Variable. Most viruses are much smaller than bacteria. The size ranges in between 100A to 250 mu. Some viruses are larger than bacteria, for example the psittacos is a virus measuring 0.75 mu in diameter.

(b) Symmetry:

Viruses occur in three main shapes. They are spherical (Cubical or polyhydral), helical (Cylinderical or rod-like) and complex. Cubical viruses may be tetrahydral (4 faces) < dodecahedral (12 faces) or icosahedral (20 faces). The Herpes virus is dodecahedral. The Tobacco mosaic virus (TMV) and the bacteriophage are, respectively, helical and complex.

1. Spherical / Cubical:

PhI X 174, Herpes virus, Tipula virus, Polyoma virus.

2. Helical / Cylinderical:

Tobacco Mosaic virus, Influenza virus Mumps virus.

3. Complex:

Vaccinia virus, ORF virus, Vesicular Stomatitis virus.

(c) Morphology:

Morphologically a virus is a core of nucleic acid (DNA or RNA) surrounded by a protein shell. An intact virus unit is known as virion. Its protein coat is called capsid. The capsid is composed of a number of subunits of a particular shape. These sub-units are known as capsomeres.

The capsid protects the nucleic acid against the action of nuclease enzyme. Some proteins of capsid help in binding the virus to the surface of host cells. Some surface proteins act as enzyme and dissolve the surface layer of host cell and thus help in penetration of its nucleic acid into the host cell.

The polio virus (Poliomyelitis) is a most extensively studied animal virus. It has a very simple organization. It consists of a protein coat built up out of 60 structurally equivalent, asymmetric protein subunits of approximately 60 Å in diameter. The spherical protein coat has a diameter about 300Å. It encloses the genetic material, RNA.

The protein coat contains about 49, 600 amino acids and RNA contains about 5200 nucleotides. The single-stranded RNA of poliovirus, thus, has triplet codes for 1700 amino acids. During infection, it alters cell metabolism drastically and leads quick death of host cell.

Tobacco mosaic virus is the most extensively studied plant virus. It is a helically symmetrical, rod-shaped virus having the length of 3000Å and diameter of 180A. It RNA is a single stranded spirally coiled molecule formed of 6500 nucleotides. The capsid is formed of 2130 capsomeres, each with a molecular weight of 18,000. The capsomeres are elliptical and remain arranged helically around to form capsid.

Classification of Viruses:

Viruses may be classified according to the type of the host, genetic material and number of strands.

On the basis of type of host, viruses are:

1. Animal Viruses:

They live inside animal cells including man. On entering the cell, these disturb the metabolism of the host cell and cause various diseases. The common animal viruses are small pox virus, influenza virus, mumps virus, polio virus and herpes virus. In many animal viruses an extra envelope surrounds their protein coat. The membrane consists of proteins, lipids and carbohydrates and is derived from the host plasma membrane.

Animal viruses may enter cells by attaching to the surface. Some are then engulfed by the cell through pinocytosis or phagocytosis. In such cases, uncoating of the viral nucleic acid might occur within the cell. Inside the host cell they may multiply and form numerous new viral particles. Usually, animal viruses release from the host cells by the rapturing and subsequent death of the host cells.

2. Plant Viruses:

They are parasites of plant cells. Their genetic material is RNA which remains enclosed in the protein coat. The most important plant viruses are tobacco mosaic virus (TMV), tobacco rattle virus (TRV), potato virus (PV), southern bean mosaic virus (SBMV), beet yellow virus (BYV) and turnip yellow virus (TYV).

3. Bacterial virus:

They are parasitic on bacteria and so also called bacteriophages. There are many varieties of bacteriophages. Their size and shape varies from species to species. Some phages are spherical, some comma-shaped whereas majority of them have tadpole-like appearance.

On the basis of nucleic acids, viruses are:

1. DNA viruses:

These viruses possess DNA as the genetic material. On replication this DNA produces new DNA. DNA transmits information for protein synthesis through RNA. (DNA → RNA → PROTEIN).

2. RNA viruses:

These viruses possess RNA as the genetic material. The RNA replicates directly to produce new RNA. Information for protein synthesis passes from RNA to protein without involment of DNA. (RNA → RNA → PROTEIN).

3. DNA – RNA viruses:

In a group of RNA tumour viruses called leukoviruses or rousviruses the genetic material is alternately DNA and RNA. In addition to the normal mode of transfer found in DNA viruses (DNA → RNA → PROTEIN) the rousviruses also transfer information from RNA to DNA (RNA-DNA-RNA -PROTEIN).

With respect to number of strands, four types of nucleic acids have been found in viruses:

1. Double stranded DNA:

Double stranded DNA has been reported in pox viruses, the bacteriophages T 2, T 4, T 6, T 3, T 7 and lamda, herpes viruses, adeno viruses, polyoma virus SV-40 and papilloma viruses.

2. Single stranded DNA:

Single stranded DNA is found in the bacteriophages ph i X 174 and M-13 and is cyclic.

3. Double stranded RNA:

Double stranded RNA has been found within viral capsid in the reoviruses of animals and in the wound tumour virus and rice dwarf viruses of plants.

4. Single stranded RNA:

Single stranded RNA is found in most of RNA viruses e.g. Tobacco mosaic virus, influenza virus, poliomylitis bacteriophage MS – 2, F – 2, Coliophage R 17 and the avian leukemia virus.