Introduction to Virus

Viruses are the ultrasonic, nucleoprotein that reproduces only inside the living things.

Characteristics of Viruses:

• All the viruses are minute and simplest organisms which can be observed only by the use of electron microscope.
• The diameter of viruses ranges from about 20-30 mm on average. They are 50 times smaller than bacteria.
• They are metabolically inactive outside the host cell because they do not have enzyme systems and proteins synthesis machinery. Thus, they are obligatory intracellular pathogens or parasite.
• They are composed of nucleic acid either DNA or RNA or both and protein. The protein that surrounds the nucleic acid is called protein coat, the capsid.
• Viruses do not have cellular structure i.e. they lack a true nucleus, cytoplasm and cytoplasmic organelles like mitochondria, Golgi complex etc.
• During reproduction, the virus particle disorganized into molecular constituents inside the host cell. This phase is called eclipse phase.
• They show respond to chemicals, heat, and temperature.
• Outside the host cell, they can be crystallized and behaves like the chemicals.
• They cannot show the independent mode of life.
• They are highly specific to their host.
• Viruses cannot reproduce by fission or conjugation.
• They undergo mutation like living organisms.

Shape:

Viruses occur in three main shapes;

• Spherical (Cubical or Polyhedral) viruses. Eg; Poliovirus
• Helical(cylindrical or rod ) viruses. Eg; Tobacco Mosaic virus
• Complex( tadpole shape) virus. Eg; Bacteriophage

Living characters of viruses

• Viruses can grow and reproduce within the host cell.
• The ultrastructure of most viruses shows similar morphological structure to bacteria like cocci.
• They undergo mutation like living organisms.
• They have their own genetic material in the form of RNA, DNA or both.
• They show intracellular parasitism and can be transmitted from the diseased host to healthy ones.
• They are infective and capable of spreading diseases.
• They respond to light, chemicals, and temperature.

Non-living character of viruses

• They lack a complete cellular structure.
• They lack enzyme system and do not have metabolic activity of their own.
• Viruses do not respire.
• Some viruses can be crystallized and stored in bottles like chemicals.
• They can be precipitated by a number of a chemical substance.
• They depend upon living host for their existence.
• They are with high specific gravity.
• Viruses are nucleoprotein entities of high molecular weight and have the power of multiplication.

Types of Viruses

The virus can be classified on the basis of the type of host and genetic material. They are of three types. They are:

• Animal viruses
• Plant viruses
• Bacteriophage

Animal Viruses:

The viruses which infect and live inside the animal cell including man are called animal viruses. Eg; influenza virus, rabies virus, mumps virus, poliovirus etc. Their genetic material is RNA or DNA.

Plant viruses:

The viruses that infect plants are called plant viruses. Their genetic material is RNA which remains enclosed in the protein coat. Some plant viruses are tobacco mosaic virus, potato virus, beet yellow virus and turnip yellow virus etc.

Bacteriophage:

Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as genetic material. There are many varieties of bacteriophages. Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria.

On the basis of nucleic acids, viruses are classified into following three types;

DNA viruses:

The viruses that possess DNA as genetic material is called DNA viruses. Bacteriophages, cyanophages and most of the animal viruses contain DNA as genetic material.

RNA viruses:

The virus that possesses RNA as genetic material is called RNA viruses. Eg; plant virus

DNA-RNA viruses

The RNA tumor viruses called Leukoviruses and Rous's viruses contain both DNA and RNA as genetic material.

On the basis of presence of number of strands, four type of nucleic acid have been in viruses

Double stranded DNA;

It is found in pox viruses, the bacteriophages T₂, T₄, T₆, T₃, T₇ and Lamda, herpes viruses, adenoviruses etc.

Single-stranded DNA;

It is found in bacteriophagesφ, X, 74 bacteriophages.

Double-stranded RNA:

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

Viroid: Small viruses without capsid.

Single-stranded RNA:

It is found in most of the RNA viruses eg: tobacco mosaic virus, influenza virus, poliomyelitis, bacteriophage MS-2, Avian leukemia virus.

Viral Genome

The viral genome is very small containing just a few genes that enable viruses to infect their respective host cells and code for the synthesis of few proteins for the capsid. Viruses are host biosynthetic machinery ( ribosomes, RNA, and enzymes ) and raw materials ( nucleotides and amino acids ), and synthesis copies of their own nucleic acid and protein.
Three stages of bacteriophages as suggested by Lwoff are extracellular virons, vegetative phage and prophage.