Introduction to Virus

Virus:

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

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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.