Virus: Virus Structure, Plant Viral Diseases, Animal Viral Diseases
Virus is a Latin word, literally meaning “poison”. Tobacco Mosaic Virus was the first Virus discovered by Russian scientist Dimitri lvanovsky in 1892. A Virus is an extreme micro, parasitic non-cellular nucleoprotein particle which can persist only if it is inside any living organism. This means that all viruses are parasites.
Salient Features
Viruses are very small acellular and infectious particles which can be seen only by an electron microscope. They can pass through bacteria-proof filters. They cannot be grown on artificial media in the laboratory. They are not cells and they behave as living organisms inside the host tissue only where they can multiply. They lack functional autonomy. They are not affected by antibiotics but can be made inactive by chemotherapy and thermotherapy. They react to stimuli such as light, radiations, chemicals, heat etc.
Viruses have been excluded from the biological classification because they are not living things in first instance. However, they do posses some properties of the both living and non-living.
| Living properties | Non-living properties |
|---|---|
| The presence of DNA or RNA (but never both) | The absence of cell. |
| Structural diversity | The lack of protoplasm. |
| Geneticity and parasitic properties | No any reproduction and growth outside the living cell. |
| Sensitivity and evolution | Stored in the form of crystal outside the living cell. |
| Capable of spreading the disease | The lack of metabolic activities like nutrition, digestio |
There are three types of Viruses viz. Plant Viruses, Animal Viruses and Bacteriophage (viruses that are parasites on bacteria).
Structure
The sizes of viruses normally range from 40 to 350 nm. The smallest virus is of Hepatitis B (42nm), while largest Virus is Pandoravirus. The shapes of Viruses are also variable ranging from spherical (polio virus), rod-shape (TMV), tadpole-like (bacteriophages), polyhedral (adenovirus) and of other types.
A virus is made of three components viz. a protein capsid, nucleic acid and a thick outer layer. Viruses may contain either DNA or RNA but not both together. Generally, plant viruses are RNA viruses while animal viruses are DNA viruses. Further, Bacteriophage is always a DNA virus. Viruses produce diseases in plants, animals and human beings.
Plant Viral Diseases
Common plant viral diseases are Tobacco mosaic, Cauliflower Mosaic Sandalwood spike, Sugarcane mosaic, Bean mosaic, Aster yellow, Bunchy top of Banana, Leaf Curl of Papaya, Potato leaf roll, Twisted leaf disease of Tomato etc.
Use of TMV in Research
The Tobacco Mosaic Virus has become a popular tool for scientific research. The main reason is that it is available in large quantities and it does not infect animals. After growing a few infected tobacco plants in a greenhouse and a few simple laboratory procedures, a scientist can easily produce several grams of virus. As a result of this, TMV can be treated almost as an organic chemical, rather than an infective agent. Tobacco mosaic virus (TMV) and Cauliflower mosaic virus (CaMV) are frequently used in plant molecular biology. Of special interest is the CaMV 35S promoter, which is a very strong promoter most frequently, used in plant transformations.
Animal and Human Diseases
Common animal viral diseases include African horse sickness, Foot and mouth disease of cattle, Virus pneumonia of pigs, Rabies etc. Common human viral diseases include Influenza, Measles, Herpes, Dengue, Smallpox, Mumps, Common cold, Hepatitis, AIDS. The recent viral pandemics / epidemics include Ebola Virus Disease, Rift Valley fever, Bolivian hemorrhagic fever, Crimean Congo Hemorrhagic Fever, SARS, and MERS etc.
