Viruses: Characteristics, Structures, Mode, Nutrition and Control

 

Viruses: Characteristics, Structures, Mode, Nutrition and Control


The position of viruses relative to living organisms has been the topic of interesting debates among many scientists over the years. The basic question has been concerned with whether they are living or not living. We now know that viruses do not fit into the classification of living organisms. They are not cells, but particles.

They differ from the five kingdoms in that they lack a nucleus, cytoplasm or cell membrane which characteristics cells of their own and cannot produce ATP. Some viruses can be crystallized like non-liting chemicals. Viruses are capable of parasiting living cells “and cannot replicate (reproduce) themselves outside a living host cell.

Every virus is made up of the basic genetic material contained in living things. (i.e. DNA or RNA) surrounded by a protein coat. Viruses are obligate intracellular parasites that are host specific.

This article will explain: what viruses are characteristics of viruses, the structures of viruses, mode of nutrition of viruses, culturing of viruses and viral diseases and their control.

 

Read: Characteristics and Classification of Lliving Organisms


Major Characteristics of Viruses



Viruses: Characteristics, Structures, Mode, Nutrition and Control

The major characteristics of viruses are given below.

1. They are smaller than bacteria so they pass through bacterial filters.

2. They are crystallisable.

3. They lack structural sites for protein synthesis. No Ribosomes, ATP and Mitochondria except in Arenoviruses.

4. They lack metabolic enzymes so they cannot generate their own energy. They depend on host's A.T.P. - generating system for their energy

5. They are made up of an outer coat of protein capsid enclosing the genetic material (no nucleus)

6. The genetic material is either RNA or DNA, never both, except the pox and retro viruses (AIDS virus). 7. All viruses contain carbohydrates.

8. They are intracellular obligate parasites requiring living cells for survival; hence they can reproduce only in other living cells by multiplication (also called replication).

9. They cannot grow on artificial media like bacteria.

10. They are highly specific, not only to host, but to a particular tissue within a specific host.

11. They do not have a cellular structure.

 

Structure of Viruses

Viruses consist of two components, nucleic acid and protein coat. The nucleic acid is the genetic material, which may be single-stranded or double-stranded ribonucleic acid. The protein material or coat capsule protects the genetic material.

Viruses: Characteristics, Structures, Mode, Nutrition and Control


Four morphological groups have been identified, on the basis of their symmetry, namely;

1. 1. Icosahedral e.g. Herpes and polio viruses

2. 2. Helical e.g. Tobacco mosaic virus and rabies virus

3. 3. Binal e.g. Bacteriophage

4. 4. Complex e.g. small pox and influenza viruses.

 

Read: Characteristics of Living Things and Non-living Things


Size of Viruses



Viruses: Characteristics, Structures, Mode, Nutrition and Control

Viruses are much smaller than bacterial cells. They can only be seen with the aid of an electron microscope. The particles range in size from about 20- 30nm — about 50 times smaller than bacteria and can pass through filters, which retain bacteria.

 

Mode of Nutrition

Nutrition Viruses are parasites in living tissues of plants and animals and bacteria.

 

Nature of Viruses (Invation of a Bacterial Cell by a Bacteriophage)

A virus invades or infects a host or bacterial cell by attaching itself to specific receptor sites on the cell. Once secured to the cell, the virus penetrates the cell wall or cell membrane.

Once inside the host cell, the viral nucleic acid takes over the metabolism and genetic machinery of the host cell to produce new virus particles in their thousands.

A free bacteriophage particle comes into contact with a bacterial cell by random collision. The phage attaches itself to a specific site on the bacterial cell wall by means of the tail fibers and end plate. The tail sheath contracts to force the DNA from the head core into the host (bacterial) cell.

This event causes the bacterial DNA to be disorganized and the bacterial cell metabolism now comes under the control of the viral DNA.

The bacterial synthetic machinery is now used to make enzymes, which replicate viral DNA and viral structural proteins. Subsequently new viral heads and tails are formed in large numbers, and the component protein parts come together, forming complete new viruses.

The process continues until the bacterial cell undergoes lysis releasing copious amounts of new virus particles into the infected host cells.

 

Culturing of Viruses

Because viruses cannot grow on artificial, they can only be cultured in living tissue. Living tissue cultures used in the laboratory include mice and incubated chicken eggs, and plant leaves/tissues.

 

Viral Diseases and their Control

1. All viruses are intra-cellular parasites.

2. Their reproduction within the host cells is rapid and results in the destruction of the cells.

3. This explains why they are so harmful.

4. The most serious diseases caused by viruses include measles, poliomyelitis, small pox, chicken pox, hepatitis, Acquired immune deficiency syndrome (AIDS).

5. Other viral diseases of man are influenza, mumps, common cold, cowpox, shingles and dengue.

6. Plant diseases due to virus infection include swollen shoot of cocoa and mosaic disease of cassava, tobacco mosaic disease, Bean mosaic disease, Tulip yellows.

 

Read: Roles and Responsibilities of Policy Stakeholders


Conclusion on Viruses: Characteristics, Structures, Mode, Nutrition and Control

The study of Viruses is very important because of its significant, and importance to man in terms of human health and agriculture.

In this article, we have learned that:

1. Viruses are visible only under the electron microscope.

2. Viruses are classified into four morphological groups according to their symmetry, namely: icosahedral, Helical, Binal and Complex.

3. All viruses are intracellular parasites.

4. They cause diseases in humans, animals and plant.

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