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
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.
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
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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