Introduction:

The place of viruses relative to living organisms has been the matter of fascinating debates among numerous scientists over the years. The fundamental question has been fretful with whether they are living or not-living.

We now recognize that viruses don't belong to the categorization of living organisms. They are not cells, however particles. They vary from the five kingdoms in that they lack a nucleus, cytoplasm or cell membrane that features cells of their own and can't generate ATP.

There are many viruses which are crystallized like non-lighting chemicals. Viruses are able of parasiting living cells and can't replicate (or reproduce) themselves exterior to a living host cell. Each and every virus is build up of the fundamental genetic material contained in living things (that is, DNA or RNA) surrounded by a protein coat. Viruses are obligate intra-cellular parasites which are host specific.

Features of Viruses:

The main features of viruses are described below.

1) They are much smaller than bacteria so they pass via bacterial filters.

2) Viruses are cellular, non-cytoplasmic infectious agents.

3) Viruses comprise of just a single kind of nucleic acid either DNA or RNA.

4) Viruses are infectious from disease to healthy organisms.

5) All the viruses are necessitate parasites and can multiply only in the living host cells.

6) Virus are host specific that they contaminate merely a single species and definite cells of the host organisms.

7) Viruses are efficient in much small doses. They are extremely resistant to germicides and limits of physical conditions.

Structure of a virus:

Virus mainly comprises of two main parts:

A) Capsid (that is, the protein coat)

B) Nucleic acid

The capsid is the external protein coat. This is defensive in function. It is often composed of many similar subunits known as capsomeres. A few viruses include an outer covering known as envelope for example: HIV. They are known as enveloped viruses. The others are known as naked viruses or simply non-enveloped viruses. The capsid is in close contact with the nucleic acid and thus known as nucleo-capsid. The nucleic acid forms the central core. Dissimilar any living cell, a virus has either RNA or DNA, nevertheless never both. The infective nature of virus is accredited to the nucleic acid while host specificity is accredited into the protein coat.

Size and Shape of virus:

Viruses are extremely small particles which they can be seen simply beneath electron microscope. They are measured in millimicrons (1 millimicron = 1/1000micron). (And 1micron = 1/1000 millimeter). Generally they distinct from 2.0 mm to 300 mm in size. They might be spherical or golf ball-like, rod-shaped, tadpole-like, helical and polyhedral.

Classification of virus:

Although viruses are not classified as members of the five kingdoms, they are varied enough to necessitate their own taxonomy scheme to assist in their study and recognition.

According to the type of the host they infect, viruses are classified mainly into the four kind as described below:

A) Plant viruses comprising algal viruses-RNA or DNA

B) Animal viruses comprising human viruses-DNA or RNA

C) Fungal viruses (Mycoviruses)-ds RNA

D) Bacterial viruses (Bacteriophages) comprising cyanophages-DNA

Rising viral infections in human beings:

Recent figure of emerging viral infections in different regions of the world include Dengue, HIV, Ebola virus, Lassa fever, Hemorrhagic fever, Rift valley fever and SARS.

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