Introduction:

Viruses are mainly acellular entities. They are genetic elements which can't replicate independently of a living cell termed as the host cell. Viruses encompass extracellular forms that facilitate them to exist exterior to the host for long periods. Although to multiply, they have to enter a cell in which they can replicate causing the infection. Viruses are the most abundant microorganisms on earth and infect all kinds of cellular organisms. The study of viruses is termed as virology.

Definition:

Viruses are simple acellular entities which can simply reproduce in living cells.

General features of Viruses:

1) Viruses are the smallest microorganisms. They range in size from 10 to 400 μm in diameter and can merely be viewed beneath an electron microscope.

2) They are acellular, that is, not cellular and non living.

3) They just reproduce when present in the living cells.

4) They are infectious agents.

5) A complex virus particle or virion comprises of one or more molecules of RNA or DNA enclosed in the coat of protein.

6) Viruses can exist in two stages: intracellular and extracellular.

7) The extracellular stage known as virion possesses few if any enzymes can't reproduce independent of living cells. This is metabolically inert and doesn't carry out respiration.

8) In the intracellular stage, viruses exist primarily as replicating nucleic acids in the host cells which induce host metabolism to synthesize virion components which are later discharged.

Virion Size:

Virions range in size from around 10 to 400µm in diameter. The smallest viruses are a little bigger than ribosomes while the pox viruses which comprise vaccinia are around the similar size as the smallest bacteria and can be observed in the light microscope. Most viruses though, are too small to be visible in the light microscope and should be viewed by scanning and transmission of electron microscope.

The Structure of Viruses:

A virus is generally made up of a central genetic nucleic acid molecule surrounded by a protein coat termed as a capsid. The combination of both is termed as the nucleocapsid. The capsid surrounds and protects the viral nucleic acid. The capsid as well gives the virus a characteristic shape and assists to establish the specificity of the virus for a specific host cells. Capsids are big macromolecular structures which self assemble from many copies of one or a few kinds of proteins. The protein employed to build the Capsids is termed as protomers. The simplest virus is a naked virus (that is, nucleocapsid) comprising of a geometric capsid assembled around a nucleic acid. On the other hand, we can contain a virus made up of a nucleocapsid surrounded by a flexible membrane termed as an envelope. This kind of virus is termed as an envelope virus.

The different morphology kinds of viruses outcomes from the combination of a specific kind of capsid symmetry with the presence or absence of an envelope that is a lipid layer external to the nucleocapsid.

Viral Genomes:

All the cells have double stranded DNA genomes. By contrary, viruses have either RNA or DNA genomes (that is, one group of viruses does use both RNA and DNA as their genetic material however at different phases of the replication cycle). Therefore, we have RNA viruses or DNA viruses.

Virus genomes can be categorized based on whether the nucleic acid in the virion is RNA or DNA and further categorized to whether the nucleic acid is single or double stranded. Linear or circular, a few viral genomes are circular however most are linear.

We can contain single stranded DNA, double stranded DNA, single stranded RNA and double stranded RNA. All four kinds are found in the animal viruses. Most of the plant viruses have single stranded RNA genomes and most bacteria viruses have double stranded RNA.

Capsids Symmetry:

There are three kinds of capsid symmetry: helical, icosahedral and complex. 

1) Helical Capsids:

They are shaped similar to hollow tubes having protein walls. The tobacco mosaic virus is an illustration of this virus. In this virus, the self assembly of protomers in a spiral or helical arrangement generates a long rigid tube, 15 to 18 nm in diameter by 300nm long.

The capsid bounds an RNA genome that is wound in a spiral and lies in a groove made by the protein molecule. The size of a helical capsid is affected by both its protomers and nucleic acid enclosed in the capsid.

2) Icosahedral Capsid:

The icosahedral is a regular polyhedron having 20 equilateral triangular faces and 12 vertices and is roughly spherical in shape. This is one of the nature's favorite shapes. A few genes at times merely one can code for protein which self-assemble to form the capsid. These Capsids are made from ringo-krob-shaped into the caller capsomers each generally made up of 5 or 6 protomers. Pentamers (pentons) encompass five subunits and hexamers (hexons) encompass six.

3) Viruses with Capsids of Complex Symmetry (Complex viruses):

Some Virions are more complex than the helical and icosahedral capsid being comprised of some parts, each having separate symmetries and shapes. The most complex viruses in terms of structures are a few of the bacterial viruses that possess icosahedral heads plus helical tails. In several bacterial viruses like bacteriophage T4 of Escherichia coli the tail itself consists of a complex structure. The complete T4 tail consists of 20 different proteins and the T4 head consists of several more protein.

Virus Reproduction:

Viruses require a host cell in which to reproduce; therefore the first step in the life-cycle of a virus is attached to a host.

This is followed by entry of either the nucleocapsid or the viral nucleic acid to the host. When the nucleocapsid enters uncoating of the genome generally takes place before further steps can take place.

Once free in the cytoplasm, genes encoded through the viral genome are expressed, that is, the viral genes are transcribed and translated. This lets the virus to control the host cell's biosynthetic machinery so that the new Virions can be made.

The viral genome is then replicated and viral proteins are synthesized. New Virions are constructed by self assembly of the coat proteins having the nucleic acids and finally, the matured Virions are discharged from the host.

The Cultivation of Viruses:

As viruses are powerless to reproduce independent of living cells, they can't be cultured in the similar manner as eukaryotic and prokaryotic microorganisms. Animal viruses are cultivated through inoculating suitable host animals or embryonated egg - fertilized chicken eggs incubated around 6 to 8 days after laying. More recently, animal viruses have been grown in the tissue (cell) culture on monolayer of animal cells.

Bacterial and Archea viruses are cultivated in either broth or agar cultures of young, actively growing cells. Plant viruses are cultivated in a diversity of ways which comprise plant tissue cultures, cultures of separated cells and cultures of protoplasts (that is, cells lacking cell wall) and growing of the viruses in whole plants.

Virus Purification and Assay:

Viral purification and Assays are essential so as to accurately study the virus structure, reproduction and other features of their biology.

Virus Purification:

This comprises getting or isolating the viral particle in its pure state, purification makes utilization of some virus properties.  

Four of the most broadly used techniques to isolate and purify viruses are:

a) Differential and density gradient centrifugation. This is frequently employed in the initial purification steps to separate virus particles from the host cells.

b) Precipitation of the viruses particles.

c) Denaturation of the contaminants.

d) Enzymatic digestion of the host cells constituents.

 Virus Assays:

The quantity of viruses in a sample can be found out either directly through counting particle numbers using the electron microscope or indirectly by the measurement of an observable effect of the virus using methods such as the hemaglutination assay.

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