A virus is a sub-microscopic infectious agent that is unable to grow or reproduce outside   a host cell this characteristic enables it to differentiate from bacteria
 
They disease producing potential, they are important biological entities.

Study of virus is known as Virology.

Historical points about virus

Term virus was coined by Louis Pasteur for any unknown agent causing disease.

The first known virus, tobacco mosaic virus, was discovered by M.Beijernick in 1899.

The existence of virus was scientifically proved by D. Iwanowski

Structure: viruses vary in size considerably. Viruses are about 100 times smaller than bacteria.
 
Most viruses which have been studied have a diameter between 10 and 300 nanometers.

Electron microscope is used to visualize virus particle.

1. The smallest plant virus is satellite tobacco necrosis virus
2. The longest plant virus is citrus Tristeza virus.
3. The smallest animal virus is foot and mouth disease virus.
4. The largest animal virus is Pox virus.
5. The most complex animal virus is Parrot fever virus.

A complete virus particle, known as a virion, consists of nucleic acid and coat of protein called a capsid. These are formed from identical protein subunits called Capsomere and have lipid envelope derived from the host cell membrane

Most viruses have several chemical distinct structural subunits that are themselves associated in specific ways to form larger assemblies called capsomeres

Inside the virion are one or more Virus specific enzymes. Such enzymes play a role during the infection and replication process. Some viruses are naked and others contain a lipid bilayer membrane surrounding the nucleiocapsid

There are four main morphological types of virus
 
1 Helical

Capsids are composed of a single type of capsomere sacked around a central axis to form a helical structure which may have a central cavity, or hollow tube

This arrangement results in rod-shaped or filamentous virions

Genetic Material of virion

Generally it is single-stranded RNA, but ssDNA in some cases, is bound into the protein helix, by interactions between the negatively-charged nucleic acid and positive charges on the protein.

Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of capsomers

The well-studied Tobacco mosaic virus is an example of a helical virus.


2   Icosahedral



Electron micrograph of icosahedral viruses (Adenovirus)

Image taken from wikipedia

Most animal viruses are icosahedral with icosahedral symmetry.

A regular icosahedron is the optimum way of forming a closed shell from identical sub-units.

The minimum number of identical capsomers required is twelve, each composed of five identical sub-units.

Many viruses, such as rotavirus, have more than twelve capsomers and appear spherical but they retain this symmetry. Capsomers at the apices are surrounded by five other capsomers and are called pentons

Capsomers on the triangular faces are surrounding by six others and are call hexons. Enveloped example is herpes zoster virus.
 
The outer membrane surrounding an infected host cell, or internal membranes such as nuclear membrane or endoplasmic reticulum.

Lipid layer known as a viral envelope.

This membrane is studded with proteins coded for by the viral genome and host genome; the lipid membrane itself and any carbohydrates present are entirely host coded

Most enveloped viruses are dependent on the envelope for their infectivity.


4.  Complex   structure of typical bacteriophage


Image taken from wikipedia

These viruses possess a capsid which is neither purely helical, nor purely icosahedral, and this may possess extra structures such as protein tails or a complex outer wall.

Some bacteriophages have a complex structure consisting of an icosahedral head bound to a helical tail which may have a hexagonal base plate with protruding protein tail fibres

Genome   Virus contain either DNA or RNA genomes one group of viruses uses both DNA and RNA as their genetic material but at different stage of their reproductive cycle

All viruses use the cells translation machinery and so no matter what the genome structure of the virus ,messenger RNA must be generated that can be translated on the host ribosome’s




Enzymes in Virion

A virion is metabolically inert; some virions do contain enzymes that play important role in infection process.

Example some bacteriophage contains lysozyme, an enzyme that makes a small hole in the bacterial cell wall.

This allows the viral nucleic acid to enter.

Some virions contain enzymes that help in their release from the host.

Retrovirus is a RNA containing virus that replicate through a DNA intermediate.

Examples are Hepatitis virus and cauliflower mosaic virus. Retrovirus is encapsidate virus

A retrovirus replicates in following way.


Entrance into the cell--->Upcoating of the viron---->Reverse transcriptase of one of the two RNA into a single stranded DNA --->Integration of the retroviral DNA into the host genome ---->Transcription of the retroviral DNA leading to formation of viral mRNA and viral genomic RNA --->assembly and encapsidation of the genomic RNA into nucleocapsid in the cytoplasm ----> Budding of enveloped virions at the cytoplasmic membrane and release from the cell.


Replication of virus:

Virus replicates by using their own genetic material, the material serves as a template for the viral proteins and genetic material required to make new viruses this replication follows the following sequence ;

Penetration ?Replication ?Transcription Protein synthesis? viral assembly ?Release



Virus family pathogenic to humans 

RNA VIRUSES




Only RNA virus havind double stranded genome is     REOVIRIDAE

Only single stranded DNA virus is PARVOVIRIDAE

Viral infection causing human cancers




Diagnosis of viral infections

Though several methods are evolved to diagnose the viral diseases, some of which are

1. Serology
2. Viral isolation
3. Virus specific antigens
4. ELISA
5. Western blot

Antiviral drugs
 
Some of the common antiviral drugs are given below

Influenza A----------Amantidine

HIV infection ------- Ziduvudine
                           Lamivudine  
                           Didinosine

Genital herpes ----------- Acyclovir

Herpes zoster--------------- Acyclovir

Chronic hepatitis----------- Interferons

 

INTERFERONS    they are used in the treatment of viral infections.

These are basically cytokines which have application in the treatment and prophylaxis of viral infections

They also exhibit immunomodulating and antiproliferative activities.

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