A virus is a smallest and ultramicroscopic particle discovered by Iwanowsky in 1892.
They are observed only by an electron microscope.
They are so smaller that they can pass through bacterial filters.
Further, they are obligate parasites on either animal or plant cells.
That means they are inactive viable forms only inside the host and inactive once outside the host cells.
Hence they have characters of non-living forms also.
Virus structure & function
The structure of a virus is a bit simple. They do not need any food, air, and water, unlike other living organisms.
They have no specific birth or death and are just particles replicating in the host cells.
Viruses do not possess any cellular constituents. They are particles that have just two biochemical substances like
1. The Nucleic acid
2. The Protein core.
The nucleic acid forms the center of the virus particle. It is usually DNA, as in animal viruses & bacteriophages.
While it is an RNA in a plant virus. This nucleic acid forming the center portion is surrounded by a protein coat termed the capsid. This capsid is made of many capsomeres.
This protein coat forms the bulk of virus mass. Which may sometimes have carbohydrates and lipids to a small extent.
Let us see the structure of the virus using an example of a bacteriophage virus. ( as shown in the fig below).
A bacteriophage has a head, tail, collar (neck), base plate (endplate), and tail fibers.
The Head is hexagonal in shape with double-stranded DNA at the center. It is surrounded by a protein coat. This protein coat extends into the collar, tail, and also tail fibers. There are 6 tail fibers. These help in the attachment of the virus to the bacteria.
There is not much to study in virus biology except for virus reproduction. Other than reproduction, they have no other physiological processes.
Virus reproduction occurs once a virus infects a compatible cell. i..e. when RNA virus attacks plant cell and DNA containing virus infect the animal cell.
Reproduction or infection happens in four stages and taking bacteriophage, for example, the steps are
a) Adsorption phase
Here, the virus particle gets attached to the bacterial cell.
Initially, the particle attaches to the bacterial wall through the ionic bonds.
This is reversible with a change in pH or salt concentration.
Later, the attachment becomes irreversible as the virus proteins on the surface link to bacterial cell surface receptors.
b) Penetration and uncoating.
The other way is the virus particle fuses to the host cell, and its genetic material is released into the cytoplasm of the host cell.
b) Eclipse Phase (replication and biosynthesis phase)
Here the virus DNA takes the chromosome-forming machinery of the cell under control and directs it to produce different parts of the virus particle.
c) Vegetative phase
In this stage, the virus components formed get assembled to form new virus particles with DNA as their core and protein capsule surrounding it.
d) Lytic phase:
Here the virus particles thus formed initiate breakage of the bacterial cell and thus get liberated out of the cell.