Virus are the smallest and ultra microscopic particles i.e they are observed only by 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 in active viable forms only inside the host and inactive once outside the host cells. Hence they have characters of non-living forms also. Structure of a virus is simple and they do not need any food, respiration and other normal cell requirements for survival. They have no specific birth or death and are just particle replicating in host cells.
Viruses do not possess any cellular constituents. They are particles which have two biochemical substances i.e
1. Nucleic acid &
The nucleic acid forms the center of virus particle and is either a DNA as in animal virus, bacteriophages etc. And is RNA in plant virus. This nucleic acid forming the center portion is surrounded by a protein coat termed as capsid.
This protein coat forms the bulk of virus mass and may sometimes have carbohydrates and lipids to some extent.
Let us see the example of bacteriophage virus as shown in the fig above.
A bacteriophage has a head, neck, tail, base plate (end plate) and tail fibers.
Head is hexagonal in shape with double stranded DNA at the center. It is surrounded by a protein coat. This protein coat extends into neck, tail and also tail fibers. These tail fibers help in attachment when attacking a bacteria.
There is not much to study on 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 infects animal cell. Reproduction happens in four stages and taking bacteriophage for an example,
a) Infection phase: Here virus particle gets attached to cell, bores a hole with lysosomes type enzyme and injects it DNA into the host cell.
b) Eclipse phase: Here the virus DNA take the chromosome forming machinery of the cell under control and directs 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 bacterial cell and thus get liberated out of the cell.