Viruses

     














 A virus is a small infectious agent that replicates only inside the living cells of other organisms.A virus is an obligate intracellular parasite that can only replicate inside a living host cell. Once inside the living host cell, a virus becomes integrated into the metabolism of its host, making a virus difficult to control by chemical means. They vary in size from 20 nanometers (poliovirus) to 300 nanometers (smallpox virus) and cannot be seen under a light microscope.

Structure of Virus

The major components of a virus are: Nucleic acid core
                                                             Capsid
                                                             Envelope
Nucleic acid core: The nucleic acid core can either be DNA or RNA that makes up the genetic information (genome) of the virus. RNA genomes only occur in viruses.
Capsid: A capsid is the protein coat that encapsulates a virus and protects the nucleic acid from the environment. It also plays a role in how some viruses attach to a host cell. A capsid consists of one or more proteins that are unique to the virus and determine the shape of the virus.
Envelope: An envelope is a membrane bilayer that some viruses have outside their capsid. If a virus does not have an envelope, the virus is called a naked virus. Examples of diseases caused by naked viruses are chickenpox, shingles, mononucleosis, and herpes simplex. A naked virus is more resistant to changes and is less likely to be affected by conditions that can damage the envelope. Naked viruses are more resistant to changes in temperature and pH. Examples of diseases caused by naked viruses include poliomyelitis, warts, and the common cold.
Shape of virus

• Helical virus: A helical virus is a rod- or thread-shaped. The virus that causes rabies is a helical virus.
• Icosahedral virus: An icosahedral virus is spherically shaped. Viruses that cause poliomyelitis and herpes simplex are icosahedral viruses.




Replication of Virus

• The easiest way to understand how viruses replicate is to study the life cycles of viruses are called bacteriophages.
•  Bacteriophages replicate by either a lytic cycle or a lysogenic cycle. 
• The difference in these two cycles is that the cell dies at the end of the lytic cycle and remains alive in the lysogenic cycle.