Viruses are a unique group of pathogens with a simple acellular organization and a distinct pattern of multiplication.
Despite their simple structure they are a major cause of disease.
They have no cytoplasmic membrane, cytosol, or functional organelles, but they can infect all types of cells, and numerous viruses can also infect bacteria, which are called bacteriophages.
Viruses and bacteriophages are not capable of metabolic activity on their own, so instead, they invade other cells and use their metabolic machinery to produce more viral molecules, nucleic acid and proteins which then assemble into new viruses.
Viruses can exist either extracellularly or intracellularly.
In the extracellular state, the virus is called a virion and isn’t capable of reproducing.
A virion consists of a protein coat, called a capsid, surrounding a nucleic acid core which contains the genetic material or the viral genome.
The nucleic acid and the capsid are collectively called a nucleocapsid.
Some virions have a phospholipid membrane derived from the host cell, called an envelope which surrounds the nucleocapsid.
The viruses that have an envelope are called enveloped viruses and these include the herpesviruses and HIV, while the ones that lack the envelope, such as poliovirus, are called non enveloped or naked viruses.
Once inside the cell, the virus enters the intracellular state, where the capsid is removed and the virus becomes active.
In this state the virus exists solely as nucleic acids that induce the host to synthesize viral components from which virions are assembled and eventually released.
Now, the viruses are surrounded by an outer protein coating called the capsid, which protects the viral genome and aids in its transfer between host cells.
Also, according to their capsid symmetry the viruses can come in many shapes and sizes.
There are three types of shapes: helical, icosahedral, and complex.
First, the helical viruses have a capsid with a central cavity or a hollow tube which is made by proteins arranged in a circular fashion, creating a disc like shape.
The disc shapes are attached helically, creating a tube with room for the nucleic acid in the middle.
An example of a virus with helical symmetry is the tobacco mosaic virus which is the most studied example.
Moving on to the icosahedral viruses which are made up of equilateral triangles fused together in a spherical shape that fully encloses the genetic material.
These viruses are released into the environment when the cell dies, breaks down and lyses, thus releasing the virions.