Viruses are the smallest obligate intracellular infectious agents containing only one type of nucleic acid, either DNA or RNA as their genome. They are Living Chemicals.
Virus differ from bacteria in that they are:
1. much smaller
2. lack the enzyme necessary for synthesis of protein and nucleic acid
3. do not have cellular organisation or ribosomes
4. do not divide by binary fission or grow in inanimate media
5. possess either DNA or RNA
6. are resistant to antibiotics
Size: The size of viruses range from 20 to 400 nm in diameter.
Largest virus: Small pox virus
Smallest virus: Coliphage virus
The simplest natully occuring infective virion consists of a nucleic acid genome packaged into a protein coat. Some viruses have an additional protective layer, the envelope. Viruses without an envelope are known as naked.
1. Nucleic acid core (genome):
a. The genome contains either RNA or DNA.
b. The nucleic acid can exist as a single stranded virus (RNA viruses except reovirus) or as a double stranded (DNA viruses except parvo-virus).
c. The genome can be linear or circular. Most viruses posses linear genomes except papova-virus which contains a supercoiled circular genome.
d. Infective DNA genome is translated by cell polymerase into mRNA and infective RNA genome is translated directly into protein by cellular ribosomes.
2. Capsid: It is a protein coat surrounding the viral genome. Together, nucleic acid enclosed by capsid is known as nucleocapsid. Viral capsids are compsed of varying number of capsomers, each of which is made up of one or more polypeptide chains, known as protomers. Its functions are:
a. protects the genome
b. mediates viral adsorption to and penetration of cells through interactions with receptors on cell membranes.
Capsid may have icosahedral or helical symmetry.
a. Icosahedral nucleocapsids: The icosahedral capsids are of two types, pentagonal capsomers at the vertices and hexagonal capsomers at the facets. Icosahedral capsid is most stable. Icosahedral viruses are mostly naked and spherical. eg: papova, herpes, retro viruses.
b. Helical nucleocapsids: The capsomers and nucleic acid are wounded together to form a helical shape. Most of the helical viruses are enveloped and rod-like in appearance. eg: rhabdovirus, paramyxovirus, orthomyxovirus
Some viruses like pox virus show neither of symmetry but complex symmetry.
3. Envelope: Virus may or may not be enveloped. The evelope is lipoproteinaceous in nature in which lipid is derived from the host cell membrane when the progeny virus is released by budding. Peplomers are the glycoprotein subunits seen as projecting spikes on the surface of the envelop. The glycoproteins serve 3 basic functions:
a. promote interaction with nucleocapsid proteins
b. act as viral attachment proteins to cellular receptors
c. are the major antigens for protective immunity
eg: Influenza virus consist of 2 kinds of peplomers.
4. Shape: Most of the animal viruses are spherical, some are irregular and pleiomorphic.
i. Rabies virus: bullet shaped
ii. Ebola virus: filamentous
iii. Pox virus: brick shaped
iv. Tobacco Mosaic Virus (TMV): rod shaped
5. Resistance: Most viruses are heat labile. They are inactivated by sunlight, uv rays and ionizing radiation. They are more resistant to chemical disinfectant compared to bacteria.