A virus is an obligate intracellular parasite containing genetic material surrounded by protein. Viruses depend on the host cells that they infect to reproduce. They cannot reproduce by themselves but once it infects a susceptible cell, however, a virus can direct the cell machinery to produce more viruses. Virus particles can only be observed by an electron microscope. Viruses cause many common human infections and are also responsible for the number of rare diseases. They contain either RNA or DNA as their genetic material.
Structure of a Virus
A fully assembled infectious virus is called a virion. The simplest virions consist of two basic components: nucleic acid (single or double-stranded RNA or DNA) and a protein coat, the capsid. The function of the capsid is to work as the shell to protect the viral genome from nucleases and which during infection attached the virion to specific receptors exposed on the prospective host cell.
Some virus families have an additional covering, called the envelop, which is usually derived in part from modified host cell membranes. Viral envelopes consist of the lipid bilayer that closely surrounds a shell of virus-encoded membrane-associated proteins.
Classification of viruses
Viruses are classified on the basis of different criteria, mainly:
- Morphology: Helical symmetry and Icosahedral symmetry
- Chemical composition and Mode of replication: RNA single/double-stranded, DNA single/double-stranded
Life Cycle of Virus consists of six stages within the host cell they are:
- Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 6.3, Viruses: Structure, Function, and Uses
- Gelderblom HR. Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 41.