General Characteristics of Viruses

General Characteristics of Viruses

Defining Viruses

  ·     Viruses are submicroscopic, acellular, obligate intracellular parasites—they replicate only inside a living host cell.

Components of Viruses

  ·     Viruses consist of a nucleic acid core and a protein capsid. Some viruses also have a membranous envelope.

  ·   Viral genetic information is contained in either DNA or RNA but never both.

  ·     Capsids are made up of subunits called capsomeres. They can have icosahedral symmetry, filamentous structure or complex structures.

  ·     A viral capsid and genome form a nucleocapsid. Such viruses are called naked viruses; those with a nucleocapsid surrounded by an envelope are enveloped viruses.

Sizes and Shapes of Viruses

  ·     Viruses have polyhedral, helical, binal, bullet or complex shapes and vary in size from 20 to 300nm in diameter.

Host Range and Specificity of Viruses

  ·     Viruses vary in host range and viral specificity. Many viruses infect a specific kind of cell in a single host species. Other viruses can infect several kinds of cells, several hosts, or both.

  ·     Viral specificity is determined by (1) whether a virus can attach to a cell, (2) whether appropriate host enzymes and other proteins the virus needs in order to replicate are available inside the cell, and (3) whether replicated viruses can be released from the cell to spread the infection to other cells.