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.

 

Timeline of Virology

Timeline of Virology

·         1500 B.C.: Polio in Egypt

·         1200 B.C.: Smallpox in Egypt

·         11^th century: Variolation practised in Asia and Africa: the deliberate infection of smallpox virus.

·         1796: Edward Jenner developed a smallpox vaccine using cowpox

·         1885: Louis Pasteur successfully developed rabies vaccination.

·         1892: Dimitri Ivanovski showed that the tobacco mosaic disease could be transmitted by extracts that were filtered by the filters fine enough to exclude the smallest known bacteria.

·         1898: Martinus Beijerinck found that the agent that caused the tobacco mosaic disease was not a mere toxin since it grew in the host.

·         1911: Francis Peyton Rous in 1911 described an oncovirus in chickens.

·         1911: Frederick Twort recognized the existence of viruses that infect bacteria.

·         1935: Wendell Stanley achieved the crystallization of the tobacco mosaic virus for electron microscopy and showed that it remains active even after crystallization.

·         1937: Max Delbruck described the basic life cycle of a virus.

·         1937: Max Theiler grew the Yellow Fever virus in chicken eggs and produced a vaccine from an attenuated virus strain.

·         1952: Hershey-Chase experiment showed that only DNA and not protein enters a bacterial cell upon infection with bacteriophage T2.

·         1955: Rosalind Franklin proposed the full structure of the tobacco mosaic virus.

·         1963: Hepatitis B virus discovered by Baruch Blumberg.

·         1965: Howard Temin described the first retrovirus.

·         1976: First recorded outbreak of Ebola hemorrhagic fever.

·         1977: Frederick Sanger achieved the first complete sequencing of the genome of any organism, the bacteriophage Phi X 174.

·         1977: Richard Roberts and Phillip Sharp showed that the genes of adenovirus contains introns and therefore require gene splicing.

·         1979: A world-wide vaccination campaign led by the UN World Health Organization resulted in the eradication of smallpox.

·         1982: Stanley Prusiner discovered prions and showed that they cause scrapie.

·         1985: Harald zur Hausen showed that two strains of the Human Papillomavirus (HPV) cause most cases of cervical cancer.

·         2002: Poliovirus was synthetically assembled in the laboratory.

·         2003: Bacteriophage Phi X 174 was synthetically assembled in the laboratory.

·         2004: Giant mimivirus was sequenced.

·         2006: Two vaccines protecting against the two cancer causing strains of the HPV were released.