Virus

• Features Common to Viruses:

  • All are intracellular parasites
  • All are small
  • All are fixed in size, no growth or development
  • All use nucleic acid as the genetic material 
  • All have a capsid made of protein
  • None have cytoplasm 
  • All have no (or very few) enzymes
• Viruses vary in:

  • Range of size
  • DNA or RNA as the genetic material
  • Single or double stranded genome
  • Circular or linear genome
  • Which genes are present
  • Size of genome
  • Proteins in and shape of the capsid
  • Presence of a lipid envelope
• Viruses are not considered to be living because:

  • They are not made out of cells
  • They can not keep themselves in a stable state
  • They do not grow
  • They can not replicate themselves
  • They can not perform independent metabolism 
• Viruses are obligate intracellular parasites. Viruses are strict molecular parasites of cells. They must infect cells and reproduce inside them
• Viruses vary in:

  • Shape
  • Genetic material type
  • Genes present
  • Enveloping
• Reproduction of Virus:

  1. Virus attaches to cell ([DNA or RNA] Nucleic acid present inside virus)
  2. Virus penetrates cell membrane and injects nucleic acid (DNA or RNA) into cell 
  3. Viral nucleic acid replicates using host cellular machinery 
  4. New viral nucleic acids are packaged into viral particles and released from cell. The host may be destroyed in the process 
• All viruses use nucleic acid as their genetic material. Some viruses use DNA and some use RNA
• All viruses have an outer coating (capsid) made of proteins
• Ebola virus is 80x90nm in size and has a spherical shape
• Parvovirus is 20nm
• Viruses are small and most range of 0.02-0.2 micrometers although some can exceed 1 micrometer in length
• Typical bacterium is 2-3 micrometers
• Typical eukaryotic cell is 10-30 micrometers in diameter
• Viruses are small so they can inject cells and because they have few (or no) enzymes and do not have cytoplasm