Introductory Microbiology

Viruses

• Virus can be defined as a genetic element containing either DNA or RNA that replicates in host cells as intracellular parasites but is characterized by having an extracellular state or are intracellular obligate parasites multiples in living host. Virus can use the metabolic machinery of the host cell and can modify the genetics of the host cell.
• Extracellular state: Outside the host cell, the virus is a minute particle containing nucleic acid surrounded by protein, which is referred as virions or virus particles. Virions are inert and have no biosynthetic and metabolic functions.
• Intracellular state: The active state in which the virus replicate in the host cell. When the virus nucleic acid is introduced to host cell and replication starts, the process referred as infection.
• Chemistry of virus: All the living cells have double strand DNA as their genetic material. In contrast viruses can either DNA or RNA as their genetic material and it can be either single strand or double strand. The third group of virus which contain RNA as genome but replicate by DNA intermediate, used both DNA and RNA as their genetic material in their reproductive stage.
• The viruses can be divided into following groups based on their nucleic acid and their form (single/double strand)

  I DNA containing virus:
  Ss DNA virus
  Ds DNA virus
  II RNA containing virus:
  Ss RNA virus
  Ds RNA virus
  III RNA – DNA virus
Virus are also divided into 3 major groups based on their host as
1) plant viruses
2) Animal viruses and
3) Bacteriophages.