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A Phage or Bacteriophage (from the Greek word phagein meaning "to devour") is a class of virus that will infect bacterial hosts. They are also used as model organisms for the study of the molecular biology and replication of viruses. Bacteriophage structures vary but all are composed of proteins containing DNA/RNA genome which allows them to replicate in the bacterial host. At a first stage the phage injects its DNA into the host bacterial cell (donor) and phage enzymes degrate host DNA. Then the cell synthesizes new phages that incorporate phage DNA and by mistake some host DNA. Transducing phage injects donor DNA and as a result donor DNA is incorporated into the recipients chromosome by recombination. A key example is the T4 phage that will infect an E. coli host cell. T4 phages are made up of a protein head containing the genetic information of the virus, along with a tail and tail fibres that permit the injection of genetic material into the host cell, these structures are made of protein[1]. T4 can only undergo a lytic instead of a lysogenic life cycle. Bacteriophages are thought to be the most abundant organisms in the world with an estimated 1031 in existence. This is a greater number than all other organisms put together, including bacteria[2].


  1. Michael Madigan, John Martinko, David Stahl, David Clark. (2012) Brock Biology of Microorganisms, Thirteenth Edition, San Francisco: Pearson. 267-268
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