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10.7B: The Lysogenic Life Cycle of Bacteriophages

Skills to Develop

  1. Describe the lysogenic life cycle of temperate phages (including spontaneous induction).
  2. Define the following:
    1. temperate phage
    2. lysogen
    3. prophage

Bacteriophages capable of a lysogenic life cycle are termed temperate bacteriophages. When a temperate bacteriophage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage (see Fig. 1). In the latter case, the cycle begins by the bacteriophage adsorbing to the host bacterium or lysogen and injecting its genome as in the lytic life cycle (see Fig. 2 and Fig. 3). However, the bacteriophage does not shut down the host cell. Instead, the bacteriophage DNA inserts or integrates into the host bacterium's DNA (see Fig. 4). At this stage the virus is called a prophage. Expression of the bacteriophage genes controlling bacteriophage replication is blocked by a repressor protein, and the phage DNA replicates as a part of the bacterium's DNA so that every daughter bacterium now contains the prophage (see Fig. 5).

 

 

The number of viruses infecting the bacterium as well as the physiological state of the bacterium appear to determine whether the temperate bacteriophage enters the lytic cycle or becomes a prophage.

In about one out of every million to one out of every billion bacteria containing a prophage, spontaneous induction occurs. The bacteriophage genes are activated and new bacteriophages are produced by the lytic life cycle (see Fig. 5A, Fig. 6, Fig. 7, Fig. 8, and Fig. 9).

Exercise: Think-Pair-Share Questions

Name a human viral infection that has a life cycle equivalent to the lysogenic life cycle of bacteriophages.

Summary

Bacteriophages capable of a lysogenic life cycle are termed temperate phages. When a temperate bacteriophage infects a bacterium, it either replicates by means of the lytic life cycle and cause lysis of the host bacterium, or, incorporates its DNA into the bacterium's DNA and become a non-infectious prophage whereby the bacteriophage DNA replicates as a part of the bacterium's DNA so that every daughter bacterium now contains the prophage. In rare cases spontaneous induction occurs. The bacteriophage genes become activated and new bacteriophages are produced by the lytic life cycle.

Contributors

  • Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)