G. BACTERIOPHAGE LIFE CYCLES
2. The Lysogenic Life Cycle
The overall purpose of this Learning Object is to learn the lysogenic live cycle of temperate bacteriophages.
LEARNING OBJECTIVES FOR THIS SECTION
Viruses are infectious agents with both living and nonliving characteristics.
1. Living characteristics of viruses
a. They reproduce at a fantastic rate, but only in living host cells.
b. They can mutate.
2. Nonliving characteristics of viruses
a. They are acellular, that is, they contain no cytoplasm or cellular organelles.
b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell.
c. The vast majority of viruses possess either DNA or RNA but not both.
As mentioned in an earlier section, bacteriophages are viruses that only infect bacteria. There are two primary types of bacteriophages: lytic bacteriophages and temperate bacteriophages.
1. Bacteriophages that replicate through the lytic life cycle are called lytic bacteriophages, and are so named because they lyse the host bacterium as a normal part of their life cycle.
2. Bacteriophages capable of a lysogenic life cycle (def) are termed temperate phages (def). When a temperate phage 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.
We will now look at the lysogenic life cycle of bacteriophages.
The Lysogenic Life Cycle of Temperate Bacteriophages
Bacteriophages capable of a lysogenic life cycle are termed temperate phages. When a temperate phage 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 (def) (see Fig. 1).
In the latter case, the cycle begins by the phage adsorbing to the host bacterium or lysogen (def) and injecting its genome as in the lytic life cycle (see Fig. 2 and Fig. 3). However, the phage does not shut down the host cell. Instead, the phage 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 phage genes controlling phage 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 phage 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 phage genes are activated and new phages are produced by the lytic life cycle (see Fig. 5A, Fig. 6, Fig. 7, Fig. 8, and Fig. 9).
To review the lysogenic life cycle of a temperate bacteriophage during which a prophage is formed, (see Fig. 2 through 9).
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Updated: Sept., 2012
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