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l phages are commonly used in DNA cloning. They can have either lytic or lysogenic cycle, depending on the environment. In the lytic cycle, l phages replicate rapidly and eventually cause lysis of the host cell. In the lysogenic cycle, the viral DNA circularizes and integrates into the host DNA. Then, l phages may replicate with the host cell. Under certain conditions (e.g., ultraviolet irradiation of cells), the l phages may transform from the lysogenic cycle to the lytic cycle. This transformation is mainly controlled by two proteins: cI (also known as l repressor) and Cro. Increase in cI proteins promotes the lysogenic cycle whereas increase in Cro proteins promotes the lytic cycle. The detailed mechanism is explained in Chapter 4 Section H.
Figure 1-F-1. The lytic cycle. (a) Before attachment. (b) Attachment. (c) Penetration and uncoating. (d) Replication. (e) Assembly. (f) Release.
Figure 1-F-2. Illustration of l phage's lysogenic cycle and its transformation into the lytic cycle. (a) Before attachment. (b) Attachment, penetration and uncoating. (c) l DNA circularizes (mechanism). (d) l DNA integrates into the host DNA (mechanism) and replicates with the host cell. l phages will remain in the lysogenic cycle if cI proteins predominate. (e) l phages will be transformed into the lytic cycle if Cro proteins predominate. (f) Release.
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