MoBio The Life Cycle of Lambda Phages Chapter 1

λ phages are commonly used in DNA cloning. They can have either lytic or lysogenic cycle, depending on the environment. In the lytic cycle, λ 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, λ phages may replicate with the host cell.

Under certain conditions (e.g., ultraviolet irradiation of cells), the λ phages may transform from the lysogenic cycle to the lytic cycle. This transformation is mainly controlled by two proteins: cI (also known as λ 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 λ phage's lysogenic cycle and its transformation into the lytic cycle. (a) Before attachment. (b) Attachment, penetration and uncoating. (c) λ DNA circularizes (mechanism). (d) λ DNA integrates into the host DNA (mechanism) and replicates with the host cell. λ phages will remain in the lysogenic cycle if cI proteins predominate. (e) λ phages will be transformed into the lytic cycle if Cro proteins predominate. (f) Release.