Passage II (Questions 1-6)

    Bacteriophage lambda is a non-enveloped DNA virus whose host is the bacterium E. coli. The infection process begins when the virus is adsorbed to the surface of the host cell. Once attached, the phage injects its DNA across the plasma membrane and into the cytoplasm of the E. coli cell, where it is transformed from a linear strand to a circular one. After this point, the infection may take one of two different pathways:

Lytic Pathway

    After injection, the circularized DNA is transcribed and translated by the host cell's DNA. The resulting protein products make up the future viral capsid. The lambda DNA then undergoes several rounds of replication, after which it is packaged into the newly assembled capsids. Multiple new viral particles are formed, ultimately causing the lysis, or bursting, of the host E. coli cell. During lysis, the new lambda phages are released.

Lysogenic Pathway

    After injection, the circularized DNA directs the production of a viral protein known as integrase. This enzyme causes the circular lambda DNA sequence to integrate itself into the bacterial chromosome. Once inserted, the lambda DNA (now termed a provirus) is replicated along with the E. coli host cell. The provirus can remain hidden and inactive in this latent state for multiple replications. If the E. coli host is damaged by UV light or ionizing radiation, the latent lambda provirus can remove itself from the bacterial chromosome and enter the lytic pathway.