If recombinant lambda phage vector is transferred to ecoli cells it will generate clear or turbid plaques?
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Answer:
The usual picture we carry in our minds of viral infection, is one of death and destruction. We think of viruses that enter a cell, take over and subvert the macromolecular machinery, and produce hundreds of progeny viruses that explode out of the dying cell.
Virulent infections = certain death!
In fact, there are a variety of viral life cycle strategies, and not all of them involve certain death for the cell. We discussed one such example in a previous lecture, when we looked at the filamentous bacteriophages such as M13, fd and f1.
Temperate bacteriophage may enter a cell and produce no progeny virus whatever! They lie silently, allowing the DNA replication machinery of the cell to copy their genomes during the course of the normal cell cycle, and having little discernable effect on the health of the host. At some point, in response to an environmental trigger, the virus leaves its cryptic state (see #7-8 below) and enters a lytic cycle that leads to host cell death and virus release (see #3-6 below).
Cryptic infection by temperate bacteriophage
Figure credit: Gary Kaiser
The most thoroughly studied temperate bacteriophage is lambda, which was pulled out of a Paris sewer 50 years ago by Lwoff, Jacob and Monod. They found that certain strains of E. coli, when exposed to ultraviolet light, generated viral plaques on a plate - that is, small areas of bacterial lysis. The word "lysogeny" was coined to describe this type of cryptic infection. Plaques generated by lambda are typically "turbid" rather than "clear" because after an initial round of lytic growth on rapidly dividing E. coli, there follows an overgrowth of bacteria carrying lambda lysogenically. These lysogenic bacteria are "immune" to lytic lambda infection, because they already harbor the virus!