Fig. 1.

The generation of mosaic and homogenous mutant plaques. In the first example (Upper), a lesion created within the cell strain forms a mutation in the bacterium during phage replication. This lesion is fixed into a mutation during the first round of phage replication, producing a mosaic plaque that is composed of a 1:1 ratio of mutant to WT phage, evident after replating. In the second example (Lower), a mutation sustained within the mouse cells directs the formation of a nearly homogenous mutant plaque, again verified by replating.