Figure 7-3: Formation of specialized transducing phages by bacteriophage lambda. The central column shows normal excision of the phage genome, mediated by integrase and excisionase. For every such excision, there are about 10(-5) abnormal excisions, where either gal or bio is effectively cloned into lambda. These are rare because they require breakage and joining or heterologous DNA. The reciprocal product (a deleted chromosome), which may or may not be formed in the same event, is not shown. The presence of the cos site allows the excised DNA to be packaged into virions and injected into other cells. Since lambda is cut at cos during packaging, part of cos occurs at each end of the packaged DNA.