If one considers the structure of a prophage as indicated below, and asks what would happen if the prophage came out imprecisely, the result would be a specialized phage. While the first two methods require little understanding of the physical structure of the phage, the last method requires a good deal of that information. Fortunately, through the diligent efforts of molecular biologists, there are a plethora of phage that have all the desired properties, at least for E. coli . Alternatively, a helper phage can be used as noted above. Finally, it can often be useful to have a drug-resistance gene cloned into the phage to serve as an easy assay for the genetic presence of the phage.
Specialized phage have two possible ways of introducing genes into a recipient.
The first and most efficient involves the normal phage integration system. By this means, the entire specialized phage DNA is integrated at the phage attachment site, possibly in conjunction with the wild-type helper phage . The other possibility involves recombination between the chromosomal DNA carried by the phage and the chromosome itself. Transferred DNA in generalized transduction can only do this by homologous recombination while DNA associated with specialized phage also has the phage int system as a means of association with the replicon. It is relatively easy.