Chapter 2 Genetic Structure

Abstract

A special feature of λ genetics is that two different linkage maps can be constructed by applying standard techniques to the virus at different stages of its life cycle: a phage map, based on the frequencies of recombinant types in the phage yield from infected cells; and a prophage map, derived from the study of lysogenic bacteria. Before describing the details of either map, I shall outline the way in which the difference is thought to arise. The reader should bear in mind that this procedure reverses the historical order of discovery: the manner of prophage insertion is inferred from the results, rather than prerequisite to their formal description.

PROPHAGE INSERTION AND EXCISION
Normal Cycle
Infection by λ has two possible outcomes, each realized in a fraction of a population of infected bacteria. In some cells, the injected DNA molecule replicates extensively, and many new phage particles are assembled intracellularly and later liberated by lysis. This sequence of events is called the productive cycle of phage development. Other cells survive infection, but harbor the phage genome in a latent state called prophage. The prophage is formed by insertion of λ DNA into the bacterial chromosome at a specific site. It is passively replicated as part of the bacterial chromosome.

Figure 1 diagrams insertion and excision of prophage. The lower line shows the gene order of the phage linkage map. The top line shows the map of the bacterial chromosome bearing the inserted prophage. The central circle is the intermediate most...