Figure 3.—

Replication-blocked λ lytic cycle crosses conducted in the absence of DNA replication produce ample phage particles for genetic analysis. When one of the two parents is heavy labeled and the other carries ordinary isotopes, the density of each of the resulting progeny particles reveals the fraction of its DNA that has been inherited from each of the two infecting parents. Selection against terminally located ts markers (Ats and Rts) allows only crossover particles to plate. If most of the crossover particles have enjoyed but one exchange, the location of that exchange is revealed by the position of the particle in a cesium formate equilibrium density gradient. The cosegregation of the cI marker with the density label implies, for most purposes, the validity of the assumption of single exchanges. Lysates centrifuged to equilibrium in a density gradient are collected as drops emerging successively through a needle hole into 1 ml of broth. Each such sample is assayed by plating at permissive temperature for total phage and at high temperature for A⁺R⁺ recombinants. Among the recombinants, the cI marker is scored from the appearance of the plaque. Phage-carrying chromosomes that have recombined by splicing the DNA duplex across the cI gene make sectored colonies interpreted as heteroduplexes (see Stahl 1994).