DNA synthesis at the site of a Red-mediated exchange in phage lambda

In phage lambda, progeny particles bearing unreplicated chromosomes are recombinant by action of lambda's Red system only near the right end of the chromosome. These recombinants are frequently heterozygous (heteroduplex) for markers located there. In replication-blocked crosses involving two heavy-labeled parents we find that particles in the solitary peak, containing progeny with fully conserved DNA, vary in density. Those on the heavy side of this peak are more apt to be heterozygous than are those on the light side. The data fit a model in which a double chain cut at cos, lambda's packaging origin, is followed by partial exonucleolytic degradation of lambda's r chain from the right end leftward. The exposed l chain, which thereby constitutes a 3' overhang, invades an intact, circular homologue after itself suffering some degradation. Completion of the recombinant chromosome sometimes involves DNA synthesis primed by the invading chain.