Joint molecules of lambda DNA as an intermediate of genetic recombination

Joint molecules of lambda DNA formed in the absence of DNA replication, which may be involved in the process of genetic recombination can be observed as branched DNA derived from different phage particles. These molecules are associated through base-pair hydrogen bonding in synaptic regions, usually with short single-stranded gaps. Furthermore, joint molecules could be accumulated up to ten fold when lambda was irradiated with ultraviolet light before infection of polI mutant of E. coli. Infection at low multiplicity did not give rise to joint molecules. These results suggest that single-strand breaks and gaps introduced in duplex lambda DNA facilitate the formation of joint molecules.