singlestranded dna gaps tails and loops are repaired in escherichia coli uniformly methylated heteroduplex plasmids which contained 6 mismatched regions including loops of 24 30 248 and 283 nucleotides as well as singlestranded gaps and free ends were introduced into a recombinationdeficient strain of bacteria and the products of repair were analyzed the results indicate that these cells are capable of repairing all of these structures although with different efficiencies repair of singlestranded gaps and free ends which occurs most efficiently is always associated with acquisition of information from the uncut strand unidirectional repair regions containing single loops or twin loops were repaired at similar efficiencies in these cases each of the two strands was capable of acting as the template for repair bidirectional repair at sites containing twin or substitution loops the larger of the loops was removed twice as efficiently as the smaller loop dna sequencing of the repaired regions indicated that the repair is precise the data also suggest that markers separated by only 58 nucleotides do not always segregate together indicating that repair tracts may be relatively short