a process to obtain linear double - stranded covalently closed dna “ dumbbell ” constructs from plasmids by restriction digest , subsequent ligation with hairpin oligodesoxyribonucleotides , optionally in the presence of restriction enzyme , and a final digestion with endo - and exonucleolytic enzymes that degrade all contaminating polymeric dna molecules but the desired construct . the invention also provides a process to obtain said dumbbell constructs employing endonuclease class ii enzymes . furthermore , the invention provides a process to obtain linear , covalently closed dna molecules , such as plasmids , free from contamination by genomic dna , by submitting the dna preparation to a facultative endonucleolytic degradation step and an obligatory exonucleolytic degradation step .