() The pairing of the 42mer oligonucleotides N1 and N2 to produce a nicked-DNA duplex with nicks in the backbone at every 21 bp. () The formation of gapped-DNA, with 2 nt gaps at every 21 bp, by the pairing of the 44mer oligonucleotides G1 and G2. () The formation of nicked-gapped-DNA, with alternating nicks and 2 nt gaps at every 21 bp, by the pairing of the 42mer oligonucleotide N1 and the 44mer oligonucleotide G2. Oligonucleotide sequences are given in Materials and Methods.