All oligonucleotides were synthesized and purified by Sangon (Shanghai, China) and the sequences were showed in Table 1. All DNA oligonucleotides were diluted by TE buffer (10 mM Tris–HCl and 1.0 mM Na2EDTA, pH 8.0). They were denatured at 95 °C for 5 min naturally cooled down to room temperature before use. The polymerase Klenow fragment exo was purchased from New England Biolabs, Inc. The deoxynucleotide solution mixture