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OBJECTIVE: To screen and characterize oligonucleotide aptamers of chronic myelognous leukemia K562 cells.METHODS: The aptamers of chronic myelognous leukemia(CML) K562 cells were screened from 88 nt random ssDNA library in vitro by SELEX method.The screened aptamters were purified and connected to pGEM-T plasmid vector, and were sequenced after screening by the blue and white.The affinities of the aptamers were detected by fluorescent primers.The primary structure and secondary structure were predicted by Clustal 2.0 and DNAMAN softwares.RESULTS: After 13 rounds of screening cycle, the absorbance(A) of the screened aptamers to CML K562 cells raised from 0.12 up to 1.25, and the absorbance(A) of the screened aptamers didn t significantly increase until the 13th round of screening.They have no homologous sequences by analysis of the primary structure, but can be divided into six families.Five families of them have their respective same series, and family six has no the same series.Secondary structure analysis shows that the stem-loops and bulges structure of the aptamters may be the structure foundation of specific binding to CML K562 cells.CONCLUSION: The oligonucleotide aptamers of high-affinity binding to ch CML K562 cells were gotten by SELEX method.