I have been struggling with ligation for long fragment(7kb+11kb).The vector is 11kb and was digested by ASCⅠ(a stick end) and NruⅠ(a blunt end).After digestion,phosphatase was added into the reaction. The insert is 7kb and was digested by the same restriction endonuclease.However,i cannot get a transformant which i need and the produce has strange fragment by agarose gel electrophoresis under ASCⅠ and NruⅠ.Why the strange fragment appeared ?How can  i do?My teacher said that i did a excessive dephosphorylation by phosphatase,but i do't understand.

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