PCR problem

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arsonkamal's picture
PCR problem

i m doing site-directed mutagenesis for DNMT1 gene. my desired size is 6kb, but after the pcr amplification i always get around 4kb,
i designed  a mutant primer (Tm=56.7,using pcr master mix as my DNA polymerase) and run pcr to mutant my gene, but i never get my desired size
i m seeking help