text: To insert a mutation into a DNA sequence, a specific primer is designed. The primer may contain a single substitution or contain a new sequence at its 5 end. If a deletion is required, a sequence that is 5 of the deletion is added, because the 3' end of the primer must have complementarity to the template strand so that the primer can sufficiently anneal to the template DNA.
  Following annealing of the primer to the template, DNA replication proceeds to the end of the template. The duplex is denatured and the second primer anneals to the newly formed DNA strand, containing sequence from the first primer. Replication proceeds to produce a strand of the required sequence, containing the mutation.
  The duplex is denatured again and the first primer can now bind to the latest DNA strand. The replication reaction continues to produce a fully dimerised DNA fragment. After further PCR cycles, to amplify the DNA, the sample can be separated by agarose gel electrophoresis, followed by electroelution for collection.
  Efficiently generating oligonucleotides beyond ~110 nucleotides in length is very difficult, so to insert a mutation further into a sequence than a 110 nt primer will allow, it is necessary to employ overlap extension PCR. In OE-PCR the sequence being modified is used to make two modified strands with the mutation at opposite ends, using the technique described above. After mixing and denaturation, the strands are allowed to anneal to produce three different combinations as detailed in the diagram. Only the duplex without overlap at the 5 end will allow extension by DNA polymerase in 3 to 5' direction.
  Following the extension of the OE-PCR reaction, the PCR mix or the eluted fragments of appropriate size are subject to normal PCR, using the outermost primers used in the initial, mutagenic PCR reactions. 
  In addition, the combination of OE-PCR and asymmetric PCR could be used to improved the efficiency of site-directed mutagenesis.