Removing a restriction site and introducing other at its place > What would you do if you want to **remove** an EcoRI restriction site and introduce BamHI restriction site at apprx. the same location ? One of the answers in my textbook was : To construct a DNA fragment with the structure: 5'AATTGGATCC3' 3'CCTAGGTTAA5' I understand that this would ligate efficiently to the sticky ends generated by EcoRI cleavage and would introduce a BamHI site but I don't understand how this will remove/not regenerate the EcoRI site.

The recognition site for EcoRI is `GAATTC`, and the enzyme cuts after the first base. See this picture from NEB:

!enter image description here

The overhang is: AATT, which is supplied by your oligo and fits into the overhang. If the next nucleotide whould be a `C` than the site would be recreated, since its a `G` its not. The new sequence of the old EcoRI site is `GGATTG` which is not recognized by EcoRI.

One technical note: To avoid religation of the digested DNA which carries this site, I would treat it with alkaline phosphatase to remove the phosphate end at the 5' end of the DNA.