as you have  stated it, i asume you know the meaning of site directed mutagenesis or point mutation. but you are ignorant enough to know that you can do that only in a thermocycler not inside a cell and we were talking about "mutation of a purpose" fool

with you attitude i think you are far away frm grad school and what ever you are talking comes straigt out of a text book or few experiments you have done. 
you havent yet understood what were we talking about.