Question
Why are genes encoding resistance to antibiotics considered useful selectable markers for E.coli. cloning vector? Explain with the help of one example.
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Answer
- Genes encoding antibiotic resistance are useful as selectable markers because the normal E.coli cells do not carry resistance against any of these antibiotics.
- The ligation of alien DNA is carried out at a restriction site present in one of the antibioticresistance genes, say a foreign DNA is ligated at the BamHI site of tetracycline resistance gene in the vector pBR 322.
- The recombinant plasmids will lose tetracycline resistance due to insertional inactivation.
- The recombinants could be selected out from the non-recombinants by plating the transformants on ampicillin-containing medium.
- The transformants growing on ampicillin-containing medium are transferred to tetracycline-containing medium.
- The recombinants cannot grow on this medium, but non-recombinants will grow on this medium too.
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