Why are genes encoding resistance to antibiotics considered useful selectable markers for E. coli cloning vector? Explain with the help of one example.
Answer. Genes encoding resistance to antibiotics are considered useful selectable markers for E. coli cloning vector because they help in selecting transformant cell from non-transformant ones.
The genes encoding resistance to antibiotics such as tetracycline, ampicillin, kanamycin or chloramphenicol, etc. are useful selectable markers for E. coli. The common E. coli cells are not resistant to any of these antibiotics. Plasmid pBR322 has two antibiotic resistance genes – ampicillin resistance (ampR) and tetracycline resistance (tetR) which are considered useful for selectable markers. The presence of restriction sites within the markers tetR and ampR permits an easy selection for cells transformed with the recombinant pBR322. For example, insertion of the DNA fragment into the plasmid using enzyme PstI or PvuI places the DNA insert within the gene ampR. This makes ampR nonfunctional. Bacterial cells containing such a recombinant pBR322 will be unable to grow in the presence of ampicillin, but will grow on tetracycline.
