Question

(a) Identify the selectable markers in the diagram of E. coli vector shown above.

(b) How is the coding sequence of beta-galactosidase considered a better marker than the ones identified by you in the diagram? Explain.

Answer 1

(a) The selectable markers in pBR322 are the ampicillin resistance gene (ampR) and the tetracycline resistance gene (tetR).

(b) The beta-galactosidase coding sequence (lacZ) gives colour change on a chromogenic substrate. Recombinants show white colonies because insertion inactivates the gene (insertional inactivation). This allows direct blue-white screening in a single step, whereas antibiotic markers need repeated plating on two antibiotic media and are slower and cumbersome.