Question

If we want to recover more copies of target (desired) DNA then it should be cloned in a vector having __________________________.
A. Large number of selectable markers
B. Large number of cloning sites
C. Many antibiotic resistance genes
D. An Ori which supports high copy number

Answer 1

Hint: If one wants to recover more copies of target (desired) DNA, then it should be cloned in a vector having a large number of selectable markers.

Complete answer: A selectable marker helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.
1. Transformation is a process through which a piece of desirable DNA is introduced in a host bacterium. Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E. coli (vector).
2. The normal E. coli cells do not carry resistance against any of these antibiotics, hence they are eliminated when cultured in a medium containing those antibiotics.
3. Therefore, a strain of antibiotic-resistant E. coli is used for this purpose as it has numerous selectable markers. One such strain is pBR322, it has resistance against tetracycline and ampicillin.
4. When the desirable DNA is inserted into the tetracycline resistance gene, the resistance against that antibiotic is lost, this can be used to extract only those bacteria from the culture that has accepted the DNA.

Additional information: The selection of recombinants due to the inactivation of antibiotics as stated above is a cumbersome procedure as it requires simultaneous plating on two plates having different antibiotics.

Correct Answer: Option A: A large number of selectable markers

Note: Therefore, alternative selectable markers are developed which differentiate recombinants from non-recombinants on the basis of their ability to produce color in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, â-galactosidase which results in its inactivation. The presence of a chromogenic substrate gives blue colored colonies if the plasmid in the bacteria does not have an insert.