Question

A piece of foreign DNA was inserted into a plasmid with an antibiotic resistance gene and lactose Z gene. The plasmid DNA was cut with a restriction enzyme, which splits the lactose Z gene and opens the circle. The foreign DNA was next inserted into the open restriction site of the plasmid. When the recombinant plasmid was introduced into bacterial cells and grown in the presence of antibiotic, some of the colonies turned blue in the presence of gene. The blue colonies contained

• A. Plasmid only
• B. Foreign DNA only
• C. Both foreign DNA and plasmid
• D. Neither plasmid nor foreign DNA

Answer 1

The correct option is A Plasmid only

In producing antibiotic resistant cloning plasmid, the ColEl plasmids of E. coli have been modified for use as cloning vectors. The original colicin genes have been deleted so that the bacteria carrying these plasmids no longer produce these anti­bacterial toxins. In addition, a gene for antibiotic resistance has been added. This provides an easily identifiable phenotype to the bacteria that carry the altered plasmid. Vectors are often designed to be selected by antibiotic resistance.

Thus, thecorrect answer is option A.