A geneticist uses a cloning plasmid that contains the lacZ gene and a gene that confers resistance to penicillin. She inserts a piece of foreign DNA into a restriction site located within the lacZ gene and uses the plasmid to transform bacteria. She then grows the bacteria on selective media containing penicillin and X-gal. Explain how the geneticist can identify bacteria that contain a plasmid with the foreign DNA. a. Only bacteria with the desired plasmid produce living colonies. b. Bacteria with the desired plasmid produce white colonies. c. Bacteria with the desired plasmid produce both blue and white colonies. d. Bacteria with the desired plasmid produce blue colonies.

Question

Biology

Sidney Garrison

4 October, 19:03

A geneticist uses a cloning plasmid that contains the lacZ gene and a gene that confers resistance to penicillin. She inserts a piece of foreign DNA into a restriction site located within the lacZ gene and uses the plasmid to transform bacteria. She then grows the bacteria on selective media containing penicillin and X-gal. Explain how the geneticist can identify bacteria that contain a plasmid with the foreign DNA. a. Only bacteria with the desired plasmid produce living colonies. b. Bacteria with the desired plasmid produce white colonies. c. Bacteria with the desired plasmid produce both blue and white colonies. d. Bacteria with the desired plasmid produce blue colonies.

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Answers (1)

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Ronnie · Answer 1

b. Bacteria with the desired plasmid produce white colonies.

Explanation:

The bacterial lacZ gene is usually used as a reporter gene, which encodes beta-galactosidase, an enzyme that breakdown lactose but can also breakdown a colourless synthetic analog X-gal, thereby producing blue colonies on X-gal-containing media. But the lacZ will be disabled when the recombinant DNA is spliced into the plasmid. This is because the LacZ protein will not be produced when the gene is disabled. Thus, X-gal will not be broken down and white colonies will be produced, which can then be separated and identified.