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A foreign DNA fragment is ligated at the Pst I restriction site of the cloning vector pBR322. The resulting recombinant plasmids are transformed into E. coli cells. If these specific recombinant transformants are plated first on a tetracycline-containing medium and then replica-plated onto an ampicillin-containing medium, what will be their growth pattern?

Options

  1. AThey will grow on the tetracycline medium and will also grow when replica-plated onto the ampicillin medium.
  2. BThey will grow on the tetracycline medium but will fail to grow when replica-plated onto the ampicillin medium
  3. CThey will fail to grow on the tetracycline medium but would grow if plated directly on the ampicillin medium.
  4. DThey will fail to grow on both the tetracycline medium and the ampicillin medium.

Correct answer

B. They will grow on the tetracycline medium but will fail to grow when replica-plated onto the ampicillin medium

Step-by-step solution

The cloning vector pBR322 has two antibiotic resistance genes: amp^ R (ampicillin resistance) and tet^ R (tetracycline resistance). The restriction site for the enzyme Pst I is located within the amp^ R gene. When a foreign DNA is inserted at the Pst I site, the amp^ R gene is disrupted due to insertional inactivation. As a result, the recombinant plasmid loses its ability to confer resistance to ampicillin, but the tet^ R gene remains intact. Therefore, these recombinant transformants are resistant to tetracycline

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