[Campbell Biology P.469] A researcher constructs a recombinant plasmid by inserting a 1.5 kb... | Practice Question

A researcher constructs a recombinant plasmid by inserting a 1.5 kb gene into a 3.0 kb plasmid using a single restriction enzyme. If this recombinant plasmid is then cut with the same restriction enzyme and analyzed by gel electrophoresis, how many distinct DNA fragments would be observed, and what would be their approximate sizes?

  • A: One fragment, 4.5 kb.
  • B: Two fragments, 1.5 kb and 3.0 kb.
  • C: Two fragments, 1.5 kb and 4.5 kb.
  • D: Three fragments, 1.5 kb, 3.0 kb, and 4.5 kb.

Explanation

When a circular plasmid 3.0 kb has a gene 1.5 kb inserted into it using a restriction enzyme, the resulting recombinant plasmid is 4.5 kb 3.0 + 1.5 . If this recombinant plasmid is then cut with the same restriction enzyme, the enzyme will cut at the recognition sites flanking the inserted gene, effectively excising the 1.5 kb gene from the 3.0 kb plasmid backbone. This will result in two distinct linear fragments: one of 1.5 kb the inserted gene and one of 3.0 kb the plasmid backbone .