If a plasmid contains two restriction sites for a given enzyme and both are cut, what is the expected result on a circular plasmid?

If a plasmid contains two restriction sites for a given enzyme and both are cut, what is the expected result on a circular plasmid?

• A. One fragment
• B. No change
• C. Two fragments
• D. Three fragments

Answer: (C)

When a circular plasmid is cut by a restriction enzyme, each recognition site that is cut opens the circle. If there is only one site cut, the circle becomes a single linear piece, so you’d get one fragment. When there are two distinct cut sites for the same enzyme and both are cut, the circle is opened at two points, producing two separate linear pieces. The plasmid’s DNA is still all present, just split into two fragments whose lengths add up to the original plasmid size. The exact sizes depend on how far apart the two cut sites are along the circle.

When a circular plasmid is cut by a restriction enzyme, each recognition site that is cut opens the circle. If there is only one site cut, the circle becomes a single linear piece, so you’d get one fragment. When there are two distinct cut sites for the same enzyme and both are cut, the circle is opened at two points, producing two separate linear pieces. The plasmid’s DNA is still all present, just split into two fragments whose lengths add up to the original plasmid size. The exact sizes depend on how far apart the two cut sites are along the circle.