In the R+ lane, which fragments are expected?

In the R+ lane, which fragments are expected?

• A. A single intact plasmid
• B. Two fragments: one corresponding to pBAD-rfp and one to ampR-ori-araC
• C. Three fragments with an extra fragment
• D. No fragments at all

Answer: (B)

The main idea here is how restriction enzyme digestion changes a plasmid’s DNA into separate pieces that you can see as bands on a gel. When the enzyme cuts the plasmid in the R+ condition, it separates the circular plasmid into two distinct fragments: one fragment is the insert carrying pBAD-rfp, and the other fragment is the backbone that contains ampR, the origin of replication, and the araC regulatory region. On a gel, you’d expect to see two bands, corresponding to the sizes of these two pieces. This happens because the restriction site used for digestion lies between those two parts, so the plasmid is split into exactly those two fragments. If you didn’t digest the plasmid, you’d expect a single intact, supercoiled band. If there were multiple cut sites, you could get more fragments, but the described setup typically yields two pieces. Complete degradation or no DNA would look very different from two clean bands.

The main idea here is how restriction enzyme digestion changes a plasmid’s DNA into separate pieces that you can see as bands on a gel. When the enzyme cuts the plasmid in the R+ condition, it separates the circular plasmid into two distinct fragments: one fragment is the insert carrying pBAD-rfp, and the other fragment is the backbone that contains ampR, the origin of replication, and the araC regulatory region. On a gel, you’d expect to see two bands, corresponding to the sizes of these two pieces.

This happens because the restriction site used for digestion lies between those two parts, so the plasmid is split into exactly those two fragments. If you didn’t digest the plasmid, you’d expect a single intact, supercoiled band. If there were multiple cut sites, you could get more fragments, but the described setup typically yields two pieces. Complete degradation or no DNA would look very different from two clean bands.