How can restriction digests be used to validate a recombinant plasmid?

• A. By sequencing the entire plasmid.
• B. By measuring plasmid color.
• C. By using a single enzyme digest to infer copy number.
• D. By comparing observed fragment sizes on a gel to the expected pattern after digestion with selected restriction enzymes.

Answer: (D)

Restriction digests validate a recombinant plasmid by turning the plasmid’s map into a recognizable barcode of DNA fragments. You choose restriction enzymes that cut at known sites so the insert and backbone produce a predictable set of fragment sizes when the plasmid is digested. After running the digest on a gel, you compare the observed fragment sizes to the sizes you expect from the designed map. If the pattern matches, it confirms that the insert is present in the right place and the overall construct matches the intended design. This is a quick, practical check that can reveal major map errors or missing pieces without sequencing every base.

While sequencing the entire plasmid would give definitive base-by-base confirmation, it takes more time and resources. Measuring plasmid color doesn’t reveal DNA structure or sequence, and a single enzyme digest to infer copy number doesn’t provide information about the insert or plasmid map. The strength of using fragment size patterns from multiple enzyme digests is that it directly reflects the arrangement of the insert within the plasmid.