What is colony PCR and how is it used to verify plasmids?

• A. A rapid PCR performed directly on bacterial colonies to check for the presence and size of the insert.
• B. A PCR performed after plasmid purification to verify sequence.
• C. PCR used to amplify plasmid replication origin.
• D. PCR to measure colony growth rate.

Answer: (A)

Colony PCR is a rapid screening method that uses PCR directly on a small sample taken from a bacterial colony, without purifying the plasmid first. By designing primers that flank the insertion site (or that span the plasmid–insert junction), you can see a product whose size indicates whether the insert is present and roughly how large it is. If the colony carries the plasmid with the expected insert, you’ll get a PCR product of the anticipated size; if the insert is missing or different, the product will differ in size or may not appear at all. This lets you quickly identify colonies with the correct recombinant plasmid before doing more involved steps like miniprep and sequencing.

Other options describe methods that aren’t testing the colony directly for the insert (such as sequencing after purification, focusing on replication origin, or measuring growth), so they don’t answer how colony PCR verifies the plasmid insert in colonies.