In a transformation protocol where two plasmids with different resistance genes were mixed, how should you split the cultures?

In a transformation protocol where two plasmids with different resistance genes were mixed, how should you split the cultures?

• A. R- lacks plasmid; R+ contains plasmid
• B. R+ is used for sequencing
• C. R+ contains plasmid with antibiotic marker
• D. R+ contains plasmid; R- lacks plasmid

Answer: (D)

The essential idea is to distinguish cells that truly carry the plasmids from those that don’t by using antibiotic selection and appropriate controls. Plasmids carry resistance genes, so only cells that retain the plasmid will grow on plates with the corresponding antibiotic. Splitting into one culture that still has plasmids (R+) and a separate culture that ends up plasmid-free (R-) gives you a positive control and a negative control. The R+ culture lets you apply selective antibiotics to confirm which resistance markers are present and to propagate the transformed cells. The R- culture should not grow on those selective plates, showing that any growth in the R+ culture is due to plasmid-borne resistance rather than other factors. This setup also accounts for the possibility that plasmids can be lost without selection, so having a plasmid-free reference helps you interpret results accurately.

The essential idea is to distinguish cells that truly carry the plasmids from those that don’t by using antibiotic selection and appropriate controls. Plasmids carry resistance genes, so only cells that retain the plasmid will grow on plates with the corresponding antibiotic. Splitting into one culture that still has plasmids (R+) and a separate culture that ends up plasmid-free (R-) gives you a positive control and a negative control. The R+ culture lets you apply selective antibiotics to confirm which resistance markers are present and to propagate the transformed cells. The R- culture should not grow on those selective plates, showing that any growth in the R+ culture is due to plasmid-borne resistance rather than other factors. This setup also accounts for the possibility that plasmids can be lost without selection, so having a plasmid-free reference helps you interpret results accurately.