After a ligation reaction, bacteria are transformed and plated on ampicillin-containing medium. Some colonies grow, but a researcher still cannot be sure which colonies contain the desired insert. Why?
AAmpicillin selects for cells with the original chromosomal DNA rather than the plasmid
BMany surviving colonies likely carry the vector that re-ligated without the insert, since restriction-cut vectors can self-ligate
CTransformation efficiency is too low to produce colonies; the antibiotic plate should have no growth
DAmpicillin resistance is encoded on the insert, not the vector, so only insert-containing cells survive
Antibiotic selection only confirms that a cell contains the plasmid — it does not confirm the insert is present. The restriction enzyme cuts the vector, but cut vectors can re-ligate without incorporating the insert, and these empty-vector cells also carry ampicillin resistance. A secondary screening step — typically blue-white screening using lacZ disruption — is needed to distinguish colonies carrying the insert from those carrying re-ligated empty vector.
Question 2 Multiple Choice
In blue-white screening, which observation confirms that a colony contains the recombinant plasmid with the insert?
AThe colony is blue, indicating active beta-galactosidase expression from an intact lacZ gene
BThe colony is white, indicating the lacZ gene was disrupted by insert cloning
CThe colony is white, indicating the insert restored lacZ function that was absent in the original vector
DThe colony fails to grow on X-gal plates, indicating successful insert incorporation
In blue-white screening, the cloning site is within the lacZ reporter gene. When no insert is present, lacZ is intact, beta-galactosidase is produced, and X-gal is cleaved to produce a blue product — the colony is blue. When the insert is cloned in, it disrupts lacZ, beta-galactosidase is not produced, X-gal is not cleaved, and the colony remains white. White colonies are the ones to pick. Option C reverses the mechanism; option D is not how the screen works.
Question 3 True / False
The insert DNA and vector must be cut with the same restriction enzyme (or compatible enzymes producing matching overhangs) so that complementary sticky ends can base-pair and be sealed by ligase.
TTrue
FFalse
Answer: True
Sticky ends produced by restriction enzymes are short single-stranded overhangs with specific base sequences. For an insert to ligate into a vector, the insert's overhangs must be complementary to the vector's overhangs — which requires using the same restriction enzyme or enzymes that generate identical overhangs. Blunt-end ligation is possible but much less efficient. Using incompatible enzymes means the overhangs cannot base-pair, and ligation will not produce the desired recombinant molecule.
Question 4 True / False
Plating transformed bacteria on antibiotic-containing medium is sufficient to identify colonies that contain the recombinant plasmid with the desired insert.
TTrue
FFalse
Answer: False
Antibiotic selection solves only the first problem: it eliminates cells that took up no plasmid at all. But it does not distinguish between cells carrying an empty re-ligated vector (no insert) and cells carrying the recombinant vector with the insert. Both types carry the antibiotic resistance gene and will grow. A secondary screening step — such as blue-white screening, colony PCR, or restriction digest of miniprep DNA — is required to identify insert-bearing colonies among antibiotic survivors.
Question 5 Short Answer
Why is a two-stage screening approach (antibiotic selection followed by blue-white screening) necessary in recombinant DNA cloning, and what specific problem does each stage solve?
Think about your answer, then reveal below.
Model answer: Antibiotic selection solves the problem of transformation efficiency: only a small fraction of cells take up any plasmid, so without selection the majority of colonies would not contain the plasmid at all. Blue-white screening solves the problem of re-ligation: among cells that did take up a plasmid, many carry the empty vector that re-ligated at the cut site without incorporating the insert. Blue-white screening distinguishes these (blue colonies, intact lacZ) from insert-bearing recombinants (white colonies, disrupted lacZ).
These two problems are logically distinct and require different solutions. Transformation is a stochastic, inefficient process — selection simply finds the winners. Re-ligation is an unavoidable side reaction in ligation — the same sticky ends that allow insert ligation also allow vector self-ligation. Understanding that two separate problems require two separate solutions is the key to designing any molecular cloning experiment.