AP Biology: Bacterial Transformation and Recombinant DNA

Bacterial Transformation and Recombinant DNA

Concept Summary

• Bacterial transformation: Uptake of naked DNA from the environment by a bacterial cell, enabling horizontal gene transfer and genetic engineering.
• Competence: Physiological state in which bacteria can take up extracellular DNA; induced naturally (e.g., stress) or artificially (e.g., CaCl?/heat shock).
• Recombinant DNA: Artificially constructed DNA molecule combining sequences from different sources, used to clone genes or produce proteins (e.g., insulin).
• Plasmid vectors: Small, circular DNA molecules used to carry foreign DNA into host cells; contain selectable markers (e.g., antibiotic resistance) and origin of replication.
• Restriction enzymes: Endonucleases that cut DNA at specific palindromic sequences, creating sticky or blunt ends for recombinant DNA assembly.

Core Questions

WHAT (definitional)

Q: What is bacterial transformation? A: The process by which a bacterial cell takes up and incorporates exogenous DNA from its surroundings, altering its genotype and phenotype. Trap/Clarification: Transformation-transduction (virus-mediated) or conjugation (cell-to-cell contact); it requires naked DNA.

Q: What is a selectable marker? A: A gene (e.g., amp? or tet?) in a plasmid vector that allows researchers to identify transformed cells by conferring resistance to an antibiotic. Trap/Clarification: Selectable markers enable survival under selection; they do not directly indicate successful insertion of the target gene.

WHY (causal/explanatory)

Q: Why is competence important for transformation? A: Competence allows bacteria to take up extracellular DNA, which is essential for genetic diversity, antibiotic resistance spread, and laboratory cloning. Trap/Clarification: Not all bacteria are naturally competent; E. coli requires artificial induction (e.g., CaCl? + heat shock).

Q: Why are restriction enzymes critical for recombinant DNA technology? A: They cut DNA at specific sequences, generating compatible ends for ligation, ensuring precise assembly of recombinant molecules. Trap/Clarification: Restriction enzymes do not join DNA fragments (ligase does); they only cut.

HOW (process/application)

Q: How do you perform artificial bacterial transformation in the lab? A: Mix plasmid DNA with competent cells, apply heat shock (42°C for 30–90 sec) to permeabilize membranes, then recover in nutrient broth before plating on selective media. Trap/Clarification: Heat shock facilitates uptake but does not guarantee transformation; selection (e.g., antibiotic plates) is required to confirm success.

Q: How is recombinant DNA constructed? A: Cut vector and insert DNA with the same restriction enzyme, mix fragments, ligate with DNA ligase, then transform into host cells. Trap/Clarification: Using different restriction enzymes for vector/insert may prevent ligation or create non-functional plasmids.

CAN (conditions/possibilities)

Q: Can all bacteria undergo transformation? A: No; only competent bacteria (naturally or artificially induced) can take up DNA, and efficiency varies by species (e.g., Bacillus > E. coli). Trap/Clarification: Gram-negative bacteria (e.g., E. coli) are harder to transform than Gram-positive due to their outer membrane.

Q: Can recombinant DNA be replicated in host cells? A: Yes, if the vector contains an origin of replication (ori) recognized by the host’s DNA polymerase, allowing plasmid propagation. Trap/Clarification: Without an ori, the plasmid will not replicate, and the recombinant DNA will be lost during cell division.

Quick Facts & Traps

• Fact: Sticky ends (overhangs from restriction cuts) increase ligation efficiency by base-pairing with complementary ends.
• Trap: "All plasmids are vectors."-Reality: Only plasmids engineered with selectable markers, ori, and cloning sites are vectors.
• Fact: Blue-white screening uses lacZ disruption to identify recombinant colonies (white = successful insertion; blue = empty vector).
• Trap: "Heat shock kills non-competent cells."-Reality: Heat shock induces competence in prepared cells; non-competent cells survive but don’t take up DNA.
• Fact: PCR can amplify DNA for cloning, but the product must be digested with restriction enzymes to match vector ends.
• Trap: "Transformation efficiency is 100%."-Reality: Typical efficiency is <1% (e.g., 10?–10? transformants/µg DNA).

Rapid-Fire True/False

• Statement: Bacterial transformation requires cell-to-cell contact. Answer: FALSE Why the common mistake happens: Confusion with conjugation, which uses a pilus for direct transfer.

• Statement: Restriction enzymes always produce sticky ends. Answer: FALSE Why the common mistake happens: Some enzymes (e.g., SmaI) cut blunt ends, not overhangs.

• Statement: Recombinant plasmids must be linear for transformation. Answer: FALSE Why the common mistake happens: Plasmids are circular for stability and replication; linear DNA is degraded by host nucleases.