AP Biology: Gel Electrophoresis – Separation of DNA Fragments, Restriction Enzymes

Gel Electrophoresis – Separation of DNA Fragments, Restriction Enzymes

Concept Summary

• Gel electrophoresis: Technique that separates DNA fragments by size using an electric field and a porous gel matrix; essential for DNA fingerprinting, cloning, and genetic analysis.
• Restriction enzymes (endonucleases): Bacterial enzymes that cut DNA at specific palindromic recognition sequences; critical for creating recombinant DNA and mapping genomes.
• Agarose gel: Polysaccharide matrix used in electrophoresis; pore size decreases with higher agarose concentration, affecting fragment separation resolution.
• DNA migration: Negatively charged DNA moves toward the anode (positive electrode) during electrophoresis; smaller fragments migrate faster due to less resistance in the gel.
• DNA ladder: Pre-sized DNA fragments of known lengths run alongside samples; used as a molecular weight standard to estimate unknown fragment sizes.

Core Questions

WHAT (definitional)

Q: What is gel electrophoresis? A: A laboratory method that separates DNA, RNA, or proteins based on size and charge by applying an electric field to a gel matrix. Trap/Clarification: Gel electrophoresis separates by size, not sequence—DNA must be linearized (e.g., by restriction enzymes) to ensure accurate size-based separation.

Q: What is a restriction enzyme? A: A bacterial enzyme that cleaves DNA at specific nucleotide sequences (recognition sites), producing either blunt or sticky ends. Trap/Clarification: Restriction enzymes cut double-stranded DNA; single-stranded DNA (e.g., in PCR products) must be denatured or hybridized to be cut.

WHY (causal/explanatory)

Q: Why does DNA migrate toward the anode in gel electrophoresis? A: DNA is negatively charged due to its phosphate backbone, so it moves toward the positive electrode (anode) when an electric field is applied. Trap/Clarification: The gel’s buffer (e.g., TAE or TBE) maintains pH and conductivity—not the gel itself—so buffer depletion can distort migration.

Q: Why are restriction enzymes important in genetic engineering? A: They allow precise cutting of DNA at predictable sites, enabling the creation of recombinant DNA (e.g., inserting genes into plasmids) and DNA fingerprinting. Trap/Clarification: Not all restriction enzymes produce compatible sticky ends—mismatched overhangs (e.g., EcoRI and BamHI) cannot ligate without modification.

HOW (process/application)

Q: How do you prepare a DNA sample for gel electrophoresis? A: Digest DNA with restriction enzymes, add loading dye (to track migration and increase density), and load into gel wells; run at 50–150V until dye reaches ~75% of the gel. Trap/Clarification: Loading dye does not stain DNA—ethidium bromide or SYBR Safe is added separately (or post-run) for visualization under UV light.

Q: How is fragment size estimated in gel electrophoresis? A: Compare the migration distance of unknown fragments to a DNA ladder (logarithmic relationship: smaller fragments = farther migration); plot distance vs. log(bp) to interpolate size. Trap/Clarification: Migration distance is inversely proportional to log(bp)—students often mistakenly assume a linear relationship.

CAN (conditions/possibilities)

Q: Can gel electrophoresis separate circular DNA (e.g., plasmids)? A: Yes, but circular DNA migrates unpredictably (often slower than linear DNA of the same size) unless linearized by restriction enzymes or heat. Trap/Clarification: Supercoiled plasmids run faster than relaxed/nicked circles, but both are unreliable for size estimation without linearization.

Q: Under what conditions do restriction enzymes fail to cut DNA? A: If the recognition site is methylated (e.g., bacterial defense against self-digestion), mutated, or obscured by secondary structures (e.g., hairpins in single-stranded DNA). Trap/Clarification: Some enzymes (e.g., DpnI) only cut methylated DNA—students assume all restriction enzymes avoid methylated sites.

Quick Facts & Traps

• Fact: Sticky ends (e.g., EcoRI: 5’-GAATTC-3’) enable directional cloning; blunt ends (e.g., SmaI) ligate in any orientation but with lower efficiency.
• Trap: "Higher voltage = better separation."-Reality: Excessive voltage heats the gel, causing band smearing or melting; optimal voltage depends on gel size (e.g., 5–10 V/cm).
• Fact: Agarose concentration determines resolution: 0.7% (large fragments, 5–10 kb), 2% (small fragments, 100–500 bp).
• Trap: "All restriction enzymes cut DNA symmetrically."-Reality: Some (e.g., NotI) cut asymmetrically, leaving 5’ or 3’ overhangs of unequal length.
• Fact: Ethidium bromide intercalates between DNA bases, fluorescing under UV light; it’s a mutagen and must be handled with gloves.
• Trap: "DNA ladders are optional."-Reality: Without a ladder, fragment sizes cannot be accurately estimated—always include one in at least one lane.

Rapid-Fire True/False

• Statement: Restriction enzymes cut DNA randomly. Answer: FALSE Why the common mistake happens: Students confuse restriction enzymes with DNases (non-specific nucleases) or assume "restriction" implies randomness.

• Statement: Smaller DNA fragments migrate farther in a gel because they are less negatively charged. Answer: FALSE Why the common mistake happens: Students attribute migration to charge differences (all DNA has ~1 charge/bp) instead of size-dependent resistance in the gel matrix.

• Statement: A 1% agarose gel can resolve a 10 bp fragment from a 12 bp fragment. Answer: FALSE Why the common mistake happens: Students overestimate agarose resolution; polyacrylamide gels (not agarose) are needed for <50 bp differences.