Gel Electrophoresis Study Guide

Background Concepts

• Restriction Enzymes

  • Definition: Enzymes that cut DNA at specific sequences.

  • Purpose: Used for DNA mapping, sequencing, cloning, and forensic analysis.

  • Types of Cuts:

    • Sticky Ends: Overhanging sequences that can bind to complementary DNA (e.g., EcoRI).

    • Blunt Ends: No overhangs, can be joined with any other blunt end (e.g., HaelII).

  • Naming: Based on the bacterial species from which they originate (e.g., EcoRI from E. coli).

  • Activity: Requires Mg²⁺ for function.

• Agarose Gel Electrophoresis

  • Purpose: Separates DNA fragments by size.

  • Mechanism:

    • DNA is loaded into wells of an agarose gel.

    • An electric current moves DNA through the gel (negative to positive).

    • Smaller fragments move faster, while larger ones move slower.

  • Visualization: DNA is stained to see banding patterns.

  • Limitations:

    • DNA fragments of similar sizes may appear as a single band.

    • Secondary DNA structures can affect movement.

• Southern Blotting

  • Purpose: Identifies specific DNA sequences.

  • Steps:

    • DNA fragments are transferred from the gel to a membrane.

    • A labeled DNA probe binds to complementary sequences.

    • The probe highlights specific bands for analysis.

  • Use in Forensics: Helps match crime scene DNA with suspects.

• Polymerase Chain Reaction (PCR)

  • Advantage over Electrophoresis: Can amplify small amounts of DNA quickly.

  • Steps:

    • Denaturation (92-96°C): DNA strands separate.

    • Annealing (45-65°C): Primers bind to target DNA.

    • Extension (72°C): Taq polymerase adds nucelotides to synthesize new strands.

• Lab Procedures

  • Day 1: Preparing the Gel and the Samples

    • Set up the gel tray:

      • Tape the ends to prevent leaks.

      • Insert combs on the negative side to form wells.

    • Prepare samples:

      • Label tubes properly.

      • Add restriction enzymes to cut DNA.

      • Incubate to allow digestion.

    • Practice pipetting:

      • Use a micropipette with water for practice.

      • Always use a new tip to prevent contamination.

    • Prepare the gel:

      • Once set, remove combs carefully.

      • Store gel in buffer solution to prevent drying.

    • Store DNA samples on ice overnight.

• Day 2: Running the Gel

  • Set up the gel electrophoresis chamber:

    • Transfer the gel into the chamber.

    • Align the wells with the black (negative) electrode.

    • Cover gel completely with buffer solution.

  • Load samples:

    • Warm DNA tubes briefly before loading.

    • Load up to 35 µL into each well.

  • Run the gel:

    • Close the chamber and connect electrodes.

    • Run at 200V for 15 min or 125V for 25 min.

  • After electrophoresis:

    • Carefully remove the gel.

    • Store in a labeled bag with buffer solution.

• Day 4: Staining and Analyzing the Gel

  • Stain the gel to visualize bands.

  • Compare banding patterns to standard DNA markers.

  • Analyze fragment sizes and compare with crime scene samples.

• Key Takeaways

  • Restriction enzymes cut DNA at specific sequences.

  • Gel electrophoresis separates DNA based on size.

  • Smaller DNA fragments move faster than larger ones.

  • Always use new pipette tips to avoid contamination.

  • Southern blotting and PCR help analyze DNA further.

  • Forensic applications use these methods to identify suspects.