Gel electrophoresis Knowledge-Based Study

Study notes

1. What is Gel Electrophoresis?

   Gel electrophoresis is a laboratory technique used to separate macromolecules, primarily DNA, RNA, and proteins, based on their size and electrical charge.

   The Role of Electric Current and Charge

   Nucleic acids (DNA and RNA) have a uniform negative charge distributed along their sugar-phosphate backbone due to phosphate groups ($PO_4^{3-}$). When placed in an electric field:

   • Molecules migrate toward the positive electrode (anode).
   • The negative electrode (cathode) is located near the loading wells where samples are introduced.

   Because the charge-to-mass ratio of double-stranded DNA is essentially constant, electric charge alone does not separate DNA fragments of different lengths. Separation depends primarily on the physical resistance of the gel matrix.

   Agarose vs. Polyacrylamide Gels

   The medium through which macromolecules travel acts as a microscopic sieve, restricting larger molecules more than smaller ones.

   • Agarose Gel: Derived from seaweed; typically used for separating medium to large DNA and RNA fragments (ranging from roughly 100 base pairs to several tens of kilobases).
   • Polyacrylamide Gel (PAGE): Possesses smaller pore sizes; offers high resolution for separating smaller DNA fragments, single nucleotides, or proteins (e.g., SDS-PAGE).

   The Molecular Sieve Effect

   As the electric field pulls negatively charged molecules through the gel pores:

   • Shorter fragments navigate through the microscopic pores easily and travel faster and farther from the wells.
   • Longer fragments experience greater frictional drag within the polymer network and travel slower, remaining closer to the wells.
   • Therefore, migration distance is inversely proportional to the log of the fragment's molecular weight or base pair length.

   Step-by-Step Procedure

   1. Gel Casting: Agarose is dissolved in a conductive buffer (such as TAE or TBE), heated, poured into a casting tray with a comb, and allowed to solidify to form loading wells.
   2. Sample Preparation: DNA samples are mixed with a loading dye containing glycerol or sucrose (to increase density so samples sink into the wells) and tracking dyes (to monitor electrical run progress).
   3. Running the Gel: An electric voltage is applied across the buffer chamber, directing migration from cathode (-) to anode (+).
   4. Molecular Weight Marker (DNA Ladder): A commercial mixture of known DNA fragment sizes is loaded into an adjacent lane to allow size estimation of unknown fragments by comparison.

   Staining and Detection

   Because DNA is colorless, fluorescent dyes such as ethidium bromide (EtBr) or safer intercalating agents (like GelRed or SYBR Safe) are incorporated into the gel or used as post-stains. These dyes bind between stacked base pairs and fluoresce brightly under ultraviolet (UV) or blue light, revealing distinct fluorescent bands.

   Practical Applications

   Gel electrophoresis is an essential tool across modern biotechnology and molecular biology:

   • Restriction Fragment Analysis: Verifying digestion of plasmids and genomic DNA by restriction endonucleases.
   • PCR Verification: Confirming the presence, specificity, and yield of polymerase chain reaction amplifications.
   • DNA Fingerprinting / Forensics: Comparing Short Tandem Repeats (STRs) or Restriction Fragment Length Polymorphisms (RFLPs) in paternity tests and forensic investigations.
   • DNA Purification: Extracting specific bands directly from the gel slice (gel extraction) for downstream cloning or sequencing.