Gel Electrophoresis

Introduction

Gel electrophoresis: technique separating charged molecules (DNA, RNA, proteins) by size using electric field through gel matrix. Ubiquitous: most fundamental technique in molecular biology. Principle: smaller molecules migrate faster through gel pores. Applications: DNA fragment analysis, protein characterization, quality control, forensics. Cost: inexpensive ($200-2,000 for equipment). Complementary: often followed by blotting, sequencing, or mass spectrometry.

"Gel electrophoresis is the molecular biologist's most essential tool. Before you sequence it, clone it, or analyze it,you run a gel. It tells you if your DNA is intact, your PCR worked, your protein is the right size. Simple, reliable, indispensable." -- Biochemistry professor

Electrophoresis Principles

Charge and Migration

DNA/RNA: uniformly negative charge (phosphate backbone). Proteins: variable charge (depends on amino acid composition and pH). Electric field: charged molecules migrate toward opposite electrode. DNA: always migrates toward anode (+) (negative charge). Proteins: migration depends on isoelectric point and buffer pH.

Gel as Molecular Sieve

Pores: gel contains network of pores (size depends on concentration). Small molecules: navigate pores easily (fast migration). Large molecules: impeded by pore network (slow migration). Separation: based on size (and shape for native gels). Resolution: depends on gel concentration, running time, voltage.

Factors Affecting Migration

Molecule size: primary determinant (smaller = faster). Gel concentration: higher = smaller pores = better small molecule resolution. Voltage: higher = faster migration (but more heat). Buffer: maintains pH, conducts current. Temperature: affects mobility (typically run at room temp or 4°C).

Mobility Equation

µ = v/E = q/(6πηr)µ = electrophoretic mobilityv = velocity (cm/s)E = electric field (V/cm)q = net chargeη = buffer viscosityr = effective radius (Stokes radius)

Agarose Gel Electrophoresis

Agarose Properties

Source: polysaccharide from seaweed (agar). Gel formation: dissolve in buffer by heating, pour into mold, cool to solidify. Pore size: large (suitable for DNA/RNA, not small proteins). Concentration: 0.5-2% (lower = larger pores, better for big DNA). Standard: 1% gel for most DNA applications.

Gel Preparation

Weigh agarose: calculate % (w/v) for buffer volume. Dissolve: microwave or heat until clear solution. Cool: ~55°C before adding stain (ethidium bromide or SYBR Safe). Pour: into casting tray with comb (creates wells). Set: ~30 minutes at room temperature. Load: place in electrophoresis tank, cover with running buffer.

DNA Size Range

Loading and Running

Loading buffer: contains glycerol (density) + tracking dye (visualize migration). DNA ladder: known size markers (100 bp, 1 kb ladders common). Voltage: 5-10 V/cm (higher = faster but lower resolution). Time: 30-90 minutes (until adequate separation). Buffer: TAE (Tris-acetate-EDTA) or TBE (Tris-borate-EDTA).

Polyacrylamide Gel Electrophoresis

Polyacrylamide Properties

Polymer: acrylamide + bisacrylamide cross-linker. Polymerization: free radical (APS + TEMED catalysts). Pore size: small, uniform (better resolution than agarose). Concentration: 4-20% (adjustable). Toxic monomer: acrylamide is neurotoxic (handle with care, polymerized form safe).

Gel Types

Native PAGE: separates by size, charge, and shape (proteins in native state). Denaturing PAGE: SDS denatures proteins (separates by size only). Urea-PAGE: denatures nucleic acids (sequencing gels, small RNA). Gradient: varying percentage (wide size range resolution).

Nucleic Acid Applications

Small DNA: <500 bp fragments (superior resolution to agarose). RNA: small RNA analysis (miRNA, tRNA). Sequencing: historical (Sanger sequencing gels, replaced by capillary). EMSA: electrophoretic mobility shift assay (protein-DNA binding). Resolution: single nucleotide for small fragments.

Gel Casting

Vertical system: glass plates with spacers. Preparation: mix acrylamide solution + APS + TEMED. Polymerization: 30-60 minutes (exothermic). Comb: creates sample wells at top. Running: vertical orientation (buffer in upper and lower chambers).

SDS-PAGE

Principle

SDS (sodium dodecyl sulfate): anionic detergent that denatures proteins. Effect: binds proteins at ~1.4 g SDS per g protein (uniform negative charge). Shape: all proteins become rod-shaped (charge proportional to mass). Result: separation purely by molecular weight. Reducing: β-mercaptoethanol or DTT breaks disulfide bonds.

Laemmli System

Stacking gel: low percentage (4%), low pH (6.8). Resolving gel: higher percentage (8-15%), higher pH (8.8). Stacking: concentrates sample into thin band (improved resolution). Resolving: separates proteins by size. Running buffer: Tris-glycine-SDS. Standard: most widely used protein electrophoresis system.

Molecular Weight Determination

Standard curve: log(MW) vs. relative mobility (Rf) is linear. Protein ladder: pre-stained markers (10-250 kDa typical range). Rf = distance migrated / distance to front. Application: determine unknown protein MW. Accuracy: ±5-10% (glycoproteins may behave anomalously).

Sample Preparation

Lysis: detergent-based cell lysis (RIPA buffer). Quantification: BCA or Bradford assay (determine protein concentration). Denaturation: heat at 95°C for 5 minutes in SDS + reducing agent. Loading: 10-50 µg total protein per lane (depends on abundance). Equal loading: critical for comparison between samples.

Staining and Detection

DNA Staining

Ethidium bromide (EtBr): intercalates DNA, fluoresces under UV. Sensitivity: ~10 ng per band. Safety: mutagenic (handle with gloves, dispose properly). SYBR Safe: safer alternative (similar sensitivity, blue light excitation). GelRed/GelGreen: non-mutagenic alternatives. Post-staining: soak gel in stain solution (or pre-cast with stain).

Protein Staining

Coomassie Blue: most common protein stain (~50-100 ng sensitivity). Silver stain: 10-100x more sensitive than Coomassie (~1 ng). SYPRO Ruby: fluorescent (linear quantification). Total protein: Ponceau S (reversible, for membrane staining before Western). Specific: immunodetection (Western blot).

Imaging

UV transilluminator: for EtBr-stained DNA gels. Blue light: for SYBR Safe (less DNA damage). Gel documentation system: camera + UV/blue light + software. Densitometry: quantify band intensity (ImageJ, Image Lab). Digital: modern systems capture high-resolution images directly.

Western Blotting

Transfer

Principle: transfer proteins from gel to membrane. Membrane: PVDF (stronger signal) or nitrocellulose (lower background). Method: wet transfer (tank), semi-dry (plates), turbo (rapid). Direction: proteins migrate from gel to membrane in electric field. Verification: Ponceau S staining confirms transfer.

Blocking and Antibody Incubation

Blocking: prevent non-specific antibody binding (BSA, milk, casein). Primary antibody: specific for target protein (incubate 1-16 hours). Washing: remove unbound antibody (TBST or PBST). Secondary antibody: conjugated to enzyme or fluorophore (anti-species specific). Detection: chemiluminescence (ECL), fluorescence, or colorimetric.

Detection Methods

Chemiluminescence: HRP-conjugated secondary + substrate → light emission. Sensitivity: ~1 pg protein. Fluorescence: fluorophore-conjugated secondary → direct imaging. Advantage: multiplexing (multiple proteins on same membrane). Colorimetric: AP-conjugated secondary + substrate → colored precipitate. Standard: chemiluminescence most common.

Quantification

Densitometry: measure band intensity (Image J, Image Lab). Normalization: divide by loading control (β-actin, GAPDH, total protein). Relative: compare treated vs. control (fold change). Replicate: minimum 3 biological replicates for statistical analysis. Limitation: semi-quantitative (dynamic range limited).

Two-Dimensional Electrophoresis

Principle

First dimension: isoelectric focusing (IEF, separate by charge/pI). Second dimension: SDS-PAGE (separate by molecular weight). Result: proteins spread across 2D space (thousands resolved). Resolution: can separate >2000 proteins per gel. Application: proteomics, biomarker discovery, protein profiling.

Isoelectric Focusing

IPG strips: immobilized pH gradient (pH 3-10 or narrow range). Principle: proteins migrate to pH where net charge = 0 (isoelectric point). Duration: 12-24 hours (focusing to equilibrium). Resolution: 0.01 pH unit separation possible.

Second Dimension

Equilibration: IPG strip soaked in SDS buffer. Loading: strip placed on top of SDS-PAGE gel. Running: standard SDS-PAGE (2-5 hours). Staining: Coomassie, silver, or fluorescent. Image analysis: software identifies and quantifies spots.

Limitations

Labor-intensive: 2-3 days per experiment. Reproducibility: gel-to-gel variation (DIGE improves). Membrane proteins: poorly solubilized. Low abundance: may not be detected. Replacement: largely replaced by LC-MS/MS proteomics (higher throughput, sensitivity).

Capillary Electrophoresis

Principle

Separation: in narrow capillary (25-100 µm diameter). Detection: UV, fluorescence, or mass spectrometry. Speed: minutes (vs. hours for slab gels). Automation: autosampler, automated injection and detection. Resolution: superior to slab gels (narrow bore = less diffusion).

Applications

DNA sequencing: Sanger sequencing (capillary array electrophoresis). Fragment analysis: PCR products, STR genotyping (forensics). Protein analysis: CE-SDS for protein characterization. Clinical: hemoglobin variants, serum protein electrophoresis. Pharmaceutical: purity analysis, charge variants.

Advantages

Speed: 10-30 minutes per analysis. Sample volume: nanoliter injection. Automation: unattended operation. Quantification: precise (UV/fluorescence detection). Throughput: 96-capillary arrays process many samples simultaneously.

Pulsed-Field Gel Electrophoresis

Principle

Problem: standard electrophoresis cannot separate DNA >20 kb (all co-migrate). Solution: alternate electric field direction periodically. Mechanism: large DNA must reorient with each pulse (larger DNA reorients slower). Result: separation of DNA up to 10 Mb. Pulse time: determines size range (longer pulses = larger DNA separated).

Applications

Bacterial typing: PFGE of restriction digests (gold standard for outbreak investigation). Genome mapping: order restriction fragments. Karyotyping: yeast chromosome separation. Rare cutter mapping: map large restriction fragments. Replaced by: whole genome sequencing for many typing applications.

Protocol

Sample: cells embedded in agarose plugs (prevent DNA shearing). Lysis: in-plug digestion (proteinase K, detergent). Restriction: rare-cutting enzymes (NotI, SfiI). Running: 18-24 hours (alternating field angles). Staining: EtBr (large DNA visible as discrete bands).

Troubleshooting

Common DNA Gel Problems

Smearing: DNA degraded (use fresh sample, avoid nucleases). No bands: PCR failed, DNA not loaded, wrong gel percentage. Fuzzy bands: overloaded, run too fast. Multiple bands: non-specific PCR products (optimize PCR conditions). Band shift: wrong buffer, voltage too high.

Common Protein Gel Problems

Smiling: uneven heating (reduce voltage, check buffer level). Fuzzy bands: incomplete denaturation (heat longer, add more SDS). No bands: protein not expressed, transfer failed (for Western). High background: insufficient blocking, antibody too concentrated. Unexpected size: post-translational modification, degradation.

Best Practices

Always run markers: size reference in every gel. Fresh buffers: reuse causes ion depletion. Clean equipment: residual detergent or protein causes artifacts. Controls: positive control confirms system working. Document: photograph every gel (data record).

Applications

Molecular Biology

PCR verification: confirm product size and specificity. Restriction analysis: verify cloning (diagnostic digest). RNA quality: assess integrity before experiments. Purification: gel extraction of specific DNA bands. Genotyping: distinguish alleles by size difference.

Clinical Diagnostics

Hemoglobin electrophoresis: sickle cell, thalassemia screening. Serum protein electrophoresis: multiple myeloma (M-spike). Isoenzyme analysis: LDH isoforms (tissue-specific). Immunofixation: identify monoclonal proteins. CSF: oligoclonal bands (multiple sclerosis).

Forensics

STR analysis: capillary electrophoresis of PCR-amplified STR loci. DNA profiling: match suspect to evidence. Paternity: compare allele patterns. Mass disaster: victim identification. Database: CODIS profile comparison.

Food and Environmental

Species identification: PCR-RFLP (fish, meat authentication). GMO detection: transgene presence/absence. Water quality: microbial source tracking. Allergen: detect allergenic proteins. Quality control: protein content verification.

References

• Sambrook, J., and Russell, D. W. "Molecular Cloning: A Laboratory Manual." Cold Spring Harbor Laboratory Press, 4th ed., 2012.
• Laemmli, U. K. "Cleavage of Structural Proteins during the Assembly of the Head of Bacteriophage T4." Nature, vol. 227, 1970, pp. 680-685.
• Towbin, H., Staehelin, T., and Gordon, J. "Electrophoretic Transfer of Proteins from Polyacrylamide Gels to Nitrocellulose Sheets." Proceedings of the National Academy of Sciences, vol. 76, no. 9, 1979, pp. 4350-4354.
• O'Farrell, P. H. "High Resolution Two-Dimensional Electrophoresis of Proteins." Journal of Biological Chemistry, vol. 250, no. 10, 1975, pp. 4007-4021.
• Schwartz, D. C., and Cantor, C. R. "Separation of Yeast Chromosome-Sized DNAs by Pulsed Field Gradient Gel Electrophoresis." Cell, vol. 37, no. 1, 1984, pp. 67-75.