Read time: ~10 min | Level: Intermediate – Advanced | Updated: 2026
Agarose Gel Electrophoresis
A concise, scannable guide covering what agarose is, what it does, how gel electrophoresis works, its key applications, and how to read results.
Table of Contents
Principle
DNA fragments are separated based on size when an electric field is applied across an agarose gel matrix. Smaller fragments migrate faster. DNA is stained with an intercalating dye and visualized under UV light.
What Is Agarose?
Agarose is a natural polysaccharide extracted from red marine algae. When heated and cooled, it forms a porous gel matrix — the molecular sieve that makes DNA separation possible.
Origin & chemistry
- Extracted from red algae speciesGelidium amansiiandGracilaria
- Repeating disaccharide unit: β-D-galactose + 3,6-anhydro-α-L-galactose
- Purified fraction of agar — agaropectin (charged component) is removed
- Dissolves above 85°C, solidifies below 35–40°C through hydrogen bonding
Why agarose works for DNA
- Electrically neutral— does not attract or repel charged DNA molecules
- Optically transparent— stained bands are directly visible under UV
- Biocompatible— does not degrade DNA or RNA during the run
- Tunable pore size— change concentration to change resolution range
- Reversible— low melting point (LMP) agarose can be melted to recover intact DNA
- Easy preparation— no chemical polymerization needed; just heat, pour, cool
Key fact — concentration controls pore size
0.5% gel = large pores → separates 5–50 kb fragments | 3% gel = tiny pores → resolves fragments below 500 bp
What Does Agarose Do?
Without agarose, all DNA migrates at identical speed in an electric field. The gel matrix creates size-dependent resistance that transforms electricity into a molecular ruler.
Why free solution doesn't work
- All DNA has the same charge-to-mass ratio regardless of size
- Larger molecules carry more charge but also more mass — effects cancel out
- Result: all DNA migrates at the same velocity → no separation possible
What the gel matrix does
- Creates an obstacle course — millions of nanoscale pores act as a physical sieve
- Smaller fragments pass through pores easily → travel far and fast
- Larger fragments must snake and deform through pores → travel slowly
- Logarithmic separation — size vs distance is log-linear, not linear
- Thermal stability — agarose melts at ~85–95°C, far above heat from a typical run
Long DNA “reptates” (creeps like a snake) through the pores. The larger the molecule, the slower and more complex this movement — which is why ladder bands compress at larger sizes rather than spacing evenly.
How Does Gel Electrophoresis Work?
1. Choose agarose concentration
- 0.5–0.8% → large fragments above 5 kb
- 1–1.5% → standard PCR products (200 bp–3 kb)
- 2–3% → small fragments below 200 bp
2. Prepare the gel
- Dissolve agarose in 1× TAE or TBE buffer by heating
- Heat until completely clear — no swirling particles
- Cool to ~55°C, add stain if pre-staining, pour into casting tray
- Allow 20–30 minutes to solidify at room temperature
3. Set up the tank
- Place gel in tank, fill with same buffer (1–3 mm above gel surface)
- Wells at the negative (black/cathode) electrode end
- DNA migrates toward positive (red/anode) electrode
4. Load samples
- Mix DNA with 6× loading dye (1:5 ratio)
- Loading dye contains glycerol (sinks sample) + tracking dyes (monitor run)
- Always load a DNA ladder in the first or last lane
5. Run the gel
- Apply 80–120 V (rule: 5–10 V/cm gel length)
- Bubbles at electrodes = current confirmed flowing
- Run 30–60 min; stop before smallest dye band exits gel
- Lower voltage = longer run but sharper bands
6. Stain and visualize
- Post-stain in EtBr (0.5 µg/mL) or SYBR Safe for 20–30 minutes
- Destain in water for 10–15 minutes to reduce background
- Image under UV (EtBr) or blue LED (SYBR Safe)
- Photograph immediately — signal fades over time
“Without the agarose matrix, all DNA races at identical speed. The gel is what turns electricity into a molecular ruler.”
