Gel Electrophoresis: How Scientists Sort Molecules by Size

Science Glossary · Explore this series ›

Tormod Guldvog

Editor & Duowriter

March 21, 2026

Key Takeaways

• Gel electrophoresis separates molecules by size using an electric field.
• The technique underpins DNA forensics, gene editing verification, and protein analysis.
• Arne Tiselius won the 1948 Nobel Prize for foundational electrophoresis research.

✓ Fact-checked · See verification details

AI assistant: Research, Drafting, QA

Gel electrophoresis is a laboratory technique that separates DNA, RNA, or protein molecules by size, pulling them through a porous gel with an electric field.

Why It Matters

Few laboratory methods appear in as many branches of biology. When researchers need to confirm that a CRISPR gene edit worked, verify the identity of a forensic sample, or check the purity of a protein preparation, gel electrophoresis is often the first step. Results appear as a pattern of bands on a gel, each band representing molecules of a particular size, readable at a glance by a trained eye.

The technique also played a role in one of the earliest molecular diagnoses. In 1949, Linus Pauling and colleagues at the California Institute of Technology used electrophoresis to show that hemoglobin from sickle cell patients carried a different electrical charge than normal hemoglobin. That finding, published in Science, was the first demonstration that a disease could be traced to a molecular defect in a specific protein.

More recently, a 2019 study in Scientific Reports showed that small insertions and deletions introduced by CRISPR-Cas9 can be detected through agarose gel electrophoresis of PCR-amplified target regions. The method remains a practical quality-control step in genome engineering.

How It Works

The process begins with a gel, typically made from agarose (for DNA and RNA) or polyacrylamide (for proteins). The gel is a mesh of cross-linked polymers with pores small enough to act as a molecular sieve. Researchers load samples into small wells at one end of the gel, then apply an electric field across it.

DNA and RNA carry a negative charge from their phosphate backbone, so they migrate toward the positive electrode. Smaller fragments slip through the pores more easily and travel farther in a given time. Larger fragments lag behind. After 30 to 60 minutes, the molecules have sorted themselves into distinct bands by size.

Proteins require an extra step. In SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis), the detergent SDS coats each protein with a uniform negative charge proportional to its mass. This removes the variable of native charge and separates proteins purely by molecular weight.

To visualize results, researchers stain the gel. Ethidium bromide or SYBR Safe dyes make nucleic acid bands glow under ultraviolet light. Protein gels are typically stained with Coomassie blue or silver stain. Size markers, loaded alongside samples, provide a reference ladder for estimating molecular weight.

Key Context

Swedish chemist Arne Tiselius developed electrophoresis as an analytical method in the 1930s at Uppsala University. Working with blood serum, he showed that proteins previously thought to be a single substance actually separated into four distinct components: albumin, alpha, beta, and gamma globulin. The discovery earned him the 1948 Nobel Prize in Chemistry.

Tiselius's original method used a liquid column, not a gel. Oliver Smithies, then at the University of Toronto, introduced starch gel electrophoresis in a 1955 Nature paper, and agarose gels followed in the 1960s. These gel-based methods gave the technique its modern form and made it accessible to virtually any biology laboratory.

Today, capillary electrophoresis offers higher resolution and automation for specialized applications like forensic DNA profiling. Yet traditional gel electrophoresis persists in most research labs because it is inexpensive, visually intuitive, and requires no specialized equipment beyond a power supply and a casting tray.

FAQ

Related Reading

When Science Catches Its Own Lies

Sources

• Primary Research:
  • Electrophoresis of serum globulin (Tiselius, 1937)
  • Sickle Cell Anemia, a Molecular Disease (Pauling et al., 1949)
  • Zone electrophoresis in starch gels (Smithies, 1955)
• Additional Context:
  • Electrophoresis definition (National Human Genome Research Institute)
  • Electrophoresis overview (StatPearls, NCBI)
  • Agarose Gel Electrophoresis for the Separation of DNA Fragments (Lee et al., JoVE, 2012)

Tormod Guldvog

Editor & Duowriter

Tormod Guldvog is the editor of Science Reader, covering the space where human and artificial intelligence meet. With a background in technology and science communication, he writes about how AI is changing scientific discovery — and what that means for us. More about Science Reader

Cite this article

Tormod Guldvog. (2026, March 21). Gel Electrophoresis: How Scientists Sort Molecules by Size. Science Reader. https://sciencereader.com/glossary/electrophoresis-gel-analysis/