What is gel electrophoresis and how does it work?

In one line

The rule

Gel electrophoresis is a laboratory technique used to separate molecules like DNA, RNA, or proteins based on their size and charge by applying an electric field to a gel matrix.

There is no single formula, but the basic idea is: Distance moved ∝ (Charge) / (Size)

• Distance moved: How far the molecule travels in the gel
• Charge: The electric charge on the molecule
• Size: How big or long the molecule is

Let's understand it simply

Imagine you are at a crowded railway station in Mumbai. People want to reach the exit gate. Some are carrying big suitcases, some have small bags, and some are empty-handed. When the gate opens, who will reach first? The ones with no luggage! The big suitcases slow people down.

Gel electrophoresis works like this. The gel is like the crowded station. DNA, RNA, or proteins are like the people. The electric field is like the open gate pulling everyone in one direction. Small molecules (no luggage) move fast and far. Big ones (big suitcases) move slowly and not very far. The gel sorts them by size.

How it works

1. Prepare the gel: A jelly-like substance (usually agarose for DNA/RNA, polyacrylamide for proteins) is poured into a tray and allowed to set, forming a slab with small wells (holes) at one end.
2. Load the samples: DNA, RNA, or protein samples are mixed with a colored dye and placed into the wells using a pipette (a dropper).
3. Apply the electric field: Electrodes are attached at both ends of the gel. One end is negative (cathode), the other is positive (anode). The gel tray is filled with a special buffer solution that conducts electricity.
4. Molecules move: The electric field pushes the charged molecules through the gel. DNA and RNA are negatively charged, so they move towards the positive end. Proteins can be positive or negative, depending on the conditions.
5. Separation by size: The gel acts like a sieve. Small molecules move quickly and travel farther. Large molecules move slowly and stay closer to the wells.
6. Visualize the results: After running the gel, a stain (like ethidium bromide for DNA) is added. Under UV light, you see bands—each band is a group of molecules of the same size.

The story behind it

Gel electrophoresis was first developed in the 1930s by Arne Tiselius, a Swedish chemist, to study proteins. Later, scientists improved the method to separate DNA and RNA. The big breakthrough came in the 1970s, when agarose gel electrophoresis became popular for DNA analysis. This technique helped scientists read genetic codes, solve crimes, and diagnose diseases. Tiselius even won the Nobel Prize in Chemistry in 1948 for his work on electrophoresis.

Where you see it in real life

• DNA fingerprinting in crime investigations: Police use gel electrophoresis to match DNA from a crime scene to suspects.
• Checking genetically modified crops: Scientists check if a plant has the right gene by separating its DNA fragments.
• Diagnosing genetic diseases: Hospitals use it to find faulty genes in patients.
• Testing food for adulteration: To check if milk or paneer contains harmful bacteria or unwanted proteins.
• Research in colleges and labs: Students and scientists use it to study genes, proteins, and viruses.
• COVID-19 testing: Some tests use gel electrophoresis to check for viral RNA.

Why it matters

• Essential for biotechnology and genetics: Without it, we could not study genes or make medicines like insulin.
• Used in forensics: Helps solve crimes and identify people.
• Medical diagnosis: Detects diseases like sickle cell anemia or thalassemia.
• Research and education: Forms the base for many experiments in biology and medicine.

Worked example

Let's say you have a mixture of DNA fragments of 500, 1000, and 2000 base pairs (bp).

Step 1: Prepare an agarose gel and set up the apparatus.
Step 2: Load the DNA mixture into the wells.
Step 3: Apply electric current (negative to positive).
Step 4: Wait for 30–40 minutes.
Step 5: Stain the gel and view under UV light.
Step 6: Observe three bands: the 500 bp fragment travels farthest, 1000 bp in the middle, 2000 bp closest to the well.

Common mistakes

• Thinking bigger molecules move faster: Actually, smaller ones move faster and farther.
• Forgetting DNA is negatively charged: DNA always moves towards the positive electrode.
• Overloading the wells: Putting too much sample can make the bands blur or mix.
• Running the gel too long: If you wait too long, small fragments can run off the gel and disappear.

Remember it

Think: "Small runs fast, big stays back"—like kids racing through a crowded market!