Gel Electrophoresis | Varsity Tutors

SECTION 1

Historical Context & Motivation

Imagine you have a mystery mixture of DNA fragments, and you need to figure out how big each piece is. Before the mid-1900s, scientists had no easy way to do this. They knew that DNA (deoxyribonucleic acid) carried genetic information, but they lacked a simple tool to sort molecules by size. The development of gel electrophoresis solved this problem and became one of the most important techniques in modern biology.

1937

Tiselius Introduces Electrophoresis

Swedish biochemist Arne Tiselius developed the first electrophoresis apparatus to separate proteins in solution. He later won the Nobel Prize in Chemistry for this work.

1955

Starch Gel Electrophoresis

Oliver Smithies introduced starch gels as a medium for separating proteins, making the technique more practical for everyday lab use.

1969

Polyacrylamide & SDS-PAGE

Ulrich Laemmli published a method using polyacrylamide gels combined with SDS (a detergent) to separate proteins by size. SDS-PAGE remains widely used today.

1975

Agarose Gels for DNA

Edwin Southern combined agarose gel electrophoresis with blotting and hybridization to detect specific DNA sequences — a method known as the Southern blot.

1984–Present

DNA Fingerprinting & Forensics

Alec Jeffreys used gel electrophoresis to develop DNA fingerprinting, revolutionizing forensic science, paternity testing, and genetic research worldwide.

Throughout these breakthroughs, scientists kept asking the same fundamental question: How can we sort invisible molecules by size so we can study them? Gel electrophoresis answered that question by using electricity and a jelly-like substance to make molecules race through tiny pores. Smaller molecules move faster, and larger ones lag behind — creating a pattern that scientists can read like a barcode.

SECTION 2

Core Principles & Definitions

Gel electrophoresis relies on a few key ideas from chemistry and physics. First, DNA and RNA molecules carry a negative electrical charge because of the phosphate groups in their sugar-phosphate backbone. When you place charged molecules in an electric field (a region between a positive and negative electrode), they move toward the positive end. Second, a gel acts like a molecular sieve — it has tiny pores that let small molecules slip through easily while slowing down larger ones.

Charge & Movement

DNA and RNA are negatively charged. In an electric field, they migrate toward the positive electrode (anode). This movement is called electrophoretic mobility.

The Gel Matrix

The gel is usually made of agarose (for DNA) or polyacrylamide (for proteins). Its pores create a mesh that separates molecules by size.

Size-Based Separation

Smaller fragments travel farther through the gel in a given time. Larger fragments get tangled in the pores and move more slowly. This creates distinct bands.

Visualization

DNA is invisible to the naked eye. Scientists use special dyes like ethidium bromide or safer alternatives that glow under UV light to make the bands visible.

✦ KEY TAKEAWAY

Think of gel electrophoresis like a race through a thick forest. Imagine tiny rabbits and large bears all starting at the same line. The rabbits (small DNA fragments) can dart between the trees easily and reach the finish line first. The bears (large fragments) keep bumping into trees and fall behind. At the end of the race, the animals are spread out by size — and that's exactly what happens to DNA in a gel.

SECTION 3

Visual Explanation — How the Gel Works

This diagram shows a gel electrophoresis setup viewed from above. The wells at the top are where DNA samples are loaded. Lane L contains a DNA ladder — a set of fragments with known sizes — used as a ruler. Lanes 1–5 contain unknown samples. Notice how smaller fragments (lower numbers) travel farther toward the bottom (the positive electrode).

In the diagram above, DNA samples are loaded into small slots called wells at the negative end of the gel. When electricity flows through the chamber, the negatively charged DNA fragments are pulled toward the positive electrode at the bottom. The gel's pores act like an obstacle course — small fragments weave through quickly and travel far, while large fragments get slowed down and stay near the top. Each horizontal band you see represents a group of DNA fragments that are all the same size.

The lane labeled "L" contains a DNA ladder (also called a size standard or molecular weight marker). It is a mixture of DNA fragments with known sizes measured in base pairs (bp). By comparing where your unknown sample bands line up with the ladder bands, you can estimate the size of each fragment in your sample.

SECTION 4

How It Works — Step by Step

The Gel Electrophoresis Procedure

Running a gel involves several steps. Understanding each step helps you see why the technique works so reliably.

Prepare the Gel

Agarose powder is dissolved in hot buffer solution. When it cools, it solidifies into a jelly-like slab with microscopic pores. A comb inserted during cooling creates the wells.

Load the Samples

DNA samples are mixed with a loading dye (which adds color and weight) and carefully pipetted into the wells. A DNA ladder is loaded in one lane for reference.

Run the Current

The gel is submerged in buffer, and a power supply sends an electric current (typically 80–150 volts) through the chamber. DNA migrates toward the positive electrode.

Stain & Visualize

After running, the gel is stained with a fluorescent dye that binds to DNA. Under ultraviolet (UV) light, the DNA bands glow brightly, revealing the banding pattern.

Analyze Results

Scientists compare band positions to the ladder to estimate fragment sizes. Gel images can be captured and measured digitally for precise analysis.

Key Factors That Affect Migration

Several factors influence how fast DNA moves through the gel. Fragment size is the most important: smaller pieces travel faster and farther. Gel concentration also matters — a higher percentage of agarose creates smaller pores, which is better for separating small fragments, while a lower percentage works better for large fragments. Voltage affects speed too: higher voltage makes DNA move faster, but too much voltage can distort the bands and reduce resolution.

ELECTROPHORETIC MOBILITY

μ = v / E

Where μ (mu) = electrophoretic mobility, v = velocity of the molecule (cm/s), and E = electric field strength (V/cm). Smaller molecules have higher mobility in a gel because they encounter less friction.

LOG-LINEAR RELATIONSHIP

log₁₀(size in bp) = m × distance + b

Within a certain size range, the logarithm of a DNA fragment's size (in base pairs) is roughly proportional to the distance it migrates. Here, m is the slope and b is the y-intercept of the best-fit line. This allows scientists to create a standard curve from the ladder and use it to estimate unknown fragment sizes.

SECTION 5

Types of Gels & Their Uses

Not all gels are the same. The type of gel you choose depends on what molecules you are separating and how precise you need to be. The two main types are agarose gels and polyacrylamide gels. Each has its own strengths.

Comparison of the two main gel types used in electrophoresis

Feature	Agarose Gel	Polyacrylamide Gel (PAGE)
Best for	DNA and RNA fragments (100–25,000 bp)	Small DNA fragments (<500 bp) and proteins
Pore size	Larger pores; adjustable by concentration (0.5–2%)	Smaller, more uniform pores; adjustable (4–20%)
Resolution	Good for large-range separation	Excellent — can distinguish fragments differing by 1 base pair
Ease of use	Simple to prepare; non-toxic	More complex; acrylamide is toxic before polymerization
Common uses	PCR product checking, restriction digests, Southern blots	DNA sequencing, protein analysis (SDS-PAGE), Western blots

A standard curve is created by plotting the log₁₀ of each ladder fragment size against the distance it traveled. The best-fit line (purple) lets you estimate the size of an unknown fragment (pink dot) based on how far it migrated. In this example, an unknown band at 28 mm corresponds to approximately 1,500 bp.

The standard curve above is the key tool for analyzing gel results. By measuring how far each known ladder band traveled and plotting those distances against the log of their sizes, you get a roughly straight line. To find the size of an unknown fragment, you measure its migration distance, find that distance on the x-axis, draw a line up to the best-fit curve, and then read across to the y-axis to get the log of the size. Taking the anti-log (10 raised to that power) gives you the estimated size in base pairs.

SECTION 6

Worked Example — Estimating Fragment Size

Let's walk through a real problem. Suppose you ran a gel and measured the migration distances of your DNA ladder bands. You also have an unknown sample band. Your goal is to estimate the size of the unknown fragment.

Estimating an Unknown DNA Fragment Size Using a Standard Curve

Step 1 — Record the Ladder DataYour DNA ladder bands migrated the following distances: 10,000 bp → 5 mm, 5,000 bp → 13 mm, 2,000 bp → 24 mm, 1,000 bp → 34 mm, 500 bp → 45 mm, 100 bp → 53 mm. Your unknown band migrated 30 mm.

Step 2 — Calculate log₁₀ of Each Ladder SizeConvert each size to its logarithm: log₁₀(10,000) = 4.00, log₁₀(5,000) = 3.70, log₁₀(2,000) = 3.30, log₁₀(1,000) = 3.00, log₁₀(500) = 2.70, log₁₀(100) = 2.00. These log values go on the y-axis of your graph.

Step 3 — Plot the Standard CurvePlot each point with distance (mm) on the x-axis and log₁₀(bp) on the y-axis. Draw the best-fit straight line through the points. Using two points — (5, 4.00) and (53, 2.00) — we can calculate the slope: m = (2.00 − 4.00) ÷ (53 − 5) = −2.00 ÷ 48 ≈ −0.0417.

Slope (m) ≈ −0.042

Step 4 — Find the Y-InterceptUsing the point (5, 4.00) and the slope: b = y − m × x = 4.00 − (−0.042 × 5) = 4.00 + 0.21 = 4.21. So the equation of the line is: log₁₀(bp) = −0.042 × distance + 4.21.

b ≈ 4.21

Step 5 — Estimate the Unknown Fragment SizePlug in the unknown's distance of 30 mm: log₁₀(bp) = −0.042 × 30 + 4.21 = −1.26 + 4.21 = 2.95. Now take the anti-log: bp = 10^2.95 ≈ 891 bp. The unknown fragment is approximately 890 base pairs long.

Unknown fragment ≈ 890 bp

💡 Pro Tip

In a real lab, you can also estimate fragment sizes by eye — just by seeing where your unknown band falls between two known ladder bands. If the unknown band sits about halfway between the 1,000 bp and 500 bp ladder bands, it is roughly 750 bp. The mathematical method is more precise, but the visual estimate is a handy shortcut.

SECTION 7

Real-World Applications & Limitations

Gel electrophoresis is used in many fields beyond basic research. From solving crimes to diagnosing diseases, this technique is a workhorse of modern biology. However, it does have limitations that scientists must keep in mind.

Common real-world applications of gel electrophoresis

Application	How Gel Electrophoresis Is Used
Forensic Science	DNA fingerprinting compares banding patterns from crime scene evidence to suspect DNA samples. Matching patterns can link a person to a crime scene.
Paternity Testing	A child's DNA bands should match a combination of both parents' bands. Gel patterns can confirm or rule out biological relationships.
Medical Diagnosis	Certain genetic diseases produce DNA fragments of abnormal sizes. Doctors can use gels to detect mutations related to conditions like sickle cell anemia.
Cloning & GMO Verification	After inserting a gene into a plasmid, scientists run a gel to confirm the correct DNA fragment was successfully inserted.
PCR Product Verification	After running PCR (polymerase chain reaction), a gel confirms that the correct-sized DNA fragment was amplified.

Limitations to Keep in Mind

Size range limits: Standard agarose gels work best for fragments between about 100 and 25,000 bp. Very large DNA requires specialized methods like pulsed-field gel electrophoresis.
Approximate sizing: The sizes you estimate from a gel are approximations. For exact sizes, DNA sequencing is needed.
No sequence information: A gel tells you the size of DNA fragments but not the order of the A, T, G, and C bases within them.
Requires enough DNA: You need a minimum amount of DNA to see bands on a gel. Very small quantities may need to be amplified by PCR first.

✦ KEY TAKEAWAY

Gel electrophoresis is like taking a photo of a fingerprint — it gives you a pattern you can compare to known patterns, but it does not tell you everything. Just as a fingerprint alone cannot tell you a person's name, a gel pattern alone cannot tell you the DNA sequence. It is best used as a powerful sorting and identification tool alongside other techniques.

SECTION 8

Connection to Advanced Techniques

Gel electrophoresis is a foundation for many advanced molecular biology techniques. Once you understand how gels separate molecules by size, you are ready to learn about more powerful methods that build on the same principles.

How gel electrophoresis connects to advanced molecular biology techniques

Basic Concept	Advanced Technique	What It Adds
Standard gel electrophoresis	Capillary Electrophoresis	Uses thin capillary tubes instead of a slab gel. Faster, automated, and can distinguish fragments differing by a single base pair — used in DNA sequencing.
Separating DNA on a gel	Southern Blotting	After gel electrophoresis, DNA is transferred to a membrane and probed with a labeled complementary sequence to detect a specific gene.
Separating proteins by size	Western Blotting	Proteins are separated by SDS-PAGE, transferred to a membrane, and detected with specific antibodies. Used to identify specific proteins in a sample.
Agarose gel for large DNA	Pulsed-Field Gel Electrophoresis (PFGE)	Alternates the direction of the electric field to separate very large DNA molecules (up to millions of base pairs), including whole chromosomes.
Manual gel reading	Next-Generation Sequencing	Modern sequencing technologies have replaced many gel-based methods, reading millions of DNA fragments simultaneously to decode entire genomes.

Even though newer technologies like next-generation sequencing are incredibly powerful, gel electrophoresis remains essential in labs around the world. It is affordable, reliable, and often the fastest way to check whether an experiment worked. Think of it as the tried-and-true tool that every biologist keeps in their toolbox.

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL

DNA is loaded into the wells at the negative end of the gel. Explain why DNA moves toward the positive electrode when the electric current is turned on.

PROBLEM 2 — BASIC CALCULATION

A DNA ladder has a band at 3,000 bp that migrated 18 mm and a band at 500 bp that migrated 42 mm. If an unknown band migrated 30 mm, estimate its size using the log-linear relationship. (Hint: set up the equation log₁₀(bp) = m × distance + b.)

PROBLEM 3 — INTERMEDIATE

A scientist uses a restriction enzyme to cut a 5,000 bp circular plasmid. On the gel, she sees three bands at approximately 2,500 bp, 1,500 bp, and 1,000 bp. Do these results make sense? Explain your reasoning.

PROBLEM 4 — APPLIED

In a forensic case, DNA from a crime scene is compared to DNA from three suspects using gel electrophoresis. The crime scene sample shows bands at 400 bp, 700 bp, and 1,200 bp. Suspect A shows bands at 400 bp, 700 bp, and 1,200 bp. Suspect B shows bands at 400 bp, 900 bp, and 1,200 bp. Suspect C shows bands at 300 bp, 700 bp, and 1,000 bp. Which suspect's DNA matches the crime scene sample? What additional testing might be needed?

PROBLEM 5 — CRITICAL THINKING

A student runs a gel and notices that the bands in all lanes are smeared (blurry and spread out) instead of appearing as sharp, distinct bands. Propose three possible causes for this problem and suggest one way to fix each.

SUMMARY

Gel Electrophoresis — Summary

Gel electrophoresis is a technique that separates DNA, RNA, or protein molecules by size using an electric field and a porous gel matrix. Negatively charged DNA migrates toward the positive electrode, with smaller fragments traveling farther and larger fragments staying near the wells. Scientists use a DNA ladder as a size reference and can create a standard curve to estimate the size of unknown fragments.

This technique has transformed fields from forensic science to medical diagnosis and serves as the foundation for advanced methods like Southern blotting, Western blotting, and capillary electrophoresis. The two main gel types — agarose for larger DNA and polyacrylamide for smaller fragments and proteins — give scientists flexibility for a wide range of experiments. Remember: gels tell you the size of molecules, but not their sequence, so they are best used alongside other analytical tools.