In modern molecular biology and genetic engineering, analysing DNA and understanding which genes are actively transcribed is central to medicine, forensics, and evolutionary research. Two vital laboratory technologies taught across UK A-Level Biology specifications (AQA, OCR A/B, Edexcel) are gel electrophoresis and DNA microarrays.

While gel electrophoresis is a physical separation technique that sorts charged macromolecules (such as DNA fragments, RNA, and proteins) according to their size, microarrays are high-throughput diagnostic tools that allow researchers to screen the expression levels of thousands of genes simultaneously.

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What Is Gel Electrophoresis?

Key Definition:

Gel electrophoresis is a laboratory technique used to separate and analyse charged biological macromolecules—specifically DNA, RNA, and proteins—based on their size (molecular mass) and net electrical charge.

The Principle of Separation

Separation in gel electrophoresis relies on an electric field moving charged molecules through a porous gel matrix:

Electrical Charge Drives Movement: DNA and RNA molecules carry an overall negative electrical charge due to the presence of negatively charged phosphate groups ( $\text{PO}_4^{3-}$ ) in their sugar-phosphate backbones. When an electric current is applied across the gel, the negatively charged DNA fragments are repelled by the negative electrode (cathode) and migrate toward the positive electrode (anode).
The Gel Functions as a Molecular Sieve: The gel (typically made of agarose for nucleic acids or polyacrylamide for proteins) forms an intricate, microscopic three-dimensional mesh of cross-linked polymer fibres.
Fragment Size Determines Migration Distance: Smaller DNA fragments experience less mechanical friction and resistance as they pass through the tiny pores of the gel matrix. Consequently, smaller/shorter fragments move faster and travel further through the gel than larger/longer fragments over a fixed period.

How Does Gel Electrophoresis Work? (Step-by-Step)

The process of running a DNA gel electrophoresis separation involves five distinct stages:

Illustration of gel electrophoresis showing the setup, DNA loading, process and resulting seperation

1. Preparing and Loading the Samples

An agarose gel slab is cast with a series of small indentations called wells moulded into one end.
The gel is placed into an electrophoresis buffer tank and completely submerged in an electrolyte buffer solution (a conductive salt solution that maintains a stable pH and conducts the electric current).
DNA samples—often generated via PCR (Polymerase Chain Reaction) or cleaved by restriction endonucleases—are mixed with a dense loading dye.
Using a micropipette, the samples are carefully injected into the submerged wells at the negative electrode (cathode) end of the gel.
A reference sample containing fragments of known lengths—known as a DNA ladder or molecular weight marker—is loaded into an adjacent lane to provide a standard calibration scale.

2. Applying the Electric Current

The electrophoresis chamber is connected to a direct-current (DC) power pack.
The negative terminal is connected adjacent to the loaded wells, and the positive terminal is connected to the far end.
When the voltage is applied, an electric field forms across the buffer solution, causing negatively charged DNA molecules to migrate toward the positive anode.

3. Separation by Size

As the DNA fragments migrate through the gel matrix, longer fragments are impeded by the agarose fibres.
Shorter fragments weave through the gel pores with ease.
Once the tracking dye nears the positive end of the tank, the electrical supply is turned off before fragments run off the edge of the gel.

4. Visualisation of the DNA Bands

Because DNA is naturally transparent and invisible to the naked eye, the separated fragments must be visualised:

Fluorescent Staining: The gel is pre-stained or post-incubated with a fluorescent dye (such as ethidium bromide or safer alternatives like GelRed) that intercalates between the bases of double-stranded DNA. When exposed to ultraviolet (UV) radiation, the dye fluoresces, revealing distinct glowing bands.
Southern Blotting & Gene Probes: For complex genomic DNA, fragments are transferred onto a nitrocellulose membrane, denatured into single strands, and incubated with single-stranded, radioactive or fluorescent gene probes that hybridise to specific complementary base sequences.

Uses of Gel Electrophoresis

1. DNA Profiling (Genetic Fingerprinting)

Humans possess highly variable, non-coding regions of repetitive DNA known as Variable Number Tandem Repeats (VNTRs) or Short Tandem Repeats (STRs). Because the number of repeats varies between individuals (except identical twins), cutting DNA around these regions produces unique sets of fragment sizes. Gel electrophoresis separates these fragments into an individual's unique DNA profile, utilised extensively in:

Forensic science: Matching suspect DNA to biological evidence left at crime scenes.
Paternity testing: Verifying parentage (a child's bands must each match either the biological mother or biological father).

2. Analysing PCR Products & Restriction Digests

Researchers verify whether a gene cloning or PCR amplification procedure was successful by checking whether the resulting band aligns precisely with the expected base-pair size on the reference DNA ladder.

3. Protein Separation (SDS-PAGE)

Unlike DNA, proteins vary widely in their native electrical charges depending on their amino acid R-groups. In SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis):

Proteins are denatured and coated with an anionic detergent (SDS), which confers a uniform negative charge proportional to their mass.
Polypeptide chains are then separated strictly according to molecular weight.
A-Level Application: Separating haemoglobin variants (e.g., distinguishing normal adult hemoglobin $\text{HbA}$ from sickle-cell hemoglobin $\text{HbS}$ ).

What Are Microarrays?

While gel electrophoresis separates physical fragments, microarrays allow researchers to monitor the activity of an entire genome in a single experiment.

Key Definition:

A DNA microarray (gene chip) is a solid substrate (glass slide, silicon chip, or nylon membrane) onto which thousands of microscopic spots of known, single-stranded DNA sequences (probes) are fixed in a precise, indexed grid.

High-Throughput Analysis: Each microscopic spot on the chip contains millions of identical copies of a unique single-stranded probe corresponding to a specific gene.

Hybridisation Principle: If a sample contains single-stranded nucleic acid fragments complementary to a probe, they will bind specifically via complementary base pairing (A-T and C-G) through hydrogen bonding.

How Do Microarrays Detect Gene Expression?

The most common application of microarrays is comparing gene expression profiles between two different biological states—such as healthy tissue vs. cancerous tissue.

Step-by-Step Microarray Protocol

1. mRNA Extraction

Active genes undergo transcription to yield messenger RNA (mRNA). Therefore, the quantity and variety of mRNA molecules present in a cell directly reflect which genes are being actively expressed. Researchers isolate total mRNA from both the control sample (healthy cells) and the experimental sample (cancer cells).

2. Reverse Transcription to Form cDNA

Because single-stranded mRNA is chemically unstable and easily degraded by environmental enzymes, it is converted into more durable complementary DNA (cDNA):

The enzyme reverse transcriptase uses the mRNA as a template to synthesise single-stranded cDNA.
This cDNA contains only coding exons, matching the expressed transcriptome.

3. Fluorescent Labelling

Different fluorescent dyes are covalently linked to the cDNA from each sample:

Healthy cell cDNA is typically tagged with a green fluorescent tag (e.g., Cy3).
Cancer cell cDNA is typically tagged with a red fluorescent tag (e.g., Cy5).

4. Hybridisation

Equal quantities of both green- and red-labelled cDNA are mixed together and washed across the surface of the microarray chip.
Any cDNA sequence that encounters a complementary probe on the chip anneals to it via complementary base pairing (hybridisation).

5. Washing the Chip

The microarray is thoroughly washed with buffer to remove any unbound or non-specifically attached cDNA. This step is critical: failing to wash away unbound cDNA would produce false-positive fluorescence across the entire chip.

6. Laser Scanning & Fluorescent Detection

A specialised confocal laser scanner excites the fluorophores at their specific absorption wavelengths and detects the emitted light. A computer analyses the colour and intensity of each spot:

Spot Colour Observed	Biological Interpretation	Gene Expression Status
Green Only	Only healthy cell cDNA hybridized to the probe.	Gene is expressed in healthy cells only (down-regulated or silenced in cancer).
Red Only	Only cancer cell cDNA hybridized to the probe.	Gene is expressed in cancer cells only (up-regulated oncogene).
Yellow (Green + Red)	Both healthy and cancer cDNA hybridized in equal amounts.	Gene is expressed at equal levels in both tissues (e.g., essential housekeeping gene).
Black / Unlit (No Signal)	Neither sample hybridized to the probe.	Gene is inactive / not expressed in either tissue type.

The intensity of the fluorescence indicates the level of gene expression: brighter spots represent a higher concentration of mRNA molecules, signifying greater transcriptional activity.

Comparison: Gel Electrophoresis vs. Microarrays

Feature	Gel Electrophoresis	DNA Microarrays
What It Analyzes	Physical length / molecular mass of DNA, RNA, or protein fragments.	Sequence presence and relative gene expression (mRNA transcription levels).
Underlying Mechanism	Migration through a porous gel driven by an electric field.	Specific molecular hybridisation between target sequences and fixed probes.
Throughput Capacity	Low to moderate (typically 10 to 30 sample lanes per gel).	High-throughput (thousands to tens of thousands of genes simultaneously).
Labelling Method	Intercalating fluorescent stains (GelRed/EtBr) or labelled Southern blot probes.	Dual-color differential fluorescent dyes (e.g., Cy3 green and Cy5 red).
Primary Applications	Forensic DNA profiling, paternity testing, sizing PCR/restriction fragments.	Cancer research, pharmacogenomics, identifying disease biomarkers, pathogen identification.