Capillary Electrophoresis: A Technique to Analyze Fluorescently Labeled Polymerase Chain Reaction Amplicons
JoVE Encyclopedia of Experiments
Biological Techniques
0 views • 3:29 min • July 8th, 2025
To determine size of DNA fragments using capillary electrophoresis, begin by taking a multi-well assay plate. Each well contains fluorescently labeled, differently sized PCR amplicons - double-stranded DNA oligonucleotides obtained after a PCR cycle.
Add a few differently sized, double-stranded DNA fragments to the well, which act as a size standard. Each of these DNA molecules carries fluorescent tag for easy identification.
Supplement the well with formamide, a strong denaturant, and heat at a high temperature. This step denatures the DNA, forming single-stranded fragments that can migrate easily through the gel pores.
Place the plate in a fragment analyzer unit that contains pre-injected polymer gel inside its capillary system.
Apply a high voltage to electrokinetically introduce the DNA from the well into the thin-diameter capillary at the cathode - the negative terminus. The negatively charged DNA molecules migrate towards the positively charged anode. The shorter DNA strands face less frictional force and move faster through the gel than the longer DNA strands.
During electromigration, the DNA crosses the path of the laser beam, which causes the dye to fluoresce. The emission signals from the dye are captured by a detector and are later translated into an electropherogram - a graph representing each fluorescent DNA as a differently colored, individual peak.
Compare the unknown DNA fragment peak with the reference peaks to analyze its size.
Following confirmation of PCR amplicons, use new PCR strips or plates, to dilute PCR products 1 to 10 in purified water. Vortex and centrifuge briefly. While working in a fume cupboard, prepare a master mix in a centrifuge tube consisting of formamide and reference size standard solution.
According to the number of PCR products to be analyzed, use reagent volumes, as detailed in the manuscript. Allow a 10% surplus volume.
Vortex briefly and distribute 9.5 microliters into individual wells on a PCR plate, appropriate for the available capillary electrophoresis system. Then, add 0.5 microliters of diluted PCR product. Seal and centrifuge briefly.
Then, load the plate containing the samples into a PCR thermal cycler, and denature the samples at 95 degrees Celsius for 3 minutes, before cooling to 4 degrees Celsius indefinitely. Centrifuge at 1,000 x g for 1 minute. Carefully remove the seal, and load the plate onto a calibrated capillary electrophoresis system according to the manufacturer's instructions.
Run fragment analysis capillary electrophoresis using reagents as appropriate for the apparatus of choice, and adjust the run conditions according to the description in the manuscript.
Overview
This article describes the process of determining the size of DNA fragments using capillary electrophoresis. It outlines the preparation of samples, the electrophoresis procedure, and the analysis of results through fluorescent detection.
Key Study Components
Area of Science
- Biotechnology
- Molecular Biology
- Genomics
Background
- Capillary electrophoresis is a technique used for separating DNA fragments.
- Fluorescent labeling allows for the identification of different DNA sizes.
- Understanding DNA fragment sizes is crucial for various genetic analyses.
- Electropherograms provide visual representation of DNA fragment sizes.
Purpose of Study
- To accurately measure the size of PCR amplicons.
- To establish a reliable method for DNA fragment analysis.
- To enhance the understanding of DNA separation techniques.
Methods Used
- Preparation of a multi-well assay plate with fluorescently labeled DNA.
- Denaturation of DNA using formamide and heat.
- Electrokinetic introduction of DNA into a capillary system.
- Detection of fluorescent signals to generate an electropherogram.
Main Results
- Successful separation of DNA fragments based on size.
- Generation of distinct peaks in the electropherogram for analysis.
- Comparison of unknown DNA fragments with reference standards.
- Confirmation of PCR amplicon sizes through capillary electrophoresis.
Conclusions
- Capillary electrophoresis is an effective method for DNA size determination.
- The technique provides accurate and reproducible results.
- Fluorescent labeling enhances the detection and analysis of DNA fragments.
Frequently Asked Questions
Executive Industry Relevance
Capillary electrophoresis provides a precise, reproducible method for sizing fluorescently labeled DNA fragments, enabling accurate verification of PCR amplicon integrity in molecular workflows. This capability supports target validation and assay development by delivering quantitative size data essential for confirming genetic constructs and detecting allelic variants. The technique enhances predictive confidence in early discovery by reducing ambiguity in fragment analysis, thereby informing go/no-go decisions in assay optimization and biomarker screening pipelines.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses through accurate sizing of PCR-generated DNA fragments, supporting functional validation of genetic targets.
- Operational Value: Provides standardized, quantitative fragment sizing that reduces mechanistic ambiguity in construct verification.
- Predictive Value: Enhances confidence in lead identification by confirming amplicon size consistency across replicates, aiding portfolio triage.
Screening & Assay Development
- Scientific Value: Prepares validated biological systems for downstream workflows by ensuring PCR product integrity through precise size confirmation.
- Operational Value: Delivers assay standardization and reproducibility via electropherogram peak comparison to reference standards, enabling reliable compound evaluation.
- Scalability: Supports platform reuse across multiple samples due to standardized denaturation, injection, and detection protocols.
Translational & Preclinical Research
- Translational Continuity: Maintains measurement consistency from discovery through preclinical validation by providing a standardized sizing method for genetic biomarkers.
- Risk-Adjusted Advancement: Supports data-driven decisions by generating reproducible electropherograms that inform construct stability and assay reliability.
- Mechanistic De-risking: Focuses on predictive value by minimizing false positives in fragment analysis through size-based discrimination.
Pipeline & Workflow Integration
The method fits within the discovery continuum from Early Discovery to Lead Identification, where accurate DNA fragment sizing informs assay readiness and construct validation prior to screening campaigns.
- Discovery Biology: Supports hypothesis testing and pathway clarification by verifying the size of PCR amplicons derived from genetic constructs.
- Screening: Ensures assay readiness through reproducible, quantitative outputs that enable reliable comparison of genetic variants.
- Analytics: Generates electropherogram peak data that allows teams to compare fragment sizes against reference standards with precision.
- Translational Research: Connects to biomarker alignment by providing consistent sizing data applicable across preclinical models.
- Enterprise Reuse: Functions as a reusable capability for genetic QC, reducing redundant optimization across projects.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence, target validation, reduction of mechanistic ambiguity in DNA fragment analysis.
- Operational Value: Standardization, reproducibility, and scalability of sizing measurements across laboratories.
- Strategic Value: Better go/no-go decisions, capital efficiency, and reduced late-stage biological risk in assay development.
- Portfolio Impact: Risk-adjusted prioritization and advancement decisions based on confirmed amplicon integrity.
Implementation Considerations
- Requires expertise in nucleic acid handling, fluorescent detection, and capillary electrophoresis instrumentation.
- Dependent on access to calibrated fragment analyzers with polymer gel capillaries and laser detection systems.
- Necessitates cross-team standardization of sample preparation, including formamide denaturation and thermal cycling protocols.
- Involves adaptation considerations for varying amplicon sizes and fluorescent dye compatibilities.
- Practical limitations include sensitivity to salt contamination and the need for optimized injection times to avoid peak distortion.
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Last updated: 29 August 2026