Capillary Electrophoresis: A Technique to Analyze Fluorescently Labeled Polymerase Chain Reaction Amplicons

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.