Fragment Size Analysis

An electric field drives negatively charged DNA through a porous medium, such as a gel or capillary system. Smaller fragments encounter less resistance and migrate more rapidly than larger fragments, producing a pattern that reflects fragment length. Comparing migration with size standards allows researchers to estimate the lengths of experimental DNA products and assess whether their composition is expected.

Fluorescent labels make selected DNA fragments detectable during analysis, while size standards provide reference points for estimating fragment length. Together, they support more accurate interpretation of the resulting profile than migration alone. This combination is especially useful when researchers need to distinguish closely sized products or compare fragment patterns between genetic samples.

Both gel and capillary electrophoresis separate DNA according to fragment size, but they use different physical formats for the separation medium. The selected format determines how the fragment pattern is produced and measured. In either case, the resulting size profile can be compared with standards to evaluate PCR products, restriction digests, or genetic markers.

A change in DNA sequence or molecular structure can produce fragments of different lengths, creating distinguishable profiles. Short tandem repeat markers are particularly useful because differing repeat patterns can separate alleles by size. Interpreting these length differences helps researchers identify genotypes and examine variation among samples without relying on the same profile for every individual.

Researchers first examine an appropriate DNA product, such as a PCR product, restriction digest, or short tandem repeat marker, and may apply fluorescent labeling when detection requires it. The fragments then undergo gel or capillary electrophoresis alongside size standards. The resulting profile is evaluated for fragment lengths, expected products, and differences that may indicate distinct alleles.

The method supports several genetics applications, including genotyping, mutation screening, linkage studies, sample identification, and molecular quality control. Researchers can use the measured profile to distinguish alleles, assess genetic identity, or verify that an experimental product has the expected composition. These uses make fragment patterns relevant both to variation studies and to checking laboratory results.