6.18—Gel electrophoresis of DNA fragments

DNA is negatively charged, so an electric field moves it toward the positive electrode. In an agarose gel, smaller fragments pass through pores more easily and travel farther than larger fragments.

Load samples with a known DNA standard, keep the polarity correct, run the gel consistently and compare band positions. A stain or fluorescent tag makes fragments visible.

A sample band aligning with the same-size standard indicates similar fragment length. A matching pattern across several markers is stronger evidence than one shared band.

Bands show fragment lengths, not whole-genome identity by themselves. Smearing, wrong polarity or overloaded wells can make a pattern unreliable.