Gel electrophoresis: sorting DNA by size — Biology, 14–17
Gel electrophoresis turns invisible DNA fragments into bands that you can compare. It is a practical way to ask whether samples match or whether a DNA experiment worked.
The idea
DNA carries a negative electric charge, so it moves through a gel towards the positive electrode. Small fragments slip through the gel’s tiny spaces more easily than large ones, so they travel farther and form separate bands.
Why it matters
A tube of DNA looks clear, so you cannot judge its fragments by eye. Scientists needed a way to compare pieces after cutting DNA or copying a region with PCR; separating them by size made hidden patterns visible.
A worked example
Imagine a ladder containing fragments of 100, 300 and 500 base pairs. Your sample shows bands at 300 and 500 base pairs, so it contains two fragment sizes; if another sample has the same two bands, the patterns match for this test, though that alone proves no person’s identity.
The common trap
It is tempting to think that a darker band means a longer DNA fragment. Darkness usually means that more DNA collected there, while distance across the gel gives information about size; amount and length are different measurements.
Where it is used
Researchers use gels to check PCR products, compare DNA from different samples and identify fragments in genetic studies. Medical and forensic laboratories may use related separation methods, but a matching band pattern is evidence to interpret, not an automatic final answer.
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