Applications of PCR and gel electrophoresisSpec D1.1.5
In short
DNA profiling is the main application of PCR and gel electrophoresis: regions of DNA that vary in length between people are amplified and separated, giving a pattern of bands. Profiles are compared in paternity tests and forensic investigations. Using more markers makes the test more reliable, because it reduces the probability that two unrelated people match by chance.
Because PCR can amplify a chosen sequence from a tiny sample and gel electrophoresis can separate fragments by length, the two techniques have a broad range of applications.
- DNA profiling for paternity and forensic investigations.
- Detecting pathogens: PCR detects DNA (or RNA copied into DNA) of a virus or bacterium in a patient sample, as in tests for viral infections.
- Genetic screening: testing for alleles that cause genetic conditions.
- Research: amplifying genes for sequencing or for comparing species.
DNA profiling
Some regions of the genome contain short tandem repeats (STRs): a short base sequence repeated many times. The number of repeats at each region varies between individuals, so the length of the region varies. In DNA profiling, several of these regions are amplified by PCR and the fragments are separated by gel electrophoresis, giving a pattern of bands (a DNA profile) that is almost unique to each person.
| Use | How profiles are compared |
|---|---|
| Paternity | A child inherits one allele of each region from each parent. Every band in the child's profile must match a band in the mother's or in the father's profile. A man whose profile lacks the non-maternal bands cannot be the father. |
| Forensic investigation | DNA from blood, hair or skin cells at a crime scene is amplified and profiled, then compared with profiles from suspects. A match links a person to the scene; a mismatch excludes them. |
Nature of science: reliability is enhanced by increasing the number of measurements. In DNA profiling, using more markers (more STR regions) reduces the probability of a false match between two different people.
(opens full size in a new tab)Quick check
Why is DNA replication described as semi-conservative?
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Each new DNA molecule contains one strand from the original molecule and one newly synthesised strand.
What is the role of helicase?
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It unwinds the double helix and breaks hydrogen bonds between bases so the strands separate.
Why is Taq polymerase used in PCR?
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It comes from a hot-spring bacterium and is not denatured at 95 °C, so it survives the denaturation step of every cycle.
Why do smaller DNA fragments travel further in gel electrophoresis?
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They pass more easily through the mesh of the gel, so they move faster towards the positive electrode.
Written and checked against the IB Biology SL specification · Updated October 2026