DNA profiling

Microbiology, Immunity and Forensics · Decay, DNA techniques and forensics · note 4 of 5

Spec 6.19

A Level only

A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean

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DNA profilingSpec 6.19

In short

DNA profiling compares variable-length repeated sequences in non-coding DNA (short tandem repeats). They are amplified by PCR and separated by gel electrophoresis, giving a band pattern. It identifies individuals and shows relationships between organisms, because offspring inherit half their bands from each parent.

Much of the genome is non-coding DNA, which contains short sequences repeated many times in a row (short tandem repeats, STRs). The number of repeats at each position (locus) differs between individuals, so the length of the fragment differs. Each person inherits one set from each parent, so, apart from identical twins, no two people have the same pattern across several loci.

DNA is extracted from a sample (blood, hair root, saliva, tissue or seed).
Regions containing STRs are amplified by PCR using primers (often labelled with a fluorescent tag).
The fragments are separated by gel electrophoresis and detected, giving a pattern of bands (the DNA profile).
Profiles are compared. Bands at the same position in two samples match.

Uses

Uses of DNA profiling
Use	How profiles are used
Identification: forensic	DNA from a crime scene is compared with DNA from suspects or a database; a match of many bands makes it very unlikely that the DNA is from anyone else
Identification: victims, species and products	Identifies remains; identifies species or individual animals, for example illegal ivory or poached animals; identifies plant varieties or checks the origin of food or timber
Genetic relationships: individuals	Paternity and maternity: every band in a child's profile must be present in the mother or the father. The father's bands that the child has but the mother does not show who the father could be
Genetic relationships: plants and animals	Measures the relatedness of individuals in a breeding programme or wild population, so that inbreeding can be reduced; used to show relationships between species and varieties

Maths skill:

Probability: if the chance of a match at one locus is 1 in 10 and loci are independent, the chance of a match at five loci is (1/10)⁵ = 1 in 100 000. More loci mean a more reliable identification.

DNA profiles for a paternity case: lanes for a DNA ladder, mother, child, man 1 and man 2 with bands A to G; the child bands E and G are not in the mother and both appear in man 1 but not man 2. — A **paternity** test: every band in the child must come from the mother or the father, so only man 1 could be the father.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How do microorganisms recycle carbon?

Saprotrophic bacteria and fungi secrete extracellular enzymes that digest dead organisms and waste, then absorb the soluble products. They respire some of this, releasing carbon dioxide into the atmosphere. Producers can then take up this carbon dioxide in photosynthesis, so carbon returns to the living world.

How does PCR work?

PCR copies a section of DNA using repeated cycles. At about 95 °C the strands separate, at about 55 to 65 °C primers anneal, and at about 72 °C Taq polymerase builds new strands from free nucleotides. Each cycle doubles the amount of DNA, giving millions of copies after 30 cycles.

How does gel electrophoresis separate DNA?

DNA is negatively charged because of its phosphate groups. In an electric field it moves through an agarose gel towards the positive electrode. The gel is a mesh, so shorter fragments move faster and further than longer ones, separating the fragments by length into bands.

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