DNA fingerprinting with short tandem repeats
DNA fingerprinting mainly tells individuals apart by comparing short tandem repeats (short sequences repeated back to back) in non-coding DNA, that is, DNA that does not code for a protein.
Remember
Fingerprinting works because the number of repeats at each short tandem repeat site differs between people, so comparing several sites gives a pattern that tells unrelated people apart.
Memory aid
Think of the repeat count at each site as one digit of a long barcode
One repeat count on its own tells you little, while the combined pattern across several sites is what is highly individual.
Between people, most of the genome is similar, yet some regions differ. DNA fingerprinting (also called DNA profiling) compares those variable regions. The usual targets are short tandem repeats (STRs): a short sequence repeated back to back, where the number of repeats varies from person to person. A site with many versions in the population is described as highly polymorphic.
Many of these repeats lie in non-coding DNA. Changes there are not selected against, so they build up over the generations, which is why these repeats vary so much. Sequences that do code for a protein must produce a working protein, so they stay very similar between people and separate individuals poorly.
The DNA is extracted from a small sample, for example a mouth swab. The chosen repeat regions are then amplified (copied many times, for example by the polymerase chain reaction, PCR) and their lengths are compared. The DNA can also be cut with restriction enzymes (proteins that cut DNA at specific sequences), and the fragments are separated by size, which gives a pattern of bands.
One site on its own could match by chance, so several sites are compared at once. A match at all of them between unrelated people is then very unlikely. It is an identity test, not a disease test: it shows who the DNA belongs to, but it does not show whether a gene is faulty.
Key facts
- Purpose
- it helps decide whether two samples come from the same person, or whether two people are related
- Target
- short tandem repeats, whose number of repeats differs widely from one person to the next
- Location
- many of these repeats lie in non-coding DNA, not in sequence that codes for a protein
- Several sites
- comparing several sites at once makes a match at all of them between unrelated people very unlikely
- Typical steps
- the DNA is extracted and copied (amplified), can also be cut with restriction enzymes, and the pattern of lengths or bands is then analysed
- Not the aim
- diagnosing a disease-causing mutation, or reading out the whole genome
Exam traps
- Conserved coding sequences, meaning sequences that stay very similar between people because they have to produce working proteins, give little to tell two people apart.
- Mitochondrial DNA is passed down the maternal line, so maternal relatives share it and it cannot separate them, although it helps with degraded samples such as bone, teeth or hair.
- Methylation is an epigenetic mark, a chemical tag on DNA that alters gene activity without changing the sequence. Levels at particular sites change with age and differ between tissues, so it is used to estimate age or tissue type, not to identify a person.
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