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DNA Fingerprinting
- Healthasion
- Nov 15, 2020
- 3 min read
DNA fingerprinting, also known as DNA typing, is a process best known for isolating and analyzing areas of human DNA. It’s most commonly found in forensic science labs to determine the perpetrator of a crime, and is performed by collecting DNA evidence that can include fingernails, toothpicks, cigarette butts, bed sheets, glasses/contact lenses, envelope seals, hair, and even phones!
DNA fingerprinting was discovered in 1984 by Alec Jeffreys and his associates at Leicester University in England. The team discovered that each individual has a unique genome — the entire collection of someone’s genetic code. Jeffreys found that areas of the genome portray polymorphism, meaning they can take on varying forms. These areas are different in everyone, and they’re also what allow scientists to compare one individual to another.
Polymorphisms can be found in non-encoded DNA, or junk DNA. Length polymorphisms are simply repeats of certain base sequences within the junk DNA. They are among the most important factors in the DNA fingerprinting process. As a result, scientists look for three types of repeated sequences:
Variable Number Tandem Repeats (VNTRs): The same base sequence repeats and can be hundreds of base pairs long.
Short Tandem Repeats (STRs): Usually three to seven base pairs long, these sections also repeat. STRs repeat over segments of the DNA strand as long as 400 bases. This gives scientists more flexibility with degraded DNA samples.
Single Nucleotide Polymorphism (SNPs): The most common DNA variations (in humans); a match is made if several SNPs are examined and a suspect’s SNPs match those of the crime scene sample.
Currently, STR analysis is the standard for DNA testing. Using polymerase chain reaction (PCR), a series of reactions that simply mirrors cellular reactions, the DNA in small samples can be replicated.. There are three stages to PCR: denaturation, annealing, and extending. During denaturation, DNA samples are heated to separate the strands. Annealing includes attaching primers to each strand’s 3’ end to prepare the original strands for replication. Extension uses the help of the primers to add new, complementary base pairs to the new strands.
After PCR, the DNA sample(s) is usually put through a gel electrophoresis machine. Electrophoresis is a process that applies an electrical current to DNA molecules, separating them depending on their fragment size and charge. To start electrophoresis, scientists create a gel solution. After the gel has been poured into the electrophoresis box, the DNA samples are loaded into small wells in the gel. Once the wells are all loaded, electrophoresis is run. Smaller DNA fragments will travel farther, towards the positive side of the electrophoresis box, while larger fragments will remain stagnant.
Once the current has been applied and the fragments have been separated, the DNA will be stained. Staining simply makes the DNA show up more distinctly, resulting in darker bands. When the DNA sample of a suspect matches the sample gathered from the scene, a match can be made, and the crime can be solved!
In the figure to the left, for instance, we can confirm that the DNA from suspect two matches the sample found at the crime scene. The bands of DNA fragments appear at the same size, whereas neither suspect one’s nor suspect three’s DNA matches the crime scene.
Resources
Lyle, D. P. (2019). Forensics for dummies: 2nd edition. Hoboken, NJ: John Wiley & Sons, Inc.
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