The Evolution of DNA Fingerprinting

By: Khushi Sheth

gel electrophoresis
Image Credit: Flickr @ Micah Baldwin

In the last century, forensic investigation has significantly advanced thanks to one effective, revolutionary technique. DNA fingerprinting has been the solution to convicting criminals, exonerating the wrongly accused, and identifying remnants found in crime scenes and after disasters, and wars. DNA fingerprinting analysis is a process that compares the similarities and differences between an unknown sample of DNA and a sample from a known source to identify matches. For example, a DNA sample from a crime scene could be compared to a DNA sample from a suspect. If the samples have the same fingerprint, then it is very likely that they came from the same person. DNA fingerprinting can also be used to establish paternity by looking for key similarities in the child and parent’s DNA.

The process of fingerprinting DNA to identify the source of a sample is a complex process that includes four steps: extraction, quantitation, amplification, and capillary electrophoresis. Since DNA is located inside the nucleus of cells, the first step in fingerprinting is extraction. To remove a sample of DNA from the nucleus, analysts must separate the DNA molecules from other cellular material. Two common ways to extract DNA include a manual Phenol-Chloroform organic method or a Maxwell® 16 robotic system method.

In the Phenol-Chloroform method, a chemical compound called isoamyl alcohol is added to the sample of cells. This separates the lipids and cellular debris from the aqueous DNA. Since alcohol and other lipids are both nonpolar, they are attracted to each other while polar objects like DNA are repelled. This difference in polarity (or the electrical charge of a molecule) essentially pulls the cell apart, as the lipids and cellular debris are attracted to the alcohol while the DNA is pushed away. Much like a magnet pushing and pulling opposite or similar charges. After centrifugation, the purified DNA is transferred to a clean tube for analysis.

The Maxwell® 16 robot method grinds solid tissue samples to separate the DNA. The cells sit in a lysis buffer in a prefilled cartridge filled with the cell sample. A lysis buffer is a solution of an acid and a base that is used to break open cells. Lysis buffers are used in molecular biology experiments to analyze the macromolecules of cells, in this case the DNA. Extracting DNA with these methods can take two to three hours at minimum.

In order to prove that the DNA extracted is human and not from bacteria or animals, the quality and quantity of the DNA sample is measured and assessed through the use of a Quantifiler DNA Human Quantification kit and the ABI PRISM 7500 Sequence Detection System. This quantification step is especially important when DNA is extracted from a crime scene or another potentially contaminated source. Quantification takes roughly three hours to complete.

gel electrophoresis
Image Credit: Flickr @Joseph Elsbernd

Because most DNA samples found at a crime scene are limited in quantity and quality the amplification step is important to DNA fingerprinting as it gives scientists more DNA to work with and analyze. DNA amplification is accomplished with the use of a Polymerase Chain Reaction (PCR) machine that produces millions of copies of a DNA sequence within a few hours. The samples are put through a thermal cycling pattern for approximately 28 cycles to produce enough samples. After the PCR reaction is completed, the large mixture of amplified DNA molecules need to be separated so they are easily differentiable. The separation of DNA molecules allows for proper analysis. This separation can be done by capillary electrophoresis. Since DNA molecules have a negative charge, it is easily attracted to a positive anode (like the negative and positive sides of a magnet being attracted to each other). In the gel capillary electrophoresis system, an electric current is applied to the negative DNA. Causing the molecules to move through a thin, capillary filled, gel-like polymer fluid and migrate towards the positive anode, away from the negative charge it was applied with. The PCR products are separated by size since it takes a greater time for larger DNA molecules to travel through the polymer than smaller DNA molecules.

DNA fingerprinting can also be utilized to identify an individual’s age, which helps narrow down a field of suspects in law enforcement. Determining age is important since it allows law enforcement to properly distinguish whether or not the individual in question is to be investigated and tried as a juvenile. One method for predicting age from DNA fingerprinting comes from epigenetics. Changes in global DNA methylation level are associated with older age as the epigenetic code alters when exposed to environmental factors. Therefore, the more DNA methylation present in a sample, the older the person is, as they’ve been exposed to more environmental factors that induce DNA methylation.

Human genetic variation and DNA fingerprinting is a major resource in forensics, however there are many questions left unanswered. Researchers are currently envisioning further forensic applications of DNA methylation analysis in order to predict a suspect’s identifiable life-style factors. Combined with cutting-edge research that aims to use DNA samples to predict appearance and biogeographic ancestry, this epigenomic lifestyle prediction will significantly increase the ability for government officials to apprehend anonymous perpetrators who are not identifiable with modern forensic DNA profiling.

Educational Content

Q: What does DNA fingerprinting do?