DNA fingerprinting is a very useful technique that has applications beyond criminal and forensic analysis.
In this lesson, you’ll learn what DNA fingerprinting is, how it is done, and different ways it can be used to determine identity.
What Is DNA Fingerprinting?
You DNA is very special because it is what makes you, you. Unless you have an identical twin, no one else has the same DNA as you. This genetic identification comes from your parents – half from mom and half from dad. But no matter how many kids your parents have, your DNA will always be unique from everyone else’s.And this is what makes DNA fingerprinting such a useful tool: it allows us to identify an individual from biological samples.
It’s called fingerprinting because, just like identifying an individual based on the unique patterns we find on their fingers, we identify an individual based on the unique fingerprint of their DNA. Because of this, you may have also heard it called DNA profiling or identity testing.For example, if you leave a hair behind somewhere, the DNA in that strand of hair can be matched to you by taking a sample of DNA from another part of your body. As you can imagine, this is really helpful for catching criminals if they leave DNA behind at a crime scene.
But we’ll get into that a little later on.
How It Works
First, let’s talk about how we go about creating the fingerprint. Again, pretty much all of your cells contain your DNA so we can get a sample from bodily fluids like blood, but it can also come from hair, bone, skin, and other types of cells.The DNA is extracted from the sample and then augmented using a technique called PCR, or polymerase chain reaction, which replicates and amplifies the DNA sample in the lab. This is necessary because there’s usually only a small amount of DNA available in the reference sample, and this step lets us copy the DNA to get enough to make a profile for the individual.This amplified DNA is then cut at specific sequences with restriction endonucleases, and this is where the magic happens. Because each person’s DNA is different, it will be cut by these enzymes at different sites, leaving us with pieces of different sizes.
Specifically, these regions are called STRs, or short tandem repeats, which are regions of non-coding DNA that have specific and repeated nucleotide sequences. The number of times the sequence is repeated is unique to each individual, giving each of us fragments of different lengths. The fragments we end up with are called RFLPs, which stands for restriction fragment length polymorphisms, and the name reminds us that we are talking about fragments cut by restriction endonucleases.But we’re not quite done yet! The next step is to separate the fragments based on their size through gel electrophoresis.
That sounds like a mouthful but it’s actually pretty simple. The DNA fragments are put into agarose gel (which is a lot like gelatin), and then an electric current is applied to the gel. The shorter fragments move to the positive pole more quickly than the longer fragments, and the pattern of the fragment separation is compared to the reference sample for a match. We can see the different fragments because they’ve been stained with fluorescent dye, and this visualization in the gel is called electrophoresis visualization.
Uses of DNA Fingerprinting
This matching makes DNA fingerprinting very useful for criminal and forensic analysis. If a criminal leaves behind a sample of DNA at the crime scene, then that sample can be compared to the DNA of various suspects to determine who was there when the crime was committed.You see this on TV dramas all the time now, but the first time this technique was used to solve a crime was only as recently as 1987.
And DNA fingerprinting is not only used to determine crimes that are committed now, but has also been used to identify criminals of past crimes and prove the innocence of many people who were put in jail for crimes they never actually committed.Because you get your genes from your parents, your DNA is very similar to theirs as well. This makes DNA fingerprinting useful for matching children with their parents.
This could be through paternity testing, which tests whether a man is the father of a certain child, but it could also be to identify children who are lost during natural disasters such as earthquakes, hurricanes, and tsunamis and reunite them with the correct family.DNA fingerprinting can also be used to identify individuals who have died but are unidentifiable – for example, identifying the victim of a crime or a natural disaster. In fact, DNA fingerprinting was used during 9/11 to identify many of the victims whose bodies were too damaged to identify otherwise.
Just like your fingerprints, your DNA is unique to you.
It’s because of this unique identifier that DNA fingerprinting can be used to identify individuals from biological samples. This may be a sample left behind at a crime scene, to determine paternity, or even to identify victims of crimes or natural disasters.To identify who the sample belongs to, DNA is extracted from the sample (which could be blood, hair, skin, bone, or other tissue) and amplified through PCR, or polymerase chain reaction.
Once the DNA is replicated, it’s cut at STRs, or short tandem repeats, by restriction endonucleases that separate the DNA into fragments unique to the individual. While we all have these nucleotide sequences found in STRs, the number of times the sequence repeats is unique to each individual and, therefore, gives each of us fragments of different lengths.These fragments (which have been dyed during PCR) are then put into agarose gel that has an electric current applied to it, a process called gel electrophoresis.
The electric current separates the unique fragments by size, and the pattern of how they separate can be compared to an individual to determine a match.