Video Transcript
A DNA fingerprint was constructed from samples of hair found at a crime scene. DNA fingerprints from samples taken from four suspects are also constructed. Based on the information in the diagram provided, state which suspect’s DNA was found at the crime scene.
This question is asking us about how DNA fingerprinting can be used to identify a suspect. So, what is DNA fingerprinting exactly? In the forensic context of this question, DNA fingerprinting is a lab technique used to determine a biological link between suspects in a criminal investigation. DNA can be extracted from the samples of hair found at the crime scene and then processed to produce the specific pattern shown here. This pattern represents the unique DNA banding of a person who was at the crime scene. When we compare it to the DNA banding patterns of each of these four suspects by looking for similarities between the sequences, we can determine whose hair it is and which suspect was at the crime scene.
To understand this a bit better, let’s look at some of the properties of DNA and how it can be manipulated to give these unique patterns that we see here. A group of enzymes, called restriction enzymes, are like molecular scissors that can cut DNA at specific sequences throughout the molecule. These cuts produce fragments of a specific size. For example, these two cuts produce a fragment that’s this size, while these two cuts produce a fragment that’s short. This pattern of cuts and the number of fragments that are produced is unique between individuals. In the DNA of another individual, the pattern of cuts will be different. In this individual, you could see more of these larger cuts than in this individual. These fragments can be sorted based on their size to give these different patterns.
Now, let’s clear some of the screen to see how this is possible using these two sets of DNA fragments. The key to generating these patterns is a laboratory technique called gel electrophoresis. This separates DNA fragments based on their size. Here, a special gel is prepared. This gel contains tiny wells to which we can add our fragment of DNA. Let’s put our first set of fragment of DNA into this well. Then, electricity is applied, so the end of the gel closest to the wells becomes negatively charged and the other end becomes positively charged. Since DNA has a negative charge, the fragments migrate through the gel towards the positive end. Small fragments of DNA migrate quickly and they end up on the bottom of the gel, whereas larger fragments migrate more slowly and they’re closer to the top.
Now, let’s see what happens when we load this second set of fragments into the gel. You’ll notice that we get a different set of bands compared to the first set of fragments. This is because the first set contains three different sizes of DNA fragments, whereas the second set only contains two. So, looking back at the question, we’re looking for the suspect whose DNA banding pattern matches the DNA taken from the crime scene. We only need to look at the darker bands because these contain the intact fragments we’re interested in, whereas the lighter bands might contain fragments that have degraded, so we’re not interested in those.
To do this, we simply look at each of these patterns from the suspects and compare it to the DNA taken from the crime scene to look for a match. We can see that this band is found in suspect 2’s DNA as well as suspect 3, while this band is found in suspect 1 and suspect 2. Comparing the rest of the bands, we can see that suspect 2 matches the best with the DNA from the crime scene. Although this doesn’t prove that suspect 2 committed the crime, it does prove that it was their hair at the crime scene, so suspect 2 might have some explaining to do. Therefore, based on the comparison of these DNA fingerprints, suspect 2’s DNA was found at the crime scene.
