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Lesson Video: DNA Fingerprinting Biology

In this video, we will learn how to outline the basic principles of DNA fingerprinting and recall some applications of it.


Video Transcript

In this video, we will outline what DNA fingerprinting is and how it works. We will also discuss some uses of DNA fingerprinting, and we will interpret some simple DNA fingerprints together.

For many years, the only way to identify an unknown suspect was by finding and analyzing fingerprints left on a crime scene because fingerprints are unique to each individual. Then, scientists discovered that the DNA contained in our cells is unique to each of us as well. This gave police detectives a modern tool to identify suspects and victims. This technology is also widely used in paternity testing, helping individuals research their family tree, and in medical research. Before we review some of the applications of DNA fingerprinting technology, let’s first understand the principles behind that technique.

DNA fingerprinting, or DNA profiling, is creating a visual profile of someone’s DNA. This process was invented in the 1980s by Sir Alec Jeffreys. He famously used DNA fingerprinting to solve a high-profile murder case at that time. The principles of DNA fingerprinting have remained largely unchanged since then. However, the techniques used have progressed a lot. So, what is the basis for DNA profiling? Well, as you probably know, all of our body cells have a nucleus, and each nucleus possesses all of our genetic material, or DNA. Our DNA contains all of the information to make us. The DNA in each of our cells is typically the same as the DNA in all of our cells and remains the same in healthy cells for all of our life.

DNA is a long, coiled, and compacted molecule. When human DNA is uncompacted, it spans about two meters in length. This DNA is divided into segments called chromosomes. We inherit half of our DNA from our biological father and the other half from our biological mother. Only identical twins have exactly the same DNA. But the DNA between any two human beings from anywhere on the planet is still similar at 99.9 percent. So, there’s only one-tenth of a percent of our DNA that makes us different from each other. Let’s continue our investigation into what makes one person’s DNA different from another.

If we took one piece of our DNA and unpacked it and magnified it, we would see that this molecule is a double helix, which contains a sequence of nucleotide bases represented here by the colored rods inside the double helix. These bases are of four types, A, T, C, and G, organized as complementary pairs. Each of our body cells, except our gametes or sex cells, has six billion of these complementary base pairs. Some sequences of these nucleotide bases are called genes. They encode information that can be decrypted by the cell machinery to make functional units such as RNA molecules and/or proteins.

All of these proteins produced from our genes determine our traits and visible characteristics. But actually only one to two percent of our DNA is coding genes. The majority of our DNA is actually called noncoding DNA. It is called this because it doesn’t directly code for the formation of functional molecules such as proteins. Early geneticists even called this DNA junk DNA as they thought it was probably useless. But forget about that. In fact, this DNA has very important functions as well and cannot be considered useless. As you will see, it’s actually with this noncoding DNA that we can really profile each individual. So, let’s learn a bit more about this noncoding DNA.

Imagine that all the DNA contained in a cell is represented by this pie. As we just said, only a tiny portion of this pie is coding for RNA and/or proteins. Our genes make up about one to two percent of the total. All of the rest is noncoding, meaning it doesn’t produce RNA or proteins. Around 20 percent of the DNA still has unidentified functions. We do know about 25 percent is associated with genes, for example, to help regulate the way that genes are expressed. The remaining 53 percent of DNA is made up of repetitive sequences. These regions of repetitions found, for example, at the extremities or ends of chromosomes called telomeres can play a protective role as part of the DNA. In DNA fingerprinting, one type of this repetitive DNA called short tandem repeats or STR can be particularly useful because the number of repetitions of these short sequences can be highly variable and unique to each individual.

In this diagram, we’re looking at the same region of a chromosome in three different individuals. This region is known to contain two sites, site 1 and site 2, where the sequence of nucleotides AGAT is repeated a variable number of times. You can see that each individual has a unique combination of repeats at these two sites. Now, imagine that our genome has many other sites like this and each site has a highly variable number of repeats. This creates billions of different combinations. Thus, the DNA fingerprint is truly unique in each individual. How is this variation possible among individuals? Because these regions of DNA are so repetitive, the machinery that duplicates it at the time that gametes are produced is more prone to make errors and thereby either increase or reduce the number of repeats of these sequences.

These mutations, that is, the increase or decrease in the number of repeats, have essentially no consequences for the offspring as they affect noncoding regions, so they are easily accumulated during the production of gametes and then passed on from parents to children. As a result, these regions can vary a lot among individuals and create a very unique fingerprint for each of us. When scientists became aware of the uniqueness of these repetitive regions in each individual, they devised a way to visualize these differences. And this process of visualization revealed itself to be very useful in many applications, for example, in solving crimes.

Let’s pretend we’re investigating a crime scene. How can we use DNA fingerprinting? First, we would extract DNA from relevant biological tissue found on the crime scene. This could be obtained from cells in a blood spatter, a hair, or even tiny amounts of skin left behind when someone touched a surface. We can then amplify the DNA, which means cause it to replicate and make many copies so that even if the initial sample is very small, we’ll have enough to work with. We would also want to do the same with the samples of DNA from our suspects, perhaps from a strand of their hair or a sample of their skin cells. Then, we would add a restriction enzyme to each vial.

A restriction enzyme is an enzyme that breaks the DNA at the locations of certain sequences of nucleotides called restriction sites. Because the enzyme will target sites and areas of noncoding DNA, which will vary in the number of repeats between the restriction sites, cutting DNA samples from multiple individuals at the same restriction sites will result in DNA fragments of very different lengths. Now, we need to find a way to separate all the fragments of DNA from each of our samples. For that, we need an electrophoresis gel.

Gel electrophoresis is a technique used by scientists to separate the DNA fragments from each sample based on their size. In this process, DNA samples are first loaded into wells at one end of the gel. These samples will include first a DNA ladder or a standardized solution of DNA fragments that will make it easier to interpret the other samples by comparison. We will also need to include a sample from our crime scene to compare against our various suspects. And finally, we need to include samples from each of our suspects. In this case, we’ve included three suspects or suspects one, two, and three.

Then, an electric current is passed through the gel. Because DNA fragments are electrically charged, they will move through the gel toward the positively charged end. However, due to the differences in sizes of the DNA fragments, they will travel at different speeds inside the gel. Smaller fragments will move more easily through the gel fibers and therefore more quickly, while larger fragments will have more difficulty moving between the gel fibers and therefore will move more slowly through the gel. After some time has passed, and at least some of the DNA fragments have been able to move all the way to the end of the gel, we can stop the current.

Contained in the gel is a special type of dye that marks the DNA. A machine which is sensitive to the DNA marker then creates an image of the results, similar to what we see here. The pattern of stripes left by each DNA sample on the gel is what we call that individual’s DNA fingerprint. Each person has a unique DNA fingerprint since the number of repeats in their repetitive DNA segments are unique and therefore the sizes of the fragments from their DNA when it’s treated with the restriction enzyme are unique as well.

Now, let’s look at the fingerprints we see on this gel and compare them to the fingerprint of the blood we found at the crime scene. Does it look like one of these suspects is a match? Yes, suspect three matches. We can tell because this individual’s DNA fingerprint has the exact same pattern of stripes as the sample found at the crime scene. This suggests that this individual was likely present at the crime scene.

DNA fingerprinting today is commonly used in forensic science as a method of helping to determine a person’s presence and likelihood of involvement in a crime. Newer genetic technologies mean that we can get a DNA fingerprint from a very small sample of a person’s DNA. Another common use of DNA fingerprinting is in determining biological parentage. Since we inherit half of our genetic material from our biological mothers and half from our biological fathers, half of our DNA fingerprint is likely to match that of our mothers and the other half is likely to match that of our fathers.

So in order to determine paternity using DNA fingerprinting, geneticists acquire three samples of DNA, one from the mother, one from the child, and one from the potential father. They then follow the same steps we’ve discussed previously to create a DNA fingerprint for each person and to determine that this potential father is actually the likely father of this child. First, the portions of the child’s DNA fingerprint that are shared with the mother are identified and eliminated. Then, the remaining portions of the child’s DNA fingerprint are compared against that of the potential father to see if they could have been inherited from him. In this case, all of the remainder of the child’s DNA fingerprint that doesn’t match the mother matches portions of the DNA fingerprint of this potential father. So, it’s very likely that this man is the biological father of this child.

Advances are being made all the time in the field of DNA fingerprinting to make it even more reliable. A lot of care is taken when samples are collected to avoid contamination with outside DNA. This is especially important for samples being collected at a crime scene and is why the investigators that collect the DNA often appear to be dressed as astronauts with all the protective clothing they wear. Care is also taken to prevent the degradation of the DNA samples before they’re analyzed. And at the point that the DNA fingerprints are analyzed, the interpretation and comparison of the fingerprints is aided by powerful computers in order to increase its accuracy. Today, DNA fingerprinting has already been used to solve many crimes and to help many people find their parents or ancestors.

Let’s see how much we’ve learned about the applications of DNA fingerprinting by applying our knowledge to a practice question.

Fill in the blank. DNA fingerprinting can be used to identify closely related organisms, analyze samples found at crime scenes using forensic technology, and blank. (A) Create recombinant DNA, (B) determine the biological parents of a child, (C) treat genetic and hereditary diseases, or (D) generate synthetic sections of DNA.

Every person with the exception of identical twins has a unique genome. DNA fingerprinting is a technique used to make a visual representation of a person’s unique genetic makeup. By visualizing and viewing a person’s genetic makeup, we can find some valuable information. For instance, we can compare samples of DNA to determine if a person was present at a crime scene. Or we can compare samples of DNA from different species to investigate evolutionary relationships.

DNA fingerprinting can also be used to settle issues of paternity in the case of a disputed lineage of a child. This is because the child will have inherited half of their genetic material and therefore half their DNA fingerprint from their mother and half from their father. By comparing the DNA fingerprint of a child to that of the potential fathers, scientists can determine whether a male is likely related to a child.

However, DNA fingerprinting is not a way to create any type of genetic material or treat any genetic disease. And we do not use DNA fingerprinting to generate synthetic sections of DNA, but only to analyze a sample of DNA that we have taken. So the correct answer is (B). DNA fingerprinting can be used to identify closely related organisms, analyze samples found at crime scenes using forensic technology, and determine the biological parents of a child.

Let’s review the key points we have discussed. The majority of DNA in humans does not code for proteins. Instead, it is considered noncoding. Noncoding regions are highly variable between individuals. This contributes toward an individual’s unique DNA. All humans, with the exception of identical twins, have unique DNA. DNA fingerprinting produces a visual representation of a person’s unique patterns of noncoding DNA. It is used in forensics, ancestry analysis, and in medical research.

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