 our fingerprints are unique to each of us. No two individuals ever have the same fingerprint. And the same goes true even for DNA sequences. The sequence of the nitrogen bases in our DNA, ATG and C, that sequence is unique to all human beings except perhaps identical twins. Just as we use fingerprints to identify, let's say culprits from a crime scene, we can use DNA sequences to do the same thing. And this is called DNA fingerprinting. DNA fingerprinting can be used not only to identify culprits amongst suspects, it can also be used to do things like paternity testing. So this video will be about the technique used to obtain DNA fingerprints of a person. But for that, I would recommend you go through these topics first. DNA polymorphism, satellite DNA, gel electrophoresis, and southern blot. You can go back to our course and take a look at the videos that we have on these topics. In our chromosomes, we have six times 10 to the nine base pairs, roughly. So if we were to analyze the whole sequence of these many base pairs, that's a huge task, right? When we are analyzing the DNA of a suspect, we want quick results. So what we do is instead of going through the sequence of the entire 46 chromosomes, we look at certain parts of the chromosomes. What we look at is satellite DNA. These DNA have repeat sequences, something like, let's say, ATC, ATC, this will keep repeating again and again. And there are many such sequences. Sometimes they are as small as this. Each repeating unit has only three nitrogen bases, whereas sometimes each repeating unit may even have 100 bases. These types of DNA sequences which have repeating units are called satellite DNA, and they have a high degree of polymorphism. And by this I mean that there is a huge amount of variation between different individuals in these DNA sequences. And how do these variations arise? Simply the number of units that is repeated varies from individual to individual. Let's say I may have only 10 of these repeats, whereas you may have 100 of these repeats. So it's the same unit that's repeating over and over again, but you have a higher number of repeats than I do. So that means your DNA containing these repeats will be longer, much longer than my DNA, my satellite DNA of this sequence, right? So this is what we are exploiting in this technique. We are differentiating the sizes containing the same DNA repeats. And how do we differentiate DNA of different sizes by doing gel electrophoresis? And once we have separated the DNA based on sizes, we do another technique called a southern blot. Once a southern blot is finished, we get a particular pattern for a particular individual, and that pattern is called the DNA fingerprint. This was just a gist of the entire procedure. Now I'm gonna take you through the entire procedure step by step. First, we obtain a sample of the person in question whose DNA fingerprint we want to try to find out. The sample can be blood or hair or saliva or some other cells from the body. And then we extract DNA from these cells. Next, what we do is we treat the DNA with enzymes called restriction enzymes. Now there are many types of restriction enzymes. Each of these, what they do is they recognize a specific sequence in the DNA and cut over there. So they make a break in the DNA strands at that particular sequence which they recognize. So we take different restriction enzymes and cut the DNA at various places. And hence the DNA is now cut at various places. We have much smaller fragments of the DNA than we originally started out with. And now we run all these fragments on a gel by doing gel electrophoresis. And what we see is we see all these different bands that run through the gel when we pass electric current through it. So there are all these bands. In fact, there are many more bands because you're taking the entire chromosomal DNA, right? So that's a lot of DNA and there will be a lot of bands. And there are way too many bands for us to focus on anything really. So we don't know what we're looking for at this point. So that's why we go through the elaborate procedure of Southern blotting next. Before that, what we have to do is we have to separate the DNA strands. You know that DNA is double stranded, right? So we take the DNA through this procedure called denaturation. It separates us two strands in the DNA. Once that happens, then we transfer the DNA, the bands that we obtain in the gel electrophoresis to a nylon membrane or a nylon sheet. And then we take the sheet and then we add a solution containing probes of satellite DNA. Now what are these probes? These probes are short DNA molecules which are single stranded and they have sequences of the satellite DNAs which we are interested in. So since right now the DNA bands that we have on this nylon membrane are all single stranded because we took care of that, right, before. So all of these DNA bands, they are single stranded and out of these, whichever belong to satellite DNAs that we are interested in, those will bind to these probes because these probes have complementary sequences to the sequences of the bands that we are interested in. Okay, so the probes go and bind there. What next? How can we see them? Well, the probes that we have used in this stage are all radioactive. So then once the probes have bound, we will wash off the solution of the probes. So only the probes that have stuck to the bands will remain and since they are radioactive, what we'll do is we'll hold them against an X-ray film. So only those bands which have become radioactive now because they have the radioactive probes attached to them, they will be shown in the X-ray film. They will produce an image in the X-ray film. And this is our DNA fingerprint because these are the images of the bands that we were looking for, which correspond to the satellite DNA that we were interested in. So this is our DNA fingerprint. Okay, so we have obtained our DNA fingerprint. What do we do with it? Suppose this is our crime scene DNA. This is the DNA fingerprint of whatever blood or hair, whatever we obtained from the crime scene and this is the DNA fingerprint that we obtained from there. Now we want to identify the criminal with this DNA fingerprint and we have three suspects. These are the DNA fingerprints of those three suspects. Now I want you to pause the video for a moment and tell me out of the suspects numbers one, two, and three, which one was present in the crime scene. Clearly suspect number two, right? So take a look at all the bands. All of the bands have to match. So this band, this band, this band, this and this, all of them match with these. Whereas with number one and number three, only some of the bands match, the others don't match with the crime scene fingerprint. So we have identified our culprit, suspect number two. So this is how DNA fingerprinting is used. In fact, the technique that I told you about is a little bit antiquated. It was used before. Nowadays they have more advanced techniques. For example, instead of using restriction enzymes, they use a technique called PCR polymerase chain reaction which you will learn about in another unit. And instead of using radioactive probes, these days they can also use fluorescent probes. And there are some other modifications which have been added to the present day technique of DNA fingerprinting.