Welcome to this session where we will see and practice the techniques used to obtain the DNA profiles of the cases studied in the course. The aim of this video is to show you how to obtain a DNA profile from a trace. You will see that it is a bit longer than in TV series. Indeed, there are six different steps that we need to follow. First, we need to detect and collect the trace. Then we extract the DNA and purify it. We quantify the DNA we have and amplify it. That is, we multiply it. We then separate the DNA molecules depending on their size and detect them. Finally, we obtain our DNA profile. Here, we can see a DNA profile, what we called, using our specialist terms, an EPG. We also see the associated table summarizing the results. This DNA profile is like an alphanumerical code that we can then compare with the database or with a person of interest. On this figure, on the EPG, we can see the letters X, Y. We can also see some numbers. For example, 12, 15, 14, 16, 9, 10, 20, 23. We also see them in the summarizing table. At the end of this presentation, you should be able to understand the table next to the DNA profile. But do not worry about it right now. We are going to see the different steps that enables us to obtain this profile. But before we go to the laboratory, let's discuss a little about DNA so that you can understand why there are these different steps. Most of you will be familiar with the aspect of DNA. As we can see here, it is a double helix. DNA stands for deoxyribonucleic acid. Like text is made of letters, DNA is made of building blocks that we called nucleotides. English has 26 letters. DNA has only four nucleotides. We call them ATGC. Part of the name of DNA comes from the fact that it can be found in a specific part of our cells, as illustrated in the image. It comes from the nucleus of the cell. Most of you will know also that we inherit half of our DNA from our mother and half from our father, so that if we analyze the whole molecule of DNA, it would be in theory unique. But what you might not know is that the molecule of DNA is very large, so large that it's packaged in small bundles that we call the chromosomes. As we see in the image, we have our cell. Inside it, we have the nucleus where we find the chromosomes, these little bundles, made up of DNA. A child will generally have 23 chromosomes from the mother and 23 from the father. We can analyze the DNA on any location of these chromosomes. But, as we will see together, we only analyze some specific parts. Indeed, as the DNA molecule is very big, forensic laboratories do not analyze all of the DNA. Well, not yet. We target specific locations on the chromosomes. We call these locations loci, which means location in Latin. These locations are selected based on the variability that exists between people. So if we target sufficient loci, sufficient localization, then we will be able to differentiate most people. If we choose locations that are on different bundles, so on different chromosomes, then it is generally considered that these loci, these locations, are independent. This offers, therefore, better discrimination. Now that we have seen a little what DNA is, let us see together the essential steps that allow us to obtain a DNA profile so that we can compare traces found on the crime scene with the profile of known individuals. All begins with a trace that has been potentially left by the offender. Before collecting this trace, we need to detect it. So if it is not clearly visible, we will perform preliminary examinations that allows us to detect the trace. There are tests for blood, for saliva, and semen. Once detected, we can collect the trace. Let us go and see how we do it in the lab. So welcome to our laboratory. So today, we'd like to show you, well, first, this is Lydie. Lydie is working on a PhD thesis on the topic of DNA transfer. She will thus show us the different steps that we do in order to collect a DNA trace and also analyze this DNA trace so that we can obtain the profile as we have seen in the cases that we studied together. So here on this bench, we have a knife that supposedly has been used in a crime. Lydie will search for the DNA of the offender on this knife and show you the techniques that we use. We also have a different type of item, which is a clothing. Because this is a porous surface compared to the knife, which is nonporous, we use different types of collection. So Lydie, do you want to show us how we proceed? Here on the knife, we use the double swab method. First, we take a cotton swab and moisture it with a little sterile water. This enables a better collection of the cells present on the knife handle. This swab is then packaged to avoid any pollution. It is also labeled to ensure the chain of custody. On substrates that are porous, such as clothing, one can either cut a piece of the material or use so many tapes as shown here. Again, once collected, the material will be secured and labeled. So now that we have collected the trace, we need to extract the DNA that it contains. This is done in the extraction lab. Let's go and see together. So now we are really in the heart of the laboratory. Well, you might wonder why I am just like this. Well, of course, because I'm not doing the analysis, I'm not wearing the face mask and other equipment that you will see Lydie is wearing. But, however, it is very important to be aware not to contaminate or sample with foreign DNA. So we want only the DNA from the trace obviously and not any contaminations. So in this lab, we are going to extract the DNA that is present on our swab, and we are going to purify it. This is what we're going to do with Lydie. Hi Lydie. So we received the sample, and I would need you to extract the DNA and purify it so that we have a sample that is as pristine as possible. To extract the DNA, we use a detergent containing liquid. This detergent breaks the cells. Once the cells are broken, the DNA is free and floats in the liquid. We have extracted the DNA that is packaged inside the nucleus of the cell, as you can see here. Now, we are quite happy as we have extracted our DNA. But with it in our liquid, there are also plenty of other molecules like proteins. This is problematic as it might interfere with the analysis. We therefore need to get rid of all these other molecules. The purification is done using different methods and chemicals so that we finally obtain what is called the extract. That is, a liquid containing the DNA of the trace, almost pure in water. The next step is to quantify the DNA in the extract. Indeed, DNA analysis has some similarity with baking. The ingredients have to be in adequate proportions for the success of the process. If you want to have a delicious cake, before you add your flour, butter, sugar, and chocolate, whereas twist, in your bowl, you'd better know how many eggs you have at your disposal. It is the same for DNA analysis. We need to know how much DNA there is in the extract so that we add the good quantity of ingredients in the subsequent steps of DNA analysis. We are now in the laboratory where we perform quantification. Again, this step allows us to now how much DNA is contained in the trace. Indeed, depending on the number of cells, we might have a lot or very little DNA. In a fresh trace, in blood or semen, we will have a lot of cells. In another stain or in a trace where the object was just touched, then, we will have much less DNA. As you will have seen, DNA analysis takes longer than in the series such as CSI. We will see, in the following video, the other steps necessary to obtain a DNA profile.
