THE BELOW OF THE SURVEY
Cathie Louvet's chronicle
CRIMINAL INVESTIGATIONS :
Folder n°5: dna
I have always wondered whether DNA is a truly reliable means of identifying a criminal. So I conducted a thorough investigation to answer that question.
1.The preferred DNA of experts:
The preferred DNA of the police is that which is nested in the heart of our cells distributed in 23 pairs of chromosomes and which is called nuclear DNA. In each pair, two almost identical chromosomes: one inherited from the father (via the spermatozoon), and the other from the mother (by its ovule).
The DNA is presented in a double helix formed of two long strands facing each other. If the double helix is detoured, it takes the form of a ladder whose bars result from the two-by-two association of chemical motifs: the nitrogenous bases A, T, G and C, the four letters of the genetic code. A can be found only in front of T, and G to C. This allows the cells (or the machines of the researchers) to make copies of the DNA: it is enough to separate the two strands then to follow the rule to reconstruct the complementary strand opposite.
On less than 10% of the total length of our DNA, the letters form "sentences" that the cell can understand. These are genes: recipes to make the most essential molecules of living organisms, proteins. Some allow vital chemical reactions to occur, others contribute to the transport of dioxygen, pigment production, etc. We all have the same genes in the same places, but sometimes in slightly different versions, which is why which we are not all identical.
In addition to this nuclear DNA, our cells harbor short mitochondrial DNA (mtDNA) in biological structures essential for their functioning called mitochondria. They come from our mother, because the mitochondria are delivered with the ovum, and then only multiply identically. All individuals of the same maternal lineage therefore share the same mtDNA.
2. The ideal analysis: from trace to identification.
• A cell phone was discovered on a crime scene. The police officer in charge of the investigation signs a request for analysis of the traces by the National Institute of Scientific Police.
• The Institute verifies that the application is in order and assigns a bar code to the applicant. Then, a technician rubs large swabs soaked with sterile liquid on the keyboard, the battery, the sim card, etc., and then places them in tubes marked with the corresponding bar code.
• The rest of the operation consists in establishing the unique genetic profile of the one who would have left his DNA on the object. In the first machines, DNA is released from its cells and its useful parts are photocopied in billions of copies so that another machine, the analyzer, has enough "material" to read those pieces of DNA that define the genetic profile.
• If several people have left their DNA on the phone, their profiles intermingle. It is up to the expert to decide whether it is possible to identify the DNA portions from a main donor. If so, he will send his profile to the National Automated DNA File (FNAEG). If the result is positive, the software detects it and gives the identity of the owner.
3. How is a genetic profile:
• For this type of analysis, the segments of interest, copied in large numbers, are found in the non-coding DNA, that which does not carry a gene. These markers are STRs: sequences of four letters, the same in all the world, but repeated one behind the other a number of times variable from one individual to another. And, in general, on the two chromosomes of a pair.
• On the pair of chromosomes 11, there is the STR THO1, whose sequence is "ATCT". It can be repeated, for example, 5 times on the chromosome of maternal origin and only three times on that of paternal origin. The analyzer recognizes all THO1 fragments that have been copied, and by their mass determines how many repetitions they make. At the same time, it carries out this search for 14 or even 16 other markers present on the other pairs of chromosomes.
• The result of the analysis is a series of peaks, two per markers, unless the number of repetitions is the same. Under the peaks, the software indicates the number of repetitions. If there are more than two peaks, the sample contains the DNA of several people.
4. How is a genetic portrait-robot made?
• This time, we are interested in the genes that intervene in the color of hair and eyes. We do not copy whole genes with their hundreds of thousands of letters, but just the short segments where we know that one of the letters varies from one person to another. Because it is these different letters (called SNPs) that make everyone not the same color of eyes or hair.
• Let us take the HERC2 gene involved in eye color on chromosome 15. On position "Rs1291383" we find either the nitrogen base C or the base T. A T on each of the two chromosomes 15 predicts with a high probability of blue eyes. On the other hand, two C or a C / T couple tilt the balance towards the brown. In an identical analysis, the machine identifies which letters are on this SNP, as well as on 5 others that strongly contribute to the color of the eyes and on 18 for the color of the hair.
• From the analysis of these 24 positions, the software defines which colors can display the hair C and the eyes D of the owner of the DNA. Another analysis carried out on 46 other SNPs makes it possible to specify its biogeographical origin. A third, out of 23 other markers, foresees the color of his skin, his predisposition to baldness and freckles. => Unlike the genetic profile used to formally identify a person and can be used as scientific evidence during a trial, the genetic profiling is only a tool to guide an investigation, indicates probabilities.
5. Where does DNA come from?
• Fluids: the most rich in DNA is sperm, which is not surprising since its vocation is precisely to convey DNA via the spermatozoa. But also the blood of which all the cells, with the exception of the red cells that have no nucleus, contain nuclear DNA. They are also found in saliva, tears, urine, sweat, and vaginal secretions, as these liquids contain, in greater or lesser quantity, dead cells which have detached themselves from the surface of the organs or the mucous membranes.
• The hair: the best is the hair plucked with its bulb, the part fastened in the skin, because it is in the bulb that are the chromosomes, and thus the nuclear DNA. Failing this, experts can use rootless hair because the stem contains the mtDNA, less precise because its brother, mother or even her maternal grandmother have the same. Some mtDNAs are even shared by 2% of the French population. And no national mtDNA file exists in France.
• The skin: Left under the nails of a victim who has struggled, the cells of the epidermis can betray the criminal since they contain its DNA. The same applies to dead skin, such as dandruff. In fact, the skin is a real plague for the delinquent who wants to remain anonymous because a simple contact with an object is enough to deposit DNA there. This is called a trace by transfer.