In this blog post, we take a closer look at how the smallest of traces left at a crime scene can become decisive evidence through DNA analysis and how it plays a role in solving crimes.
No matter how big or small the incident, it always happens where there are people. Perhaps people and incidents are inseparable. Various incidents, whether small collisions or major crimes, occur at the scene where people’s desires and conflicts are intertwined. The incidents themselves are interesting, but the various investigative techniques used in solving them are even more interesting. In particular, in modern society, scientific investigation techniques are not just a tool for solving cases, but play a major role in revealing the relationships and dynamics between people. In the past, the collected evidence was often not properly used, but in modern times, evidence is being properly used through various scientific investigation techniques. In the case of Korea, the National Forensic Service is conducting investigations using science in various fields.
Among them, the genetic identification technology developed in the late 1980s brought about an innovative change in the investigation of violent crimes, especially sex crimes. Micro-evidence that was previously unknown, such as a single hair or trace of saliva, has been turned into powerful evidence through the power of science. Evidence such as semen and saliva, which were previously ineffective, have gradually become key in revealing the culprit. This has made it much easier to identify suspects and prove their involvement in crimes. The speed of solving violent crimes has increased significantly, and the credibility of investigations has also increased significantly as science and technology have become legal evidence.
In the 1986 rape and strangling case in Leicestershire, England, police sought the help of Professor Alec Jeffries of the University of Leicester, who had published a DNA fingerprinting method to prove that the crime was committed by the same perpetrator as a crime three years earlier. This case was the first time DNA was used in a criminal investigation, and since then, forensic science has gradually developed into a fusion of life sciences and investigation. DNA is the most commonly used word in forensic science cases such as “DNA of the criminal was found under the victim’s fingernails” or “saliva presumed to be from the criminal was found.” So, how important is DNA analysis in solving real-life cases? Why is DNA considered such an accurate and reliable piece of evidence? Where and how can this DNA be obtained, and why is it so often mentioned? This article will take a closer look at the life sciences used in investigations, especially DNA.
Wherever there are people, there are unintentionally dropped cells. And in those cells is human DNA. DNA can be obtained from body fluids, including tears, snot, and saliva, or from small amounts of cells found on surfaces that have been touched or on clothing that has been worn. Modern genetic identification technology can analyze genetic information from as little as 20 cells, or 100 pg (1 pg = 1 trillionth of a gram) of DNA. DNA analysis technology is very practical and an important asset that enables fast and efficient investigations because it can be collected from even a small amount of cells. The “amplified DNA analysis” commonly seen in scientific investigations raises the question of how it is possible to identify a person from such a small amount.
How can so little genetic material be used for DNA testing? A life science technology called PCR (Polymerase Chain Reaction) amplifies small amounts of DNA into large quantities. If a specific region of a DNA molecule is deemed worthy of investigation, primers are designed for PCR. PCR is the process of repeating denaturation, cooling, and polymerization. In simple terms, PCR is a technology that “copies” a small amount of DNA and amplifies it as much as desired, making it possible to use DNA evidence practically.
The sample is incubated at 94-97 degrees to separate the DNA helix into two independent strands (denaturation). The temperature is lowered to 50-60 degrees to allow the primer to bind to the DNA (cooling). The temperature is raised again to 70-72 degrees, allowing the primer to initiate polymerization by Tag DNA polymerase, which produces a complementary copy of the template using G, A, C, and T bases (polymerization). The amount of DNA doubles with each repetition of the PCR process. Even if it is repeated 20 times, the number of DNA molecules in the target area will increase by the square of 20. This technology has made it possible to easily obtain DNA samples in numerous cases and verify their relevance to the suspect. PCR cannot copy long DNA, but it can accurately replicate short DNA of a few thousand bases, so it can produce a large amount of DNA from even a small amount of cells in genetic identification.
In addition, DNA identification techniques have been developed over time and are overcoming existing limitations. Compared to early DNA analysis, accurate results can be obtained with much less sample, and the analysis speed has also been significantly improved. So why is DNA used in investigations? What is so special about DNA that it is used in scientific investigations? To answer this question, let’s start with the conclusion: the genetic information in DNA is unique to each person. Therefore, DNA can be used to identify a specific person as a suspect. For example, even a single hair containing a hair root or a single speck of skin cells on a fingernail can be used to identify a suspect through DNA analysis.
DNA analysis is particularly powerful because of its high specificity. Each individual’s DNA sequence is unique, and it consists of billions of base sequences without repetition. Human cells contain 23 pairs of chromosomes of varying shapes and sizes. Of these, 22 pairs are autosomes that contain information that determines the development of the body, and the remaining pair is the X and Y sex chromosomes that control the development of the reproductive system. Each chromosome consists of DNA with a double helix structure, and each DNA strand is divided into small units called genes. A gene consists of about 3,000 nucleotides, and a nucleotide is divided into three parts: phosphate, sugar, and base. Among these, the bases are most directly related to genetic information. Four bases, G (guanine), A (adenine), T (thymine), and C (cytosine), are arranged in a specific order to encode proteins that determine hair and eye color, enzymes, and genetic traits. As such, the arrangement of specific regions alone can determine an individual’s characteristics, making DNA a powerful piece of evidence in investigations.
When an incident occurs, samples of the evidence, suspects, or related parties at the scene are obtained and their DNA profiles are analyzed. If there is a suspect, the DNA profile of the evidence and the suspect are directly compared, and if there is no suspect, the DNA profile of the evidence is compared to the database. When comparing the DNA in the evidence with the DNA of the suspect, the probability of the genotype matching in each of the markers is quite low, at less than 0.2. Therefore, the probability of all 13 markers (13 markers are compared in the US) matching is only 2.380*10^-16. In other words, if all the cover pages match, the suspect is very likely to be the culprit.
In 2006, an infant was found dead in a freezer in Seorae Village, South Korea. It was the DNA-based analysis by the National Forensic Service that shut up the French couple who denied their crimes. As such, scientific investigation is providing decisive evidence in numerous cases and contributing to catching the real culprits. As technology continues to advance, it is playing a more and more powerful role in solving cases, and perfect crimes are becoming increasingly difficult. The world is gradually becoming a place where perfect crimes are becoming more difficult to pull off. Scientific investigation is bringing justice to innocent victims and giving the bereaved families more opportunities to face the truth. I hope that technology will continue to advance so that no case will ever be left in a mystery, and that the bereaved families will be able to find some comfort.
