This blog post takes a detailed look at how minute traces left at crime scenes become crucial evidence through DNA analysis and what role they play in solving crimes.
Regardless of scale, incidents occur wherever people are. Perhaps people and incidents are inseparable. In places where human desires and conflicts intertwine, various incidents occur, whether minor clashes or major crimes. While the incidents themselves are intriguing, the process of solving them, utilizing diverse investigative techniques, is even more fascinating. Especially in modern society, forensic science techniques go beyond being mere tools for solving cases; they play a key role in revealing the relationships and dynamics between people. In the past, collected evidence was often underutilized, but today, various scientific investigation techniques ensure evidence is properly leveraged. In South Korea, the National Forensic Service conducts investigations using science across multiple fields.
Among these, DNA profiling technology, developed in the late 1980s, brought revolutionary changes to violent crime investigations, particularly sex crimes. Evidence once too minute to detect—a single strand of hair, trace amounts of saliva—has returned as powerful evidence through the power of science. Evidence like semen and saliva, previously deemed ineffective, has gradually become crucial in identifying perpetrators. This has also made it much easier to pinpoint suspects and prove their connection to the crime. The speed of solving violent crimes has increased significantly compared to before, and as scientific technology has become established as legal evidence, the reliability of investigations has also greatly improved.
In a 1986 rape and strangulation case in Leicestershire, England, police sought the help of Professor Alec Jeffreys from the University of Leicester, who had announced DNA fingerprinting, to prove the crime was committed by the same perpetrator as a case three years prior. This case marked the first use of DNA in criminal investigations, and since then, forensic science—the fusion of life sciences and investigation—has steadily advanced. In forensic cases, the most frequently mentioned term is DNA, as in “DNA from the perpetrator was found under the victim’s fingernails” or “Saliva believed to belong to the perpetrator was discovered.” So, how crucial is this DNA analysis in the actual process of solving cases? Why is DNA accepted as such accurate and reliable evidence? Where and how can this DNA be obtained, and why is it so frequently mentioned? This article will delve into the life sciences used in investigations, particularly DNA.
Wherever people have been, cells are unintentionally shed and left behind. And within those cells lies human DNA. DNA can be extracted even from minute quantities of cells found in bodily fluids like tears, nasal secretions, or saliva, or from surfaces touched by hands, or clothing worn by individuals. With modern genetic identification technology, genetic information can be analyzed from as few as about 20 cells, or just 100 pg (1 pg = one trillionth of a gram) of DNA. This ability to extract DNA from such minimal cellular quantities makes DNA analysis technology highly practical, serving as a crucial asset enabling swift and efficient investigations. The ‘amplified DNA analysis’ commonly seen in forensic science raises the question: how can such a small amount identify a specific person?
How is genetic identification possible with such a small amount? A life science technology called PCR (Polymerase Chain Reaction) amplifies small amounts of DNA into large quantities. When a specific region within a DNA molecule is deemed worthy of investigation, primers for use in PCR are designed. PCR involves repeatedly cycling through denaturation, annealing, and polymerization stages. Simply put, PCR is a technique that ‘copies’ small amounts of DNA, amplifying it to the desired quantity, making DNA evidence practically usable.
The sample is incubated at 94-97°C to separate the DNA double helix into two independent strands (denaturation). The temperature is lowered to 50-60°C to allow primers to bind to the DNA (annealing). The temperature is then raised again to 70-72°C, where Tag DNA polymerase initiates polymerization using the primers, producing a complementary copy of the template using G, A, C, and T nucleotides (polymerization). Each repetition of the PCR cycle doubles the amount of DNA. After just 20 cycles, the number of DNA molecules at the target site increases by 2 to the 20th power. This technology has made it possible to easily obtain DNA samples in numerous cases and verify their relevance to suspects. While PCR cannot copy long DNA strands, it can accurately replicate short DNA fragments of a few thousand bases. This capability allows for the large-scale replication of DNA from minute cellular samples in genetic profiling.
Furthermore, DNA profiling techniques have progressively advanced over time, overcoming previous limitations. Compared to early DNA analysis, accurate results can now be obtained from much smaller samples, and analysis speeds have significantly increased. So, why is DNA used in investigations? What unique characteristics does DNA possess that make it useful for forensic science? To put it simply, the genetic information within DNA is unique to each individual. Therefore, DNA can be used to identify a specific person as a suspect. For example, even a single hair containing a root or a single skin cell found under a fingernail can be analyzed to identify a suspect.
The primary reason DNA analysis is particularly powerful is its high specificity. Each individual’s DNA sequence is unique, composed of billions of base pairs arranged without repetition. Human cells contain 23 pairs of chromosomes, varying in shape and size. Of these, 22 pairs are autosomes carrying information determining physical development, while the remaining pair consists of the X and Y sex chromosomes, which regulate the development of reproductive organs. Each chromosome consists of DNA in a double-stranded structure, and each DNA strand is divided into small units called genes. A gene is composed of approximately 3,000 nucleotides, which are themselves divided into three parts: phosphate, sugar, and base. Among these, the base is the part most directly related to genetic information. The four bases—G (guanine), A (adenine), T (thymine), and C (cytosine)—are arranged in specific sequences to encode proteins that determine traits like hair and eye color, enzymes, and genetic characteristics. Because the arrangement in a specific region alone can determine an individual’s characteristics, DNA is used as powerful evidence in investigations.
When a crime occurs, evidence from the scene or samples from suspects or related individuals are obtained, and their respective DNA profiles are analyzed. If a suspect exists, the DNA profile from the evidence is directly compared with the suspect’s profile. If no suspect is identified, the DNA profile from the evidence is compared against a database. When comparing DNA from evidence to a suspect’s DNA, the markers in the DNA profiles are compared. The probability of a single marker matching is quite low, 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 markers match, the likelihood that the suspect is the perpetrator is extremely high.
In 2006, there was a case in Seorae Village, South Korea, where an infant was found frozen to death inside a freezer. What silenced the French couple who denied their crime was the National Forensic Service’s analysis, which established a blood relationship based on DNA. In this way, forensic science provides crucial evidence in countless cases, contributing to the capture of the real culprits. Technology, advancing day by day, plays a powerful role in solving cases, making the perfect crime increasingly difficult. We are moving toward a world where committing the perfect crime grows ever harder. Scientific investigation also brings justice for innocent victims and provides bereaved families with more opportunities to face the truth. I hope that scientific technology continues to advance so that no case remains unsolved, bringing even a little comfort to the hearts of bereaved families.
