The forensic technique of extracting and analyzing DNA from microscopic evidence left at a crime scene is a powerful tool that can identify suspects from even the smallest amount of cells. PCR technology amplifies evidence DNA to identify the culprit and plays an important role in solving crimes. Advances in science and technology are making investigations faster and more accurate, reducing the likelihood of a complete crime.
No matter how big or small the case, where there are people, there will always be crimes. Perhaps people and events are inseparable. Whether it’s a small conflict or a major crime, the interplay of people’s desires and conflicts can lead to a variety of events. Crimes are interesting in and of themselves, but it’s even more interesting to see how different investigative techniques are utilized to solve them. Especially in the modern world, forensic science isn’t just a tool for solving crimes; it’s also a key way to uncover relationships and dynamics between people. In the past, the evidence collected was often not utilized properly, but nowadays, various forensic investigation techniques are used to properly utilize evidence. In Korea, the National Institute of Forensic Science and Investigation is conducting investigations using science in various fields.
Among them, genetic forensic technology developed in the late 1980s has revolutionized the investigation of violent crimes, especially sexual crimes. Microscopic evidence, such as a single hair, trace amounts of saliva, etc. that were previously unrecognizable, have become powerful evidence through the power of science. Previously ineffective evidence such as semen and saliva have gradually become key to identifying criminals. This has made it much easier to identify suspects and prove their connection to the crime. Homicide cases were solved much faster than before, and the credibility of investigations was greatly enhanced as science and technology became accepted as legal evidence.
In a 1986 rape and strangulation case in Leicestershire, England, police sought the help of Professor Alec Jeffreys of the University of Leicester, who had published DNA fingerprinting, to prove that the perpetrator was the same as three years earlier. The case was the first use of DNA in a criminal investigation, and has since evolved into a forensic science that merges life sciences and investigative techniques. DNA is the word most often mentioned in forensic stories, such as “the killer’s DNA was found under the victim’s fingernails” or “saliva was found that is believed to be the killer’s”. But how important is DNA analysis in solving real-life cases? Why is DNA such an accurate and reliable source of evidence, where and how is it obtained, and why is it so often mentioned? In this article, we’ll take a closer look at the life sciences used in investigations, specifically DNA.
Wherever a person has been, there are cells left behind that have been dropped unintentionally, and they contain human DNA. DNA can also be obtained from bodily fluids, including tears, snot, and saliva, and from small amounts of cells found on touched surfaces and clothing. Modern genetic forensic technologies require only about 20 cells, or 100 pg (1 pg = 1 trillionth of a gram) of DNA, to analyze genetic information. The ability to collect such small amounts of cells makes DNA analysis technology very practical and an important asset in enabling fast and efficient investigations. The common use of “amplified DNA analysis” in forensic investigations raises the question of how a person can be identified from such a small amount.
How can genetic forensics be done in such a small amount? A life science technique called polymerase chain reaction (PCR) amplifies a small amount of DNA into a large amount. Once a specific region of the DNA molecule is determined to be worth investigating, primers are designed for the PCR. In short, PCR is a technique that “copies” a small amount of DNA and amplifies it to the desired amount, making DNA evidence practical.
The sample is incubated at 94-97 degrees to separate the DNA helix into two independent lines (denaturation). Lower the temperature to 50-60 degrees to allow the primers to bind to the DNA (cooling). Raise the temperature back up to 70-72 degrees to allow the Tag DNA polymerase to start polymerizing with the primers and produce complementary copies of the template using the G, A, C, and T bases (polymerization). Each time the PCR process is repeated once, the amount of DNA doubles. After just 20 iterations, the number of DNA at the target site will increase by 20 to the power of 2. This technology has made it easier to obtain DNA samples in many cases and verify their relevance to suspects. While PCR does not have the ability to copy long DNA, it can accurately replicate short DNA of a few thousand bases, which allows genetic forensics to copy large amounts of DNA from trace amounts of cells.
Furthermore, DNA forensic techniques have become increasingly advanced over time, overcoming their limitations. Compared to early DNA analysis, it is now possible to obtain accurate results from much smaller samples, and the speed of analysis has increased significantly. So why is DNA used in investigations? What is it about DNA that makes it so unique that it can be used in forensics? The short answer is that the genetic information in DNA is unique to each person. This means that DNA can be used to point to a specific person as a suspect. For example, a single hair with its root or a single skin cell on a fingernail can be analyzed to identify a suspect.
What makes DNA analysis particularly powerful is its high degree of specificity. Each person’s DNA sequence is unique, consisting of billions of sequences without repeats. Human cells have 23 pairs of chromosomes, which vary in shape and size. Twenty-two of these pairs are autosomes, which carry information that determines the development of the body, and the remaining pair are X and Y sex chromosomes, which regulate the development of the reproductive system. Each chromosome is made up of DNA in a double helix structure, and each strand of DNA is divided into smaller units called genes. A gene is made up of about 3000 nucleotides, which are divided into three parts: phosphate, sugar, and base. Of these, bases are the most directly related to genetic information. The four bases G (guanine), A (adenine), T (thymine), and C (cytosine) are arranged in specific ways to encode proteins that determine hair and eye color, enzymes, and genetic traits. This specific arrangement of bases can determine an individual’s characteristics, making DNA a powerful source of evidence in investigations.
When a crime occurs, evidence from the scene or samples from suspects or people involved are collected and analyzed for their DNA profiles. If there is a suspect, the DNA profile of the suspect is compared directly to the evidence, and if there is no suspect, the DNA profile of the evidence is compared to a database. When comparing the DNA in the evidence to the suspect’s DNA, the markers on the DNA profile are compared, and the probability of a genotype match at each marker is quite low, less than or equal to 0.2. Therefore, the probability of all 13 markers (13 markers in the United States) matching is only 2.380*10^-16, which means that if all the markers match, the suspect is very likely to be the culprit.
In 2006, an infant was found frozen to death in a freezer in the South Korean village of Seorae. The French couple, who denied any wrongdoing, were silenced by the National Institute of Forensic Science, which confirmed their kinship based on DNA. In many cases, forensic investigations have provided crucial evidence that has led to the capture of the real culprit. With technology playing a powerful role in solving cases, it’s becoming increasingly difficult to pull off the perfect crime. The perfect crime is becoming increasingly difficult. Forensic investigations are bringing justice to innocent victims and giving families more chances to face the truth. We hope that science and technology will continue to improve so that no matter what the case, the bereaved families will be able to find some solace.
