Advances in Forensic Science: How has DNA evidence impacted criminal investigations?

Modern forensic techniques have revolutionized criminal investigations by utilizing DNA evidence, especially PCR technology, which allows for accurate genetic detection from small amounts of DNA, playing a key role in identifying criminals.

 

No matter how big or small the case, where there are people, there are always crimes. 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 using science in many areas. Among them, genetic forensic technology developed in the late 1980s has revolutionized the investigation of violent crimes, especially sexual crimes. Evidence such as semen and saliva, which were previously ineffective, have gradually played a major role in identifying criminals.
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 where and how is this DNA obtained, and why is it mentioned so often? In this article, we’ll take a closer look at the life sciences used in forensics, specifically DNA.
Wherever a person has been, there are cells left behind that have been dropped unintentionally, and inside them is human DNA. DNA can also be obtained from bodily fluids, including tears, snot, and saliva, as well as from small amounts of cells found on touched surfaces and clothing. With modern genetic detection technologies, only about 20 cells, or 100 pg (1 pg = 1 trillionth of a gram) of DNA, are needed to analyze genetic information.
How can genetic detection be done with 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. The process of denaturation, cooling, and polymerization is then repeated for PCR.
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 only 20 iterations, the number of DNA at the target site will increase by 20 to the power of 2. While PCR does not have the ability to copy long DNA, it can accurately replicate short DNA of a few thousand bases, making it possible for genetic forensics to copy large amounts of DNA from trace amounts of cells.
So why is DNA used in investigations? The short answer is that the genetic information in DNA is unique to each person. Therefore, DNA can be used to pinpoint a specific person as a suspect. Human cells have 23 pairs of chromosomes that vary in shape and size. Twenty-two of these pairs are autosomes, which carry information that determines the development of the body, and the other 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 hereditary traits. Only a small portion of the genome (about 3%) is encoded, and between the coding regions are long, repetitive non-coding regions that show great diversity. It is the short tandem repeats (STRs) of these non-coding regions that are used for genetic detection. Only a few STRs are typically used for forensic DNA profiling.
When a case occurs, evidence from the scene or samples from suspects or people involved are obtained and analyzed for their respective 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 culprits. The perfect crime is becoming increasingly difficult. We hope that science and technology will be further developed so that no matter what the case, the bereaved families will be able to get a little peace of mind.