December 18, 2024

Uy Pham 

12th Grade

Fountain Valley High School

Introduction

You’ve seen the classic crime-solving in laboratory scenes in your favorite shows and movies. The scientist places a sample of DNA into a machine, and the sample yields a match within a database — the DNA has been matched, and the crime has been solved. Yet, how does this machine work? How is the DNA sample collected, and how is the DNA matched with a person to solve countless crimes in the world? 

The first use of DNA profiling in a court procedure occurred in 1986. Two young girls had been raped and murdered in England, and police sought to confirm the identity of the killer. University of Leicester professor Alec Jefferys, who was already famed for his discoveries regarding the concept that DNA could serve as a genetic fingerprint based on unique patterns, analyzed over 4,000 voluntarily provided DNA samples in the area. There were no matches, but a discovery that one person had provided a false sample led to a verification of the sample; the new sample matched DNA at the scene, and the suspect was identified and eventually convicted. 

What is DNA?

Deoxyribonucleic acid (DNA) is one of the essential components and unique to every person. Stored inside the nucleus of every cell, DNA is long strands of nucleotides, which are composed of three molecular parts: a 5-carbon sugar, a phosphate group, and a chemical base. There are four options for the chemical base: adenine, thymine, guanine, and cytosine. Nucleotides are joined together by covalent bonds known as phosphodiester bonds, and the order in which the nucleotides are arranged, including which chemical bases are in that particular order, is the genetic variation that makes each DNA unique to every person. 

Eventually, DNA plays an important role in several biological functions, such as the 23 chromosome pairs (two DNA strands are arranged antiparallel to each other and wrapped on the exterior with histone proteins) with sections called genes that direct protein production within the living organism. The two DNA strands are bonded together by hydrogen bonds and at points in which the complement nucleotides are paired across both DNA strands; guanine of one strand is paired with cytosine of the other strand, adenine of one strand is paired with thymine of the other strand, and vice versa. The produced proteins instruct the organism on how to develop, produce, and function, and the DNA is replicated as part of a process called DNA replication to transfer genetic material to newly created identical cells. 

DNA Extraction

As seen in popular entertainment, DNA can be extracted from a variety of sources that have a biological connection, such as skin cell remnants, hair, or a bodily fluid. Investigators at a crime scene will search and preserve such samples so that DNA can be extracted. However, since samples collect entire cells and DNA is located in the nucleus of cells, the DNA must first be extracted away from the cell membranes and other organelles.

Therefore, the membranes within the cell, both the cell membrane and nuclear membrane, must be broken apart in a process known as lysis. Lysis can involve either chemical or physical processes, with chemical lysis involving the utilization of a detergent that separates the membrane’s lipid molecules. Furthermore, with the addition of the physical grinding of cells, membranes can be broken via physical lysis as well. Once the membrane’s lipid molecules are broken down, a concentrated salt solution is added to form a precipitate, or solid. In a process known as centrifugation, the solution is spun at rapid speeds and the precipitate sinks to the bottom and can be filtered out. To remove the remaining histone proteins surrounding the exterior of the DNA, protease is added as an enzyme to break down these proteins. Lastly, to complete the extraction of DNA from the remaining liquid solution, cold alcohol is added to prevent denaturing. Since DNA is not soluble in the presence of alcohol, DNA precipitates and will remain once the alcohol is removed. 

Polymerase Chain Reaction and Gel Electrophoresis

The next step is to duplicate many copies of the DNA extracted from the biological sample, and the current most-used method is known as polymerase chain reaction (PCR). PCR utilizes 25-35 variations of rapid heating and cooling cycles in order to denature the DNA strands or unbind the two DNA strands covalently bonded in a chromosome pair. This allows for specific primers, or a short sequence of nucleotides, to bind to specific complement nucleotide pairings and indicate where Taq polymerase (a heat-tolerant DNA polymerase) begins to replicate DNA. Once each DNA strand has its complement strand bonded to create a new pair, the heating and cooling cycle repeats to separate these two strands of DNA and generate two new complementary strands of DNA. Billions of copies can be generated from a few original copies by repeating this cycle of heating and cooling, binding primers, and synthesizing complementary DNA strands to create new pairs utilizing Taq polymerase. Throughout this process, primers indicate where the Taq polymerase will synthesize complement DNA strands, allowing specific fragments of the DNA to be analyzed in gel electrophoresis.

Once these billions of copies of specific DNA fragments can be generated, gel electrophoresis is a process where the DNA strands are put through a gel matrix with an electric current. By utilizing an electric current, the parts of the DNA will travel at different distances through the gel based on its size. Therefore, DNA segments of the same size will travel the same distance and gather to form a band, which can be compared against a DNA standard ladder. The ladder is used as a method of comparison or reference to determine the various sizes of the DNA segments based on how far they traveled through the gel matrix. This is the process by which the different chromosomes, which are different lengths, can be separated and further analyzed. 

Matching DNA Profiles to Identification Systems

In the United States, the FBI uses thirteen core short tandem repeat (STR) loci to differentiate between individuals. STRs are a short sequence of three to four nucleotides that repeat within a particular chromosome in a DNA sequence, and the number of repeats of that STR is the allele (particular gene expression) that can be used to identify the number of people. For example, for one of the STRs, the sequence GATA (guanine adenine thymine adenine) can repeat between five and sixteen times on chromosome seven, meaning there are twelve different possible alleles based on the number of repeats a person has. A person can have two alleles of a STR sequence (two different numbers of repeats), one from each parent. 

In PCR, primers can target specific segments of the DNA where these STRs or repeat sequences are typically located and create billions of copies for analysis in gel electrophoresis. Gel electrophoresis can separate the STR alleles by size, as different alleles have different numbers of repeats and will move different distances through the gel. After electrophoresis, the agarose gel, in which the gel matrix is submerged after the process, can illustrate the patterns of bands and identify the particular STR alleles one may have through comparison to the standard DNA ladder. 

The alleles a person has for the thirteen STR loci identified by the FBI can be used to create a profile, and this profile can be compared against existing databases to determine matches for the thirteen STR loci.  Although it is possible for two people to have the same thirteen STR profile, the chance is extremely unlikely. For example, for Caucasians, it is estimated by the FBI that 1 in 575 trillion people would have the same STR profile, meaning other factors can be considered to determine whether a match in the STR profile matches a particular person to be identified. 

Modern Uses of DNA Profiling 

While DNA can be useful in determining parents’ heritage (as half of the chromosomes are inherited maternal and the other half paternal) or identifying genetic conditions (based on the order of nucleotides that lead to a certain protein function), DNA profiling specifically has helped bring new evidence to the criminal justice and investigation systems. DNA profiles can be compared to people available in databases including missing people, convicted offenders, arrested people, and human remains — all of which can serve as reference points to eventually potentially identify a person’s DNA profile that matches the DNA thirteen STR loci profile found within a biological sample. 

Although DNA profiling is a method of science and a powerful piece of evidence to present a t trial, DNA profiling is not the sole piece of evidence used in criminal investigations. Other aspects of the investigation, such as considering the motive of a person, the identity of the person, and the location of the person are all pieces of evidence that can be combined with the DNA profile in the database and in the sample analyzed towards a criminal conviction. This is essential, as two people can have identical DNA profiles, despite independent inheritance, and also privacy concerns with how DNA profiles are stored within law enforcement databases. With the importance of DNA profiles, the MIT Technology Review has called it essential for security measures to be implemented to protect the DNA profiles of a country’s citizens and the samples of those involved in intelligence operations. The uses of DNA profiling in forensics have helped reopen many cases where a suspect could not be previously identified, as DNA profiling allows for the analysis of biological DNA samples left at the scene of the crime; however, as with other pieces of evidence used at trial, it ultimately remains important the  DNA profiles are correctly preserved and identified to ensure that the appropriate person is matched to the appropriate sample.