Question

What is a short tandem repeat? How are short tandem repeats used in forensics?

Short answer

A short tandem repeat (STR) is a repetitive DNA sequence of 2-6 base pairs in length that occurs at specific locations in the genome and varies in the number of repeats among individuals. STRs are crucial in forensics due to their high polymorphism, allowing for unique DNA profiling. Forensic scientists use techniques like polymerase chain reaction (PCR) to amplify specific STR regions from DNA samples and then compare profiles to determine a match. Multiple STR regions are analyzed to increase reliability and accuracy, making STRs vital for criminal investigations.

Step-by-step solution

Definition of Short Tandem Repeat (STR)

A short tandem repeat (STR) is a repetitive sequence of nucleotides in DNA where the repeating unit is relatively short, usually 2-6 base pairs long. These repeating units occur at specific locations in the genome and vary in the number of repeats from individual to individual. For example, the sequence might look like this for two different individuals: Person A: AGAGAGAG Person B: AGAGAGAGAGAG In this case, the repeating unit is "AG", and it repeats four times in Person A's DNA and six times in Person B's DNA.

STRs and Forensic Science

The use of short tandem repeats (STRs) in forensics revolves around their high degree of polymorphism, meaning that these sequences differ among individuals. In human populations, STRs show significant variation in the number of repeat units, allowing for unique DNA profiling. In forensic science, a technique called polymerase chain reaction (PCR) is used to amplify specific STR regions from a DNA sample, such as those found at a crime scene or from a suspect. This amplified DNA is then separated and detected using gel electrophoresis or capillary electrophoresis, creating a DNA profile. The DNA profiles from the crime scene and the suspect can be compared to determine if they match or not. Multiple STR regions are typically analyzed in these profiles to increase the reliability and accuracy of the comparison. The more STRs match between the crime scene and suspect, the higher the likelihood that the DNA came from the same person. In summary, short tandem repeats are essential in forensic science for DNA profiling and comparing DNA samples. The variation found in these DNA sequences makes it possible to differentiate between individuals and provide vital information for criminal investigations.

Key concepts

DNA Profiling

DNA profiling, also known as genetic fingerprinting, is a process used to identify individuals based on their unique DNA characteristics. It relies heavily on analyzing specific regions of DNA known as short tandem repeats (STRs). Since these regions vary significantly among people, they serve as a powerful tool to distinguish one individual from another.
In the world of forensic science, DNA profiling plays an integral role. When a crime occurs, investigators may collect biological samples such as blood, saliva, or hair from the crime scene. These samples contain DNA, and with DNA profiling, scientists can create a detailed genetic map that helps to identify a suspect.
The uniqueness of STRs ensures that each person's DNA profile is nearly one-of-a-kind. Thus, when a DNA profile from a crime scene matches a profile from a suspect, it provides compelling evidence regarding the involvement or presence of the individual at the crime scene. However, to ensure the profile's accuracy, multiple STR regions are typically examined, making the probability of a coincidental match extremely low.

Forensic Science

Forensic science encompasses a wide range of disciplines used to examine and analyze evidence from crime scenes. One critical component within this field is the use of DNA for solving crimes. By utilizing scientific principles, forensic experts can reconstruct crime scenes, identify individuals, and determine the timeline of events.
The introduction of DNA analysis into forensic science revolutionized the way crimes are investigated. Its high accuracy in identifying individuals provides law enforcement with a powerful tool to link suspects to crime scenes conclusively.
When a crime is committed, forensic scientists collect and preserve samples that may contain DNA. By analyzing these samples and comparing DNA profiles, they can either confirm a suspect's presence at the scene or potentially exonerate the wrongfully accused. This process not only aids in solving crimes but also upholds justice by ensuring the right individuals are brought to account.

Polymerase Chain Reaction

Polymerase chain reaction (PCR) is a fundamental technique used in the analysis of DNA samples, especially in the context of forensic science. The main purpose of PCR is to amplify small quantities of DNA, making it possible to analyze even minute traces from a crime scene.
PCR mimics the natural process of DNA replication, but it does so in a controlled laboratory setting. Specific regions of DNA, particularly STR regions, are targeted and copied through a series of heating and cooling cycles that prompt the replication process.
The beauty of PCR lies in its ability to produce millions of copies of a specific DNA segment in a matter of hours. This amplification allows forensic scientists to have sufficient material to conduct thorough analyses, such as creating a DNA profile or testing genetic markers. Its precision and efficiency make PCR an indispensable tool in forensic investigations.

Gel Electrophoresis

Gel electrophoresis is a scientific technique used to separate DNA fragments based on their size and charge, and it is a critical step following the amplification of DNA via PCR. After the DNA has been amplified, it is loaded onto a gel matrix and an electric current is applied.
This current causes the negatively charged DNA fragments to move towards the positive electrode. Smaller fragments move faster and farther through the gel than larger ones, leading to a separation of the DNA based on size.
In the context of forensic science, gel electrophoresis allows for the visualization of STRs in a sample. The resulting pattern can then be compared to other samples, such as those collected from a suspect or a crime scene. By examining the number and length of repeats, investigators can ascertain whether a suspect's DNA matches the sample from the crime scene, thus providing a crucial piece of evidence in forensic investigations.