Activity 8: Proving Your Innocence When Accused Of Crime

Activity 8 Proving Your Innocenceyou Have Been Accused Of Committing

Prove your innocence in a criminal case involving DNA evidence by analyzing STR (short tandem repeat) sequences from your own DNA, your parent’s DNA, and suspects related to the crime. Determine the number of repeats at four specific STR sites for each individual, interpret gel electrophoresis results to compare DNA profiles, and evaluate whether the DNA evidence matches you or the suspects. Additionally, explain key forensic techniques such as DNA amplification and the importance of analyzing multiple STR sites to ensure accurate identification.

Paper For Above instruction

In forensic science, DNA analysis plays a crucial role in criminal investigations, often providing definitive evidence to identify or exclude suspects. A common method involves analyzing short tandem repeats (STRs), which are regions in DNA where a specific sequence, such as "GATA", is repeated multiple times. The number of repeats at particular STR loci varies among individuals, making them useful markers for forensic identification. This paper explores how analysis of STRs was used to determine the guilt or innocence of a suspect in a murder case, details the techniques used in DNA profiling including PCR amplification and gel electrophoresis, and discusses the significance of multiple STR sites in forensic investigations.

The case involves a murder that took place at 1:54 am on March 10, 2019. Police collected DNA evidence from the crime scene and from the individuals accused—namely, the suspect and their parent. The DNA sequences at four STR sites were examined, with the focus on counting the number of "GATA" repeats. For each individual, the specific number of STRs at each site was provided. For the suspect ("Your" DNA), the counts at the four sites were 11, 6, 27, and 15. Corresponding counts for the parent, potential suspects William and Theodore, and an analysis of their DNA was also provided.

Determining the Number of STRs

From the given sequences, the number of STR repeats at each site was determined by locating the "GATA" sequence and counting how many times it occurs consecutively. For example, in the suspect's DNA, Site 1 had 11 repeats, containing a stretch of "GATA" repeated 11 times. Similarly, the counts for the other sites were derived from the sequences provided.

DNA Amplification via PCR

The process of DNA amplification in forensic science is known as Polymerase Chain Reaction (PCR). PCR allows scientists to create millions of copies of specific DNA regions, including STR loci, enabling detailed analysis even from minute samples. PCR involves cycles of denaturation, annealing of primers specific to the target regions, and extension. This process is critical because it increases the amount of DNA available for gel electrophoresis, making it possible to visualize and compare the STR profiles of different individuals.

Gel Electrophoresis and DNA Profiling

After amplification, gel electrophoresis separates DNA fragments based on size. The DNA samples are loaded into a gel matrix, typically agarose or polyacrylamide, and subjected to an electric current. Smaller fragments migrate faster and travel farther, producing a pattern that can be compared across samples. Gel images display bands corresponding to the number of STR repeats at each locus. In this case, the crime scene DNA was already analyzed, with bands plotted along the left side for reference.

By comparing the number of repeats at each locus between the suspect’s DNA and the crime scene DNA, forensic analysts can determine if there is a match. If the patterns align across multiple STR sites, it significantly supports the hypothesis that the suspect’s DNA was at the crime scene. Conversely, differing profiles suggest the suspect is not the source of the DNA evidence.

Analysis of Additional Suspects

In this investigation, two additional suspects, William and Theodore, were identified as potential perpetrators based on their DNA profiles at the same four STR loci. Their sequences indicated that William had 19, 6, and 7 repeats at three of the sites and 6 at another, while Theodore had 7, 6, and 6 repeats at the same sites. By plotting their profiles alongside the crime scene evidence, investigators can determine whether either matches the DNA found at the scene. In this case, William’s profile (with higher repeat counts at certain sites) did not match the crime scene DNA, but Theodore’s profile closely resembled the evidence at multiple loci.

The Importance of Multiple STR Sites

Analyzing multiple STR sites enhances the reliability of forensic DNA analysis. Relying on a single locus can lead to false positives or negatives because of the probability that two unrelated individuals might share the same number of repeats at one site. By examining multiple loci—typically at least 13 in forensic standards—the combined probability of two individuals having identical profiles across all sites becomes astronomically low. For example, if only one site is considered, coincidental matches are more likely, which could lead to wrongful convictions or misidentification.

In this case, the analysis of four STR loci provided a strong differentiation capability. If only Site 2 was used, there would be a higher risk of misclassification due to its lower variability. The use of multiple sites ensures that forensic conclusions are based on robust, statistically significant data, increasing judicial confidence in DNA evidence.

Conclusion

The forensic investigation exemplifies how analyzing STR regions through DNA profiling can be decisive in criminal justice. The process involves extracting DNA, amplifying STR regions via PCR, separating fragments using gel electrophoresis, and comparing the resulting profiles. Accurate interpretation hinges on examining multiple STR loci, which assures high specificity and minimizes errors. In the presented case, the DNA evidence at four STR sites allowed investigators to exclude the suspect and identify Theodore as a probable perpetrator, illustrating the power of forensic genetics in solving crimes. As DNA technology continues to evolve, its role in both exonerating the innocent and securing convictions remains paramount.

References

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