Court is in session and you're guilty

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CSI is what I would assume to be one of the first exposures to forensic sciences to most of the North American population. Although it was one of the most predominant examples in media for the area of forensic studies, it is not the best place for an accurate representation of the scientific techniques. The first season of the show, CSI: Las Vegas, is widely known in the forensic community to have the most faithful depiction of forensic science, with the least “fudging”, so to say, of the true scientific process (although there are still many instances of bad technique and incorrect handling of materials!). The following post consists of excerpts from an essay I wrote in 2014 detailing the representation (or rather, the misrepresentation) of DNA profiling in episode 8 of the first season of the show. I have cut out the argumentative parts of my paper, leaving only the informative parts that are hopefully coherent and educational to the everyday population.

DNA Profiling 

DNA is the genetic code of life (Anderson, 2014). It is a double helix made of the sugar-phosphate backbone and the four nucleobases, adenine, guanidine, cytosine and thymine, which serve as the information storage part of the structure (Weaver, Robert Franklin, 2012). DNA contains both coding and non-coding regions, and coding regions are made up of genes. DNA is organised into chromosomes and humans have 23 pairs of chromosomes (Weaver, Robert Franklin, 2012). DNA profiling is based on the assumption that every single individual in the world –except for identical twins- has a unique DNA sequence that can be analysed (Taupin, 2013, p. 9). Basic DNA analysis steps as described in Taupin et al (2013) are

• Isolate the crime stain or other biological sample. 
• Separate the DNA and clean the sample from the other material. 
• Measure the quantity and quality of the DNA. 
• Target the specific areas of interest within the DNA molecule. 
• Produce multiple copies of the DNA pieces. 
• Sort the DNA pieces according to size. 
• Measure the sizes of the DNA pieces (p. 45)  

DNA analysis in the forensic lab was traditionally done using restriction fragment length polymorphisms (RFPLs). RFPLs is a technique that utilizes enzymes called restriction endonucleases to cut double stranded DNA at specific known sequences. The DNA fragments can then be separated by electrophoresis (electrical current, as DNA molecules are charged and can be moved with an electrical current) and the determined variations in the patterns of repeated nucleotides in DNA sequences can be visualised by dyes or ultraviolet light (McDonald and Lehman, 2011). DNA analysis techniques have advanced to polymerase chain reaction (PCR) to amplify short tandem repeats (STR) (Saferstein, 2015, p. 383). After the PCR technique was invented in 1983, the RFPL technique was replaced as PCR was much more sensitive, thus less starting material is needed (remember, it usually very hard to find good quality DNA evidence in crime scenes) and even DNA molecules that have been degraded can be used and analyzed for DNA typing (McDonald and Lehman, 2011). PCR of STRs are used by DNA profilers as it uses much smaller repeat units that allows them to be amplified more easily, be less prone to degradation and require even smaller quantities of DNA (McDonald and Lehman, 2011). 

STRs emerged as a standard to forensic profiling due to its high degree of variability, ease of use in multiple amplifications and its ability to be highly individualizing (Taupin, 2013 p. 46). STRs are individualising due to, as mentioned, the number of repeats but also the contents of the repeats. As STRs are pieces of DNA, they have the chance of accumulating random mutations that can be highly individualizing; identifying variations can include changes in nucleobase or a deletion of a nucleobase (Taupin, 2013, p. 46). To further increase the resolving power of STR, a technique called multiplexing is used (Saferstein, 2015, p. 386). Combining selectors for different STR and resolving them will decrease the likelihood of two individuals selected at random to have the same STR types (Saferstein, 2015, p. 387). A typical STR technique examines at least 9 different STRs and will give a very high statistical value (Taupin, 2013, p. 20, 46).

After PCR amplification with STR specific primers, the result is a sample with over 20 different sized fragments that must be separated, thus the sample is subjected to electrophoresis (McDonald and Lehman, 2011). Electrophoresis involves a solid phase and an electric potential, it can separate molecules according to their weight and charge, resulting in DNA molecules separated by size charge (but as all DNA molecules are the same charge, the main condition of sorting is size) (Saferstein, 2015, p. 385). The most preferred method of electrophoresis capillary electrophoresis, though slab gel electrophoresis is also common (McDonald and Lehman, 2011).  STR alleles with more repeats has larger molecular weight, and therefore will take longer to navigate the solid phase and thus, when visualised with fluorescence detection, show up as a band closer to the source on the gel (Saferstein, 2015, p. 385). Fluorescence detection on the capillary electrophoresis then displays as peaks on an electropherogram that can be printed ((Saferstein, 2015, p. 389) (McDonald and Lehman, 2011). The forensic scientist can then compare and match the crime scene DNA to the DNA ladder, and the suspect DNAs.

 Strengths and Weaknesses of Forensic Profiling   

DNA demonstrates a high degree of discrimination power and DNA profiling is based on the assumption is DNA is unique to every individual (Taupin, 2013, p. 9). DNA is also more robust and resistant to degradation than its forensic processor, serology markers (Taupin, 2013, p.18). Its most impressive aspect is its high statistical discrimination based on the STR sites (Taupin, 2013, p.19). It is also possible to keep records of DNA databases in order to exonerate suspects (Taupin, 2013, p. 22). The DNA of every individual is unique (Saferstein, 2015, p. 380). DNA also remains unchanged throughout the person’s lifetime and the DNA composition remains the same throughout the body, ie the DNA sequence seen in cells from the brain is the same DNA that can be seen in cells from skin cells and so on (Taupin, 2013, p.33). 

 Although DNA evidence has reached general acceptance as an important tool for the pursuit of justice, the bottom line is that it is not infallible and it is indeed a tool (Walsh, 2005)(Taupin, 2013, p.13). There are many authors in the forensic community that continue to argue and challenge the applicability and reliability of DNA as a forensic tool (Walsh, 2005). Although many of their arguments are ideological in nature and will not be covered in the scope of this paper, forensic scientists must remember to make certain that the limitations and potential of DNA evidence is not exaggerated nor is it infallible (Walsh, 2005). 

 The science gives results that must be interpreted by the forensic scientist who are human. Thus there have been numerous cases where DNA evidence has been compromised by: quality issues, contamination, interpretation issues, and just intrinsic error rates associated with the DNA processes themselves (Taupin, 2013, p. 131-144). Technical issues remain the most common type of error in faulty DNA analysis to (Bennett, 1995, p. 146). 

Another common weakness of DNA analysis is the quality of the DNA sample. The first question the evidence recovery unit would ask is whether there is enough DNA present for the analysis (Bennett, 1995, p.155). In recent times, this is not a paramount problem to succumb as recent DNA technology has made it possible to analysis DNA from just 18 cells (Taupin, 2013) (Saferstein, 2015). The next question asked is whether the DNA is too contaminated for further testing (Bennett, 1995, p. 155). Samples that were subject to harsh outdoor conditions, extreme heat, bacteria or chemicals are often too damaged for analysis efforts (Bennett, 1995, p. 155). In some crime scene samples, the DNA specimen may be contaminated with foreign DNA that may make interpretation difficult (Bennett, 1995 p. 155). In addition, when cold cases are re-examined, DNA evidence may be highly degraded and even destroyed thus losing most of its forensic value due to evidence is not stored correctly because there were less stringent regulations during that time or simply because so much time has passed (Taupin, 2013, p. 131). At the end of the day, DNA must be present in adequate quality and quantities in order for the analysis to occur. 

 

There are many concerns with false DNA evidence or planted DNA evidence. As there is no way at the laboratory level to distinguish if the DNA is endogenous to the crime scene or not, this is a difficulty that forensic laboratories face (Anderson, 2014). Advancement in scientific development has also brought difficulties as well. With current technology, it is technically possible to manufacture DNA and plant it at crime scenes (Anderson, 2014)! Fortunately, there are still methods of distinguishing synthetic DNA from natural DNA (Frumkin et al, 2005).

 

CONCLUSION!!! 
Forensic biology and investigations are not infallible. Although there are very strict standards and regulations for laboratory results from accredited forensic labs, those results still need to be interpreted in court by the judgement of scientists who are still human and can make mistakes. It is important to remember that in the end, laboratory results are just tools and aids towards uncovering the truth of the crime and they are never definite indicators or "smoking guns" of any crime. It is near impossible to deduct motives of a crime from laboratory results alone and a through investigation is still needed to reach a verdict.


References  

 

1.       Weaver, Robert Franklin. "Chapter 2." Molecular Biology. New York: McGraw-Hill, 2012. 18-19. Print. 

2.       Saferstein, R. (2015). Criminalistics: An introduction to forensic science 11^th edition. 

3.       McDonald, J., & Lehman, D. C. (2011). Forensic DNA analysis. Clinical laboratory science: journal of the American Society for Medical Technology,25(2), 109-113. 

4.       Taupin, J. M. (2013). Introduction to Forensic DNA Evidence for Criminal Justice Professionals. CRC Press. 

5.       Hopkins, B., Williams, N. J., Webb, M. B., Debenham, P. G., & Jeffreys, A. J. (1994). The use of minisatellite variant repeat-polymerase chain reaction (MVR-PCR) to determine the source of saliva on a used postage stamp. Journal of forensic sciences, 39(2), 526-531. 

6.       Warshauer, D. H., Marshall, P., Kelley, S., King, J., & Budowle, B. (2012). An evaluation of the transfer of saliva-derived DNA. International journal of legal medicine, 126(6), 851-861. 

7.       G. Anderson (Crim.), CRIM 355: Forensic Science. Course Material: Fall 2014. Burnaby BC: Simon Fraser University. 

8.       Elkins, K. M. (2012). Forensic DNA Biology: A Laboratory Manual. Academic Press. 

9.       Caprette, David R. "Guidelines for Keeping a Laboratory Record." Guidelines for Keeping a Laboratory Notebook. Rice University, n.d. Web. 14 Nov. 2014. 

10.   Walsh, S. J. (2005). Legal perceptions of forensic DNA profiling: Part I: A review of the legal literature. Forensic science international, 155(Saferstein, 2015), 51-60. 

11.   Bennett, Margann. "Admissibility Issues of Forensic DNA Evidence." Kan. L. Rev. 44 (1995): 141-1045. 

12.   Frumkin, D., Wasserstrom, A., Davidson, A., & Grafit, A. (2010). Authentication of forensic DNA samples. Forensic science international: genetics, 4(2), 95-103.