Forensics and DNA

Forensics is the application of scientific techniques and evidence to solve crimes used in the legal system. The use of DNA profiling is an example of modern forensic evidence

The name Colin Pitchfork from Leicestershire, England is the name that holds weight in the forensic science community. He was the first murderer to be caught using DNA analysis for raped and killed 15-year-old Dawn Ashworth. The genetics professor; Alec Jeffreys was able to find the real culprit using his experience and knowledge based on DNA technology.

DNA profiling has revolutionized criminal investigations since that first case 30 years ago, although the DNA evidence alone is not enough in modern conviction. Hence, DNA profiling methods have become faster, more sensitive, and more user-friendly since the first murderer was caught with help from genetic evidence on Sept. 19, 1987.

The main steps in forensic science are collecting evidence left at the crime scene, isolate DNA from samples, determine DNA profile, and finally data analysis. DNA can be isolated from a wide range of evidence left at a crime scene – from skin, hair, and semen samples to bacteria in dirt

Forensic scientists or SOCO (scene of crime officers) are required to collect biological material from a crime scene. The blood is known as the rich source of DNA. DNA is collected only from WBC (white blood cells) since mature red blood cells do not contain DNA. DNA can also be obtained from hair follicles, any other cellular tissue, and the heads of individual sperm cells. The small trace of the sample is enough for isolating genomic DNA in the laboratory under precise conditions.

The second step is to determine the DNA profile which is like a genetic fingerprint. Thus, it is unique to each individual (the only exception is the identical twins) while making it very useful for identifying and verifying criminals. A DNA profile is a list of numbers that indicate how many repeat units are in each copy of 20 marker regions located throughout the genome

The chromosomes contain markers where short DNA sequences are repeated multiple times. It is also known as short tandem repeats (STRs). The STRs used for forensics range from three to five bases long. The number of repeats at each marker varies from individual to individual. Hence, provide even more specificity to a person’s DNA profile. There are two copies of each marker.

One is inherited from mother and other from father. The repeats serve as a marker for DNA profiles rather than exact sequences. The main reason is repeated length is sufficient for distinguishing among individuals. Scientists choose a small set of markers to analyze among many STR loci in the human genome.

The one selected loci are distant from other selected ones. It interprets the one locus is inherited independently from another locus. Hence, it also increases the rarity of a particular DNA profile.

Figure1: Example of the distribution of repeats in two alleles. Source; Adapted from NIST  https://www.nist.gov/

To do a DNA profile, the extracted DNA initially undergo polymerase chain reaction (PCR) to amplifying the amount of DNA in a sample by copying it many times. The modern scientists use a different pair of PCR primers for each locus. It allows amplification of all loci in the same reaction without interfering with one another.

The copied markers are separated by using capillary electrophoresis. The modern forensic scientists measure the size of DNA fragments with capillary electrophoresis rather than X-ray-based gel electrophoresis. Though, the principle is the same as gel electrophoresis (Small fragments travel more quickly than large fragments through a gel when voltage supplies).

As the separated DNA bits pass a fluorescence detector, they are registered as a series of peaks in an electropherogram. The electropherogram is a plot of DNA fragment sizes, typically used for genotyping such as DNA sequencing achieved by an instrument called automated DNA sequencer. The position of the peaks in the electropherogram correlates with the number of repeats in the two alleles for each marker.

Figure 2: – example for an electropherogram. Source; Adapted from NIST https://www.nist.gov/

The resulting DNA profile contains information like the number of repeats in two alleles for every 20 markers. Then Forensic scientists enter DNA profiles into law enforcement databases. The units are RFU (relative fluorescence units).

The analyzed crime scene sample is not benefited without a suspect. Therefore, intense governments decide to maintain databases of DNA profiles. Thus, once databases are established, they can be able to identify the suspect/s and if there are no suspects, can be able to match DNA from other solved or unsolved cases which might lead to revealing serial killers or rapists.

There is a common standard for selecting loci in DNA profiles since they are shared internationally. The number of loci for CODIS (Combined DNA Index System – established in 1998 by the FBI) profiles was increased to 20 from 13 in 2017. 7 primary additional loci are currently used by forensic scientists in Europe.

Figure 3: – National DNA Index System (NDIS) of CODIS. Source; Adapted from NISThttps://www.nist.gov/

Recently, the FBI introduced a valid commercial kit that contains the PCR primers and other reagents needed for amplifying the CODIS-designated loci. If there are no 20 loci, the repeated trial is required as there is no reason for not getting 20 loci in a fresh and known source. CODIS contains profiles of approximately 16 million convicted offenders and arrestees and 750,000 crime scenes.

The main challenge in this method is the sensitivity of PCR. The smaller the amount of DNA found in only a few cells, may lead to less sensitivity of PCR. Nowadays, the analysis of a single sample is much more likely to lead to multiple DNA profiles due to improved methods. They are sensitive enough to detect DNA that might have been in the background. Previously and after the crime.

The scientists are dealing with mathematical methods that allow them to incorporate all the data in their analysis. The algorithm is used in software packages to determine which combinations of DNA profiles better explain the observed data. The mixture challenge is overcome by Cybergenetics.

Figure 4: – the logo of cybergenetics. Source; https://www.cybgen.com/

Cybergenetics is a Pittsburgh-based bio information software using advanced mathematics. TheSTRmix, the other major DNA mixture characterization software package. These new approaches lead to better match statistics.

Another interesting fact is the Rapid DNA program. It simply moving out of the lab. It works without the intervention of humans. This allows to conduct CODIs and software generates a profile as the DNA from a reference sample comes from a single source. Thus, the system doesn’t need to perform mixture deconvolution

The next-generation sequencing (NGS) is also a ladder for revolutionized criminal investigations. The main advantage of NGS over conventional methods being able to run many samples in parallel and thus being much faster. This method is still evaluating the provided results and check whether the results are indeed compatible with existing databases.

Forensic DNA analysis has come a long way since the first case almost 30 years ago. No one cannot tell where it will go in the next 30.

Author: Lakma Rahubaddha
B.Sc. (Honors degree in Biochemistry and Molecular Biology)
Undergraduate
Faculty of Science
University of Colombo

References;
Science, N., 2020. Introduction. [online] Ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK234533/ [Accessed 17 June 2020]. Forensic and DNA
Science, N., 2020. Introduction. [online] Ncbi.nlm.nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK234533/ [Accessed 17 June 2020]. Thirty years of DNA forensics: How DNA has revolutionized criminal investigations
Arnaud, C. (2020). Thirty years of DNA forensics: How DNA has revolutionized criminal investigations | September 18, 2017 Issue – Vol. 95 Issue 37 | Chemical & Engineering News.
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