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During the last 3 decades, DNA has risen to become one of the key tools for Forensic experts in solving crime. The technology was first applied to forensic science in 1980 and since then, it has become a major growing area of research in the forensic world (1). This assignment will discuss the structure of DNA, the types of DNA and what makes them so useful in forensic investigations. Also discussed are the various technologies used for DNA profiling which depend on the characteristics of the forensic case itself. The various uses of DNA in forensic investigations have also been mentioned in this assignment along with the legal and ethical issues associated with DNA evidence.
DNA stands for Deoxyribonucleic acid. In 1953, Watson and Crick proposed a double helical structure for DNA as shown in Figure 1 below. Each chain is made up of a phosphate backbone, deoxyribose sugar and four nitrogenous bases: Adenine (A), Guanine(G), Thymine(T) and Cytosine (C). (2)
Figure 1: Structure of DNA (3)
From the above figure we can see that the two chains run in opposite directions and the bases of these chains are connected together by hydrogen bonds. Adenine is connected to Cytocine with two hydrogen bonds, while Guanine and Thymine are connected to each other by 3 hydrogen bonds (2).
Types of DNA
Nuclear DNA is located in our chromosomes which are stored in the nucleus of each cell. Every individual receives half number of chromosomes from his/her mother and the other half from the father (4). This means that DNA is transferred from both parents to their offspring's which makes it responsible for the transmission of characteristics and traits from one generation to the next. Every human being has a unique copy of their nuclear DNA, i.e. they do not share the same DNA characteristics with other humans. The only exception to this is in the case of Identical twins where they have the same nuclear DNA, however it might differ in minor ways (4).
DNA is also found in the mitochondria's of cells, which are the power house of the cell. This is termed as Mitochondrial DNA or simply mtDNA. Unlike nuclear DNA, mitochondrial DNA is not unique to every individual. It is inherited only from the mother and there are over 1000 copies in each cell (5). This means that the mitochondrial DNA in an individual would be the same mitochondrial DNA found in the individual's mother, maternal grandmothers and siblings (from the same mother).
Why is DNA used in Forensic Science?
Dr. Edmond Locard was one of the early pioneers in the field of forensic science. He proposed the 'exchange principle' which states that when two objects come into contact there is always a transfer of material between them(6). In the forensic science world, the material of interest that is transferred at a crime scene is the offenders DNA. It can be transferred onto the victim or onto a tangible object which makes it physical evidence. Physical evidence may include things such as glass, tools, firearms, projectiles, clothing etc. DNA can also be extracted from any body fluid such as blood, saliva, sweat, mucus. It can also be extracted from the fragments of a body which included skin, hair, bone etc (6).
Once the DNA is found at a crime scene, it is sent for analysis to determine which individual this DNA profile most likely belongs to. This process has many different names which include DNA fingerprinting, DNA typing or DNA profiling. The idea behind DNA fingerprinting is identical to the idea of standard fingerprinting. Every individual has a unique set of finger prints and in the same way; every individual has a unique DNA profile (except identical twins). The only difference is the fact that in DNA fingerprinting involves comparing DNA strands as seen in Figure 2 below. These features make DNA extremely useful in forensic investigations.
Figure 2: DNA Fingerprinting (7)
DNA technologies used in Forensic Science
There are various technologies used in forensic science to perform DNA profiling. The technology used depends on the amount of DNA sample found at the crime scene, amount of degradation the DNA has suffered and also the type of case that is under investigation, i.e. whether it is a missing persons case, or a sexual assault case etc.
Restriction Fragment Length Polymorphism (RFLP)
Restriction Fragment Length Polymorphism involves the extraction of DNA from blood or semen samples. Figure 3 illustrates the various steps involved in RFLP analysis.
Figure 3: RFLP process (8)
The DNA is cut into fragments which are placed onto an Agarose gel. They undergo a process called electrophoresis. This process separates the fragments based on their size since large fragments move slower than the smaller fragments. Once the fragments are separated they are placed on a nylon membrane for Southern Blotting. Short pieces of single stranded radioactive DNA probes bind to specific DNA fragments on the membrane which contain the complementary base sequences (9). The membrane is then washed and processed against an X-ray film to reveal the radioactive pattern. It is this DNA pattern that is then compared to the DNA profiles of known individuals.
It is highly accurate and is mainly used to discriminate between multiple samples. Despite having these qualities, RFLP has its draw backs as well. The technique is a long process, requires high quantity and high quality DNA, i.e. degraded samples of DNA cannot be used. This technique used to be one of the standards and is still used in DNA forensics(10).
Polymerase Chain Reaction analysis (PCR)
The Polymerase Chain reaction technique was developed in 1986 by Kary Mullis (10). It results in the amplification or duplication of DNA. It gives forensic experts the power to generate more DNA from the DNA evidence found at the crime scene. Hence, one of the major advantages of this technique is the fact that it can be done on small and degraded amounts of DNA (6).
The development of this technique was one of the major advancements in technology used for DNA profiling. PCR improved the sensitivity of analysis and it took significantly less time as compared to the RFLP. It is also more cost efficient as less labor is required (4). This technique is very suitable for majority crime scenes because physical evidence such as single hair strand, cigarette buds, saliva traces etc contain only small amounts of DNA. (11)
Short Tandem Repeat analysis (STR)
This technique is based on the Polymerase Chain Reaction. STRs are DNA regions with small repeat units. There is a lot of variation in these repeat units which is why they exhibit a high degree of length polymorphism(12). Because of this great variation in STR repeats between individuals, it makes STR analysis an effective technique in forensic science for identification of humans. Another advantage of this technique is the fact that degraded and low amounts of DNA can also be typed with high accuracy (13).
Mitochondrial DNA analysis
Mitochondrial DNA analysis is done in the case of missing individuals. This is because there is a high mutation rate between mtDNA which makes it highly variable between unrelated individuals. Due to its maternal inheritance pattern, mtDNA from the mother, grandmother or siblings can be used to identify victims. Also, there are large number of copies of mtDNA in each cell which means that it can be recovered from small, old and degraded samples(5). Despite these advantages, mtDNA analysis is done in forensic investigations only if the nuclear DNA analysis fails to show positive results. This is because mtDNA posses some technical and interpretational challenges(1).
The Y chromosome is present in males only and it consists of several genetic markers which are of use in forensic investigations. Y-chromosome analysis is an excellent technique used to analyze biological evidence which has traces of multiple male contributors. It can also be used to track family relationships amongst males since fathers pass on their Y chromosomes to their sons (7).
Possible Outcomes of DNA profiling
Once the analysis of the DNA is complete using these technologies, it results in three possible outcomes (14).
Match: This indicates that that there is no unexplainable difference between the samples that have been compared.
Non-Match: This indicates that there exist differences between the DNA profiles thus suggesting that the compared samples have different sources.
Inconclusive: This indicates that the results of DNA profiling do not confirm a match.
Uses of DNA in Forensic Science
As mentioned in sections 2.2.2 and 4.4, mtDNA is passed onto an individual from the mother. This means that mitochondrial DNA from the mother, maternal grandmothers or even siblings can be analyzed to identify individuals. An example of where DNA analysis was done to identify victims was after the 9/11 incident. The only remains of most of the victims were bones and small tissue fragments. These samples were collected and a new data base of the 9/11 victims DNA profiles was made. These profiles were studied and analyzed which resulted in a majority of victims being correctly identified (15).
DNA can also be used to identify burnt victims. This is done using Mitochondrial DNA because it has an advantage over nuclear DNA as mtDNA can survive harsh environmental impacts such as burns, which nuclear DNA cannot (4). This makes mtDNA an excellent tool for Forensic experts to identify burnt victims. It has also extensively been used in archaeology and anthropology. A study conducted in 2001 showed that mtDNA was successfully recovered from Pleistocene skeletal remains of Australian Aboriginal, putatively dating the skeleton beyond 60,000 years (16).
Tracking Viral Transmissions
Another use of DNA in forensic investigation is to track the transmission of viruses from person to person. Transmission of microbes intentionally or negligently is an offence. Some of the transmitted viruses include Human Immunodeficiency Virus (HIV). These can be tracked from sexual assault cases or even attempted manslaughter. But tracking these virus transmissions accurately is a challenge because viruses tend to evolve rapidly and as a result their genome changes quickly over time. Also, the genetics of the host has an impact on the diversity of these virus strains. The methodology to track these viruses from a sexual assault case involves collecting the samples from the male /female suspects and sequencing the HIV genome. This is compared to the HIV sequences of a control group from the same geographic area, which can determine the relationship of virus strains (5).
Forensic Entomology involves the study and use of insects in forensic investigations. It is used to estimate the time of death of a body and to determine if the corpse was moved in anyway. Traditionally, forensic experts use the morphological features of the insects to determine these details. However, there are a number of draw backs associated with morphological identification of specimens such as; damage to specimens can affect their morphology resulting in inaccurate results and the fact that rearing is time consuming (5).
A study conducted in 2002 was aimed at distinguishing species using DNA. The results of the study suggest that mtDNA can be used to identifying species in forensic entomology (17). Using DNA over morphological features in forensic investigations has many advantages which include that fact that DNA can be extracted even if the specimens die and it is a faster at identifying than rearing (5).
Legal aspect of DNA evidence
Even though DNA fingerprinting can identify the persons involved at a crime scene, DNA evidence cannot be used to prove that a person has committed a certain crime. But DNA evidence can be used to overturn previous serologically based guilty verdicts because of its higher discriminatory power (10).
Another legal issue relates to the developments of databanks which store DNA from known criminals. This can prove to be very useful for future forensic investigation because the DNA found at crime scenes could be compared to the DNA from the data banks which would most likely identify the criminal. However this system is considered to be a biased system because DNA from individuals with no criminal record is not collected. This implies that people with no criminal records will not commit any crimes in the future, which is clearly not true (18). This can lead to ethical issues.
Occasionally, defendants challenge the forensic laboratory in which the DNA tests were done. They can accuse the lab of not following appropriate protocols and procedures to accurately type samples. If this is proven to be true, then the DNA evidence against the criminal no longer holds and gives the defendant the opportunity for a second test by an independent laboratory (11)
Based on the material discussed above, it is clear that DNA is one of the biggest assets that forensic investigators have. It has a wide variety of uses in forensic investigations and a number of DNA profiling techniques can be used depending on the quality and quantity of the DNA found at the crime scene. DNA technologies used in forensic science investigations have improved significantly since the 1980s and still continue to do so due to ongoing research in the field.