Provided its uniqueness, DNA has found many uses in the medical and criminal investigation fields. In the medical field, scientists use DNA analysis to determine paternity. In the criminal investigation field, scientists use DNA analysis to identify suspects in criminal investigations. Nonetheless, identification of people using DNA analysis is not foolproof.
DNA testing, also known as DNA profiling, came because of two separate breakthroughs in molecular biology. Kary Mullis invented the Polymerase Chain Reaction in the USA while Professor Sir Alec Jeffreys discovered DNA fingerprinting at the University of Leicester in the UK (Kilpatrick).
Professor Sir Alec Jeffreys was the first to employ DNA analysis in a criminal investigation in 1986. He used DNA fingerprinting techniques in two rape and murder cases that took place in 1983 and 1986 in Leicestershire, UK. Officers from a small local police service approached Professor Jeffreys to assist them in solving two potentially linked rape and murder cases of two young girls. Professor Jeffreys analyzed samples from both girls in the two murder cases and came up with DNA profiles of the culprits in both cases. The DNA profiles were identical, and this led them to the conclusion that they were dealing with one culprit for both cases (Prime, and Newman 12). A mentally challenged porter, 17 years of age, had earlier confessed to commit one of the murders. However, when Professor Jeffreys took his DNA profile and compared it to the DNA profile of the culprit obtained earlier, they did not match.
The police had to find the perpetrator. The police organized the first ever screening project that was carried out for DNA profiling. They obtained blood samples from the close to 5,000 men living in a small community where the murders had taken place. The samples, unfortunately, did not identify the suspect directly. The suspect, Colin Pitchfork, had asked one of his friends to give a blood sample on his behalf. However, the friend eventually gave in and admitted he had stood in for Pitchfork. Police arrested Pitchfork and his DNA profile obtained from his blood sample. His DNA profile matched the DNA profile obtained by Professor Jeffreys in the two murder cases. The probability of such a match occurring was 5.8 x 10-8, and this provided credibility to his conclusions. His findings led to the conviction of the culprit, Colin Pitchfork, and the exoneration of the innocent man who was previously the main suspect in the crimes (Prime, and Newman 13).
Though Professor Jeffreys’ method was accurate and reproducible, it required large quantities of high quality DNA, which forensic scientists did not always recover during forensic investigations. Further research on DNA analysis led to two major breakthroughs in the 1980s and 1990s. These breakthroughs formed the basis for modern DNA analysis techniques that are in use today.
The concept behind DNA testing is finding the differences in the DNA sequences of two or more samples of DNA. Thus, scientists use two or more sets of DNA in the analysis. In forensics, these samples are samples of DNA from the crime scene compared with DNA samples from suspects. The initial step in testing varies depending on the testing techniques. However, the first step is normally the isolation of a small part of a DNA sequence. Forensic scientists then isolate the DNA fragments and sort them based on size by gel electrophoresis (Whiting). The forensic scientists then make a blot of gel, and they release a probe onto the sample, which finds and binds onto a DNA sequence that is similar with that which is associated with the probe (Jones 527).
The common techniques used in DNA analysis include Restriction Fragment Length Polymorphism (RFLP), Polymerase Chain Reaction (PCR), Short Tandem Repeats (STR), and Amplified Fragment Length Polymorphism (AmpFLP).
Restriction Fragment Length Polymorphism (RFLP) is a widely used method of isolating DNA. It requires large quantities of undegraded DNA. Because of this, this method is ideal for fresh crime scenes that have a lot of DNA evidence. Once scientists obtain adequate samples, they introduce restriction enzymes to isolate the AATT sequence. Scientists carry out gel electrophoresis on the fragments, make a blot, and probe the sample as mentioned earlier. The scientists analyze the sizes of the DNA fragments that the probe has found. They then compare the results of each DNA sample. Forensic scientists declare a match if the sizes of the two bands (bands of DNA from the crime scene and suspect’s DNA) are within five percent of each other.
Polymerase Chain Reaction (PCR) is a method of DNA amplification, which has the ability to use information from minute and degraded DNA samples. This method amplifies specific parts of DNA. The method makes use of two short DNA pieces known as thermostable DNA polymerase and oligonucleotide primers to isolate the part of DNA, which scientists copy. The scientists carry out gel electrophoresis as with Restriction Fragment Length Polymorphism (RFLP). In Polymerase Chain Reaction, contaminated DNA samples may bring about false results. Therefore, scientists only use uncontaminated samples.
Short Tandem Repeats refer to tiny regions of short repeated sequences. In these regions, DNA varies immensely between people who are not related. In this method, scientists seek to identify these regions in both samples and compare the two regions from the two samples. Scientists can achieve higher accuracy by isolating and testing many short tandem repeats. The amplifying effects of Polymerase Chain Reaction make it the ideal method for isolating short tandem repeats.
The Amplified fragment length polymorphism makes use of the variable number tandem repeats to differentiate alleles. Unlike in other tests where the criterion of separation of DNA sequences is size, scientists use polyacrylamide gel to separate alleles. This method is cheaper than the other methods and is highly automated.
The development of DNA testing has revolutionised the justice system in the US. It has led to close cooperation between forensic services and police services (Prime and Newman 11).DNA testing assists detectives in solving difficult cases where other investigative techniques prove insufficient. DNA testing can help provide clues where no witnesses are forthcoming, and it can drastically reduce wrongful arrests. More importantly, DNA testing makes evidence more reliable. It is interesting that the first use of DNA to solve crimes led to the exoneration of the main suspect of the crimes, who had earlier confessed to committing one of the crimes (Prime and Newman 13).
DNA testing has helped solve a wide variety of cases ranging from forced entry and homicide to gang crimes, sexual assaults, and murders. A study carried out by the National Institute of Justice in the United States sought to determine the reliability of DNA evidence as a technique of solving crimes. The study involved examining cases and the availability of evidence for DNA profiling. In 55% of the cases that had biological evidence, researchers were able to identify a DNA profile. Of those cases that investigators fed into the DNA database, 41% provided a lead in identifying the suspect (Johns and Rushing). DNA testing in investigations led to two-and-a-half more success rates as compared to traditional investigation methods. The rates of arrest and prosecution were double with DNA testing (Johns and Rushing).
In spring 2003, Holly Jones, a 10-year-old girl, went missing from her neighborhood in Toronto. The days that followed her disappearance, investigators found parts of her mutilated body on the shores of Lake Ontario (Prime and Newman 13). Investigators positively identified these body parts as belonging to Jones through DNA typing. Moreover, the investigators developed a foreign DNA profile, most probably belonging to the perpetrator, from fingernail scratchings they obtained from the body parts.
The investigation of this case posed a great challenge to the Toronto Police Service. The lead investigators had the option of undertaking door-to-door canvassing in order to identify possible sexual offenders, whose number ran into hundreds. They had also received more than 2,000 tips from the public. The huge amount of work awaiting the investigators led them to considering forensics to assist them in solving the case. A trace evidence scientist at the forensic science laboratory examined the victim’s tapings, which led the investigators to the clue that the victim had been in touch with a green carpet (Prime and Newman 12). With this lead, police officers went on a door-to-door canvassing. Police officers identified a potential suspect. The suspect, however, declined to give a voluntary DNA sample to assist the police officers in the investigation. Undercover police officers were able to obtain DNA samples from the suspect, which the investigators compared to the DNA samples obtained from the body parts. The DNA profiles matched.
The police officers apprehended the suspect, and they searched his apartment. DNA analysis showed traces of Jones’ blood in the suspect’s apartment. With the amount of evidence obtained, the suspect entered a guilty plea (Prime and Newman 12).
Today, countries have established DNA databanks to help investigators with information with possible suspects. In Canada, the DNA databank had an almost immediate effect. With the formation of the National DNA databank, investigators reopened a 12-year-old unresolved case, which had involved more than 1,000 suspects. The police had arrested the culprit on an unrelated crime. When investigators fed the culprit’s DNA profile into the databank, the investigators found important leads that guided them to solving the 12-year-old case.
The future of DNA profiling will test the delicate balance between law enforcement needs and the public’s right to privacy. Scientists are developing techniques to predict a person’s physical traits through DNA analysis. This will provide investigators with a genetic “eyewitness” (Prime and Newman 13). In addition, technology on identifying culprits through kinship relationships is under research. The future of DNA testing is both delicate and fascinating, and it will require a cautious approach. DNA testing, when used properly, has great benefits to crime investigators and the society in general.
Johns, Susan, and Patricia Rushing. “Using DNA to Solve Property Crimes.” Community Policing Dispatch 2.12 (2009): n. pag. Community Policing Dispatch. Community Policing Dispatch, Dec. 2009. Web. 21 Mar. 2014. .
Jones, Stephen T. “DNA Profiling Cross-examined DNA Technology in Forensic Science.” Trends in Biotechnology 11.12 (1993): 527-28. Print.
Kilpatrick, Robert. “History of DNA Profiling.” University of Leicester. University of Leicester, n.d. Web. 15 Mar. 2014. .
Mandal, Ananya, MD. “What Is DNA?” News Medical. News Medical, 13 Dec. 2013. Web. 15 Mar. 2014. .
Prime, Raymond J., and Jonathan Newman. “The Impact of DNA on Policing: Past, Present, and Future.” The Police Chief 74.11 (2007): 11-13. The Police Chief. The Police Chief, Nov. 2007. Web. 21 Mar. 2014. <http://www.policechiefmagazine.org/magazine/index.cfm?fuseaction=archivecontents
Separating Binary Mixture of Naphthalene and Biphenyl
Napthalene and biphenyl mixture is a binary mixture of substances where the substances are very similar to each other as can be observed from their structures. Due to their similarities in physical properties their separation cannot be accomplished by simple crystallisation solvent extraction. Therefore, in such case, there must be a chemically modification of one of the components to render it significally different from the other constituents. Then separation of the derivative from the mixture; and finally the derivative is transformed back from the mixture.
When several benzene rings are fused together to give more extended pi systems, the molecules are called polycyclic benzenoid or polycyclic aromatic hydrocarbons (PAHa). In these structures two or more benzene rings share two or more carbon atoms.
The fusion of one benzene ring to another results in a compound called naphthalene. Further fusion can occur in a linear manner to give anthracene, tetracene, pentcacene, a series called the acenes. Angular fusion (annulation) results in phenanthrene, which can be further annulated to a variety of benzenoid polycycles.
In contrast to benzene, which is a liquid, naphthalene is a colourless crystaline material. It is best known as a moth repellent and insecticide. The spectral properties of naphthalene strongly suggest that is shares benzeneâ€™s delocalised electronic structure and thermodynamic stability. Basically when observing spectra and peaks, it is concluded that the electrons in napthalene are delocalised more extensively than in benzene. Therefore the added four pi electrons enter into efficient overlap with those of the attached benzene ring. Hence, several resonance forms can be drawn:
Resonance Forms of Naphthalene
Naphthalene is an example of a polycyclic aromatic molecule where resonance forms are used to describe the reactivity of these molecules. Resonance is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula. Is is noted that a molecule or ion with such delocalized electrons is represented by several contributing structures, called resonance structures or canonical forms.
Basically the aromatic character of naphthalene is proven in this reactivity. Naphthalene undergoes electrophilic substitution rather than addition. For example, treatment with bromine, even in the absence of a catalyst, results in smooth conversion into 1-bromonaphthalene. The mild conditions required for this process prove that naphthalene is activated with respect to electrophilic aromatic substitution. It is noted that these reaction can occur in the absence of a catalyst. However if benzene instead of naphthalene, an aluminium chloride catalyst is required for the reaction to occur. Similarly, whereas both benzene and naphthalene can be alkylated using Friedel-Crafts reactions, naphthalene can also be alkylated by reaction with alkenes or alcohols, with phosphoric acid as the catalyst.
Regarding electrophiles, it is noted that these attack substituted naphthalenes regioselectively. This basically refers to the preference of one direction of chemical bond making or breaking over all other possible directions. It is noted that an activating group usually directs the incoming electrophile to the same ring anda deactivating group directs it away. This ring carrying the substituent is the most affected.
It is important to also mention the picric acid apart from the naphthalene. This is because this acid was useful for the formation of the derivative, solid naphthalene picrate. Picric acid is actually the chemical compound called 2,4,6-trinitrophenol (TNP). This yellow crystalline solid is one of the most acidic phenols. It is an explosive, like other highly nitrated compounds such as TNT. Picric acid is called acid because of the unusually high acidity of its hydroxyl group (pKa 0.38) which is increased beyond that of acetic acid (pKa 4.7) and even hydrogen fluoride (pKa 3.2) by the electron-withdrawing effect of the three nitro groups. It is noted that this property was in part responsible for its replacement by TNT in military uses.
Moving on to the derivative formed, one should first say that a picrate is actually a salt or an ester of picric acid. However, it is important to say that it could also be an additional compound which picric acid forms with many aromatic hydrocarbons, as in this case in this experiment, where the aromatic hydrocarbon is naphthalene. Hence, additional compounds are also called picrates, even though they are not a salt of picric acid.
Discussing the results obtained in this experiment it is noted that no data of the results obtained could be recorded for the biphenyl. This is because the biphenyl did not crystallise. For recrystallisation the solubility of the substance is required to be high in the hot solvent but low when it is cooled, so that the desired product will crystallise whilst impurities remain dissolved. However, this did not happen for the biphenyl. After cooling in ice-cold water for several minutes, it did not crystallise. A reason for this could be due to the solubility of the biphenyl, which is found to be very soluble at room temperature and so most of the biphenyl remained dissolved. Hence poor recovery is obtained such that the biphenyl was discarded. Hence no vacuum filtration was carried out since no crystals were formed.
One should say that in order to increase the yield, a second miscible solvent could be added. This would alter the solvent polarity sufficiently to decrease the solubility of biphenyl. As the second solvent diffuses into the mixture, the biphenyl should crystallise out.
Sources of error:
The naphthalene crystals to be collected on the Hirsch funnel simply passed through the filter paper into the Buchner flask during vacuum filtration. This might occurred due to the crystals not being formed completely or due to the crystals which might dissolved a bit in the methnaol (solvent) during the process and so passed through, even though a minimum amount of solvent was used. The process was then repeated so as to collect as much product as possible, and the yield claculated was quite reasonable.
The solid naphthalene picrate crystals formed and collected on a watch glass were not completely dry when they were weighed and the melting point temperature range was determined. This was due to lack of time available. Hence the mass obtained might varied a bit and was slightly greater due to some solvent still present. This was noted during the melting point determination process where the powdered sample was very difficult to be tapped down to the bottom of the melting tube. It just kept being blocked in the middle within the tube, because it was not completely dry.
Conclusion It was concluded that the naphthalene (50 % w/w) and biphenyl mixture was separated and the separation involved the formation of a derivative of naphthalene, a solid napthalene picrate, which was isolated from biphenyl. It was also concluded that at the end of the day, it was the individual components which were required to be obtained individually, i.e. the purified biphenyl and the purified naphthalene which was reconverted from picrate. Hence since this was achieved the experiment was quite successful since the aimwas reached.