Dna Profiling –a Forensic ToolBy valeri On August 3, 2010 Under Criminal Profiling
Author: SREEKUMAR CHATHUAR
Every object, natural or manmade, has an individuality, which is not duplicated in any other object .It is unique. Neither the nature has duplicated itself, nor man can. This is the law of individuality, which is of fundamental importance in forensic science. Anything and everything involved in a crime has individuality. The culprit is unique, his modus operandi is unique, his weapon of offence is unique, scene of crime is unique, evidentiary clues, left over or picked up by the culprit, are unique. The main task of the investigator is to identify the uniqueness to link the crime with the criminal. It was a tedious task till the advent of the DNA Profiling technique .The advancement in biotechnology has gifted the forensic scientist with the most powerful genetic marker, the DNA, for individualizing and discriminating the biological evidence in forensic investigations. The term DNA Fingerprint was a trade mark name given by Cell mark Diagnostics, a company in America which licensed the technique, developed in U.K.Later this term was evolved to be replaced by the term DNA Profiling.DNA profiling is an absolute method for identification as on today for individualization of biological sample to the probability value of 99.9998 percent. The theory of the technique lies of the fact that each individual inherits DNA from its parents in the proportion of 50:50 DNA in one person does not match with other person in the world.
DNA - STRUCTURE AND UNIQUENESS.
DNA stands for deoxyribonucleic acid which is found in the chromosomes of the cells of living beings is the blue print of an individual.DNA determines the characteristics of the person such as the color of the skin ,type of hair, nails and so on. Using this genetic fingerprinting identification of an individual is done like in the traditional method of identifying fingerprints of suspects. This method of identification provides hundred percent precision. Structurally, DNA is a double helix-two thread like long strand of genetic material spiraled around each other. DNA is a polymer of deoxyribonucleotides composed of Base [Adenine (A), Thymine (T), Guanine (G), and Cytosine (C)], Sugar and a Phosphate. The sequential arrangement of the individual nucleotides is responsible for giving uniqueness to any individual living form, be it humans, animals, plants, or microbes. The DNA fingerprinting technique was developed by Sir Alec Jeffreys of the Leicester University Sir Alec first made his world-changing discovery by separating strands of DNA into different sizes and showing them as bands on a photograph. What first seemed to him to be ‘a complicated mess’ has now become invaluable for police investigation, ranging from settling immigration and paternity disputes to solving rape and murder cases. He observed that while large sections of DNA show similar base sequence for every human being as they are made of same body components, a few sections of human DNA are found varying from individual to individual. This observable variation in human DNA is called polymorphism. These polymorphic segments in the DNA molecule serve as a means to identify individuals. To determine the individuality the DNA has to be first extracted from the specimen. The DNA is found in every living cell of our body, such as blood, semen, hair roots, bone, saliva, urine, a postmortem specimen etc.
DNA PROFILING PROCEDURE.
The DNA is first extracted from a specimen by an elaborate chemical process. It is then mixed with a special enzyme called restriction endonucleasis which function as biological scissors which cut the DNA at specific sites .The restricted fragments are then separated according to size using gel electrophoresis technique. The fragments are put on a gel and electric current is applied. The shorter fragments move across on the gel faster towards the positive pole than the longer fragments. This separates the DNA fragments on the basis of their length. The next process is southern blotting. This process fixes the DNA fragments to the membrane firmly and in the same position. This is followed by hybridization process.Hybridisation involves bringing in contact a radioactive labeled probe , which is a particular type of DNA molecule that binds specifically with its complimentary base sequence present I the membrane. The probe bound fragments being
Radioactive can then be recorded on an X-ray plate .This process is known as autoradiography. The X-ray plate will show dark bands appearing very much like bar code. The comparison of the bands in the DNA profiles of different specimen from the same individual will show similarity whereas DNA specimen of two different individuals will show different DNA profiles. This is the DNA profiling technique. Most recently another technique known as polymerase chain reaction has been developed for DNA profiling. This method is recommended when there is insufficient DNA for analysis or is in degraded form. The PCR technique takes into account polymorphic DNA segments and through a polymorphic chain reaction produces multiple copies of a particular polymorphic DNA segment which would be sufficient to meet the requirements of DNA profiling test.
APPLICATION OF DNA PROFILING IN FORENSIC IDENTIFICATION.
If the DNA fingerprint of a person matches with that of a sample, it means that the sample has come from that person only. The probability of two persons except identical twins having the same DNA finger print is around 1 in 30 billion world population it means that DNA test gives the perfect identity .It is a very advanced science.
The main advantage of this method is its ability to analyze very minute and environmentally challenged samples and to accurately establish their origins with a high degree of certainty .DNA is much resistant to degradation caused by environmental conditions like temperature humidity time which lead to the growth of micro organisms .It generates the same genetic pattern irrespective of the biological material like hair ,seminal stains, fresh blood, soft tissue, hard tissue etc.In fact this unique feature makes it a powerful tool in forensic identification. DNA can be successfully obtained fro blood and blood stains, vaginal and anal swabs, oral swabs, clothing, bone teeth, most organs and to some extent urine. Material objects obtained from the scene of crime like liquor bottles, glasses, cigarettes, stamps and envelope flaps have all been found to provide varying amounts of DNA .This helps in linking the suspect with the crime scene.
Difficult problems connected with sexual offences can be solved by DNA profiling test. The vaginal swabs ,seminal stains and blood stains can be subjected to DNA technique for identifying the rapist or excluding the suspect with near certianity.In homicidal crimes , assault, hit and run accidents any type of body sample may be found at the crime scene such as hair ,teeth, saliva etc, though DNA may be insufficient in hair or saliva specimen by the application of the PCR technology, the offender can be identified.DNA profiling is used in identifying dead bodies in mass disasters, accidents etc.Likewise postmortem tissues or organs of the deceased can be identified by DNA test to establish the likely parentage of the deceased . DNA profiling is used in resolving paternity dispute which involve issues like maintenance suits, inheritance disputes or immigration problems. This technique has been used in Britain to screen immigrants claiming citizenship on the basis of their relations holding British citizenship. Cases which were closed due to lack of evidence can be re opened in the light of the DNA profiling technique.
Modi’s Medical Jurisprudence-22nd Edition, page 540 to 542.
Taylor’s Principle and Practice of Medical Jurisprudence.
Pantangi Balarama Venkata Ganesh v. State of A.P. 2003 CRI.L.J.4508.
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