One of my favourite pass times these days on Facebook is a game called criminal case. One of the various tools used to nab the criminal there is of DNA profiling and it set me thinking as to really can we find out the green colour eyes or blonde hair that the game lets us find out. Is DNA fingerprinting the solution to so much? How does it work?
What is DNA?
DNA stands for Deoxy ribo Nucleic Acid. Famously recognized by its intertwining double helical structure, the DNA is the basic code to an organism’s individuality. The DNA is embedded deep in a cell’s nucleus and is the source code which works as a template for amino acid synthesis through a process comprising of transcription and translation. Transcription is the copying of the code on to a RNA from the DNA, and then translation is the amino acid synthesis in ribosomes of a cell from the code in the RNA. Now amino acids work as the building block for proteins which are the primary messenger’s within the body. Thus the DNA controls the messenger system of the body, which therefore influences the way the body functions. The same process is essential in cell division to retain the gene code of the original cell in the new daughter cells. By maintaining the same cell code in a multicellular organism, the individuality, including quirks like colour, disease and such are created and maintained.
Basis of DNA fingerprinting
DNA fingerprinting as a forensic technique owes its origins to Alec Jeferreys, and is based on the concept of Variable Number Tandem Repeats (VNTR)sequence within a DNA string. Often referred to as mini-satellites these VTNRs are found to be repeated with in a gene in a unique pattern and do not contribute to the functioning of the genes. However the pattern of repetition is generally unique, though becomes similar as the relation of the people providing the DNA becomes nearer and/or stronger. Thus if you are related you may have common mini-satellites, and this pattern grows stronger in blood relatives, such that it might be identical in identical twins. However in 99.99% this may be used to determine a unique pattern in an individual.
Establishing a DNA fingerprint
The primary process of establishing a DNA fingerprint starts with obtaining a sample of DNA. This is ideally done from the source say through a buccal swab. Blood samples or other cell/tissue samples from items of personal use may also be used. Once the tissue is isolated, the DNA is extracted from it and purified. A typical technique referred to as restriction fragment length polymorphism (RFLP) then uses an enzyme is used to cut the DNA at specific points and then sorted by means of gel electrophoresis. The double stranded DNA is then split into single strands and the mini-satellites are tagged with radioactive synthetic DNA, and then exposed to an X-ray film. This leads to the identification of the mini satellites and their pattern. This establishes a reference against which the test sample may now be compared.
Polymerase Chain Reaction (PCR) is an alternative technique that mimics the natural DNA replication process, though limits it to a specific sequence of interest. This not only allows a referencing and identification tool, but also helps in reconstruction from partial decomposed/damaged sequence or very small samples. PCR uses an amalgamation of denaturation of DNA sample to polynucleotide, hybridization from a primer to replicate the sequence and then extension to multiple copies.
Once a referral DNA profile is created, it can be used to compare against an alternative sample to identify the person, to establish relation in cases of paternity suites, lineage and such. Anomalies in terms of diseases amy also be identified using this DNA database to help determine characteristics which are manifested in an individual and thus help in identification or tracking of a suspect.
Considerations when using DNA fingerprinting
One of the most important considerations while using DNA fingerprint as a basis of evidence is that this is an extremely critical and complex technique, where contamination of sample may lead to a completely awry result. Thus the possibility of a wrong result is a definite reality. The probability of a match when stated in numbers as 1 in 5 milllion often exposes, to statistical sample size questions. Thus for a country like india with a population of over a billion, this match would mean more than 20 people could match the same DNA profile. Therefore it is always suggested that the DNa profile should always be considered in conjunction with other evidence and not just as a deciding tool on a standalone basis. Another major consideration is of identical or monozygotic twins. When a zygote formed after the fusion of an ova and a sperm divides to form two embryos in the embryo, such that both develop and are given birth, we have a pair of identical twins. Since they are originated from the same zygote, they have an identical match of their DNA code. DNA fingerprinting will not work in such a case, and the greater circumstances of the case needs to be taken into account.
Thus my musings on DNA fingerprinting lead me to believe that while it is a defining tool in the hands of todays forensic scientist, it still is not a standalone crime busting tool. In terms of its usefulness in building up the identity of an individual or using certain quirks in his gene pool to identify or set him aside it is indeed useful. However there is still no alternative of the proverbial legwork that an investigating officer needs to do to tie up everything long with the evidence provided from the DNA profile. So back to my game of criminal case I go.
