The process of identifying an organism by means of sample of DNA through series of chemical test, distinguishing unique patterns present in its DNA is called as DNA fingerprinting. This process is creating a great buzz in the field of biology as we can use DNA fingerprinting from looking for cure for diseases to solving a criminal investigation. We have found vital information in application to research of mating systems, parental investment and cooperative behavior. We also have a huge prospect of measuring population differentiation which is becoming a reality due to DNA fingerprinting as we can make deduction about level of genetic dissimilarities within and between populations.
In DNA fingerprinting, a set of DNA fragments arrangement is required from specific DNA sample. There are various methods of DNA fingerprinting, but the most common way is to use PCR (polymerase chain reaction) for detection of fragments. This creates a large number of copies of DNA to make a DNA fingerprint. A DNA fingerprint is a pattern of bands on a photographic film where each band represent specific fragments from an individual’s DNA.
Brief History of DNA Fingerprinting
The first reference of DNA fingerprinting can be seen in Arthur Conan Doyle’s novel “A study in Scarlet” in 1886 and nearly 100 years later, in late 1984, this became a reality when Professor Sir Alec Jeffreys invented DNA fingerprinting while he was working at the University of Leicester in the Department of Genetics. He discovered human DNA had variations in the form of minisatellites. Minisatellites are short sequence of repetitive DNA that shows differences from one person to another than other parts of the genome (complete set of).
The first DNA fingerprint was produced using samples of human blood using molecular ‘scissors’ called restriction enzymes to cut the DNA. The DNA resulted were thousand in pieces with numerous length which were then separated using process called gel electrophoresis. A single strand of DNA was produced by sorting pieces of DNA and transferring them onto a robust piece of nylon membrane where they were “unzipped”. Radioactive probes were used to incubate nylon membrane. Finally, the nylon membranes were exposed to X-ray film to visualize the minisatellites which were attached by the probes.
Principle of DNA Fingerprinting
Every individual is unique even if 99.9% DNA of all human beings are the same. What makes us unique is that about 0.1% or 3 x 10 ^6base pairs (out of 3 x 10^9 bp) of our DNA are different. There are numerous inheritable sequences of small noncoding bases that every human genome possesses called satellite DNA that show polymorphism and their repetition are in tandem. Satellite DNAs are categorized into microsatellites and mini satellites based on length, base composition and numbers of tandemly repetitive units. Satellite exhibit polymorphism when mutation occurs with time that causes variations when the mutated non-coding sequences are piled up.
The physical length of DNA molecules has variations that can be shown by the junk DNA regions (about 95 % of our genetic DNA) made up of length polymorphism. There are variations between individuals in the number of tandem repeats at particular loci on the chromosome. The size of repeats determines the repeat regions. If there are 2-5 base pair repeats, we classify them as short tandem repeats (STRs)and if the number of repeats of base pairs are 9-80, we classify them as variable number of tandem repeats (VNTRs). These repeats are usually inherited by a child from his/her parents that makes them distinguishable which is the main principle of DNA fingerprinting.
Procedure of DNA Fingerprinting
- DNA Extraction
- DNA is released by breaking down the cells.
- Samples are usually collected by means of blood, hair, saliva, semen, body tissue cells, etc.
2. DNA Cutting
- We use restriction enzymes to cut the DNA into small fragments.
- The DNAs are cut at a particular base sequence at individual restriction enzymes.
- DNA fragments are separated on the basis of their size.
- We load DNA samples at one end of indentations (cavity) and on other end we apply electric current.
- Electric currents results DNA samples to move towards positive electrode. Short fragments move faster while the large ones move slower.
4. Southern Hybridization
- Nylon membrane or filter paper are used to transfer DNA fragments.
- An alkaline solution splits DNA into single strands.
- Finally, DNA is bounded by radioactive probe in solution.
- Filter paper or nylon membrane are placed on the x-ray film.
- Dark spots are made on the film by the radioactive probes.
Advantages of DNA Fingerprinting
- Affordable and reliable technique
- Easy and less time consuming
- No age boundary for testing
Disadvantages of DNA Fingerprinting
- Samples should be handles with much care.
- Complex process and difficulty may occur during interpreting results.
- Sometime difficult to maintain result confidential.
Applications of DNA Fingerprinting
Some of the major applications of DNA fingerprinting are:
- Reveal biological identity of a person.
- Dead bodies that are badly damaged can be identified.
- Paleontology, archaeology, etc.
- Diagnosis of medical conditions such as inherited disorders and even AIDs.
- DNA fingerprinting as a forensic tool for crime scene investigation and criminal verification.
- STR analysis to identify blood relatives in transplantation procedures such as allogenic bone marrow transplantation, organ transplantation, etc.
- Identify maternal cell contamination during pregnancy genetic testing through VNTRs and STRs markers with PCR-gel electrophoresis.
Since the development of DNA fingerprinting nearly 25 years ago, it has played a major role from identifying someone to a crime to saying someone’s life in one way another. There has been advances in DNA fingerprinting and countries are desiring to create a DNA database that could speed up and simplify any process whether finding a link to a crime, identifying bodies that are badly damaged, finding blood relatives for organ transplantation. Not only this, but it can also help identify evolutionary change and relationship on the molecular level in biological classification.
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