There are several methods for examining DNA to determine if two samples are the same or different from one another. DNA fingerprinting is a term that is sometimes used to describe this process. A cloned bit of DNA can be analysed in the laboratory to see if it has any sections in common with another piece of DNA, and therefore if they overlap with one another. The collection and analysis of DNA samples can be performed in a different location, such as a crime scene, to evaluate whether or not they match DNA samples taken from suspects of the crime. There is a very high statistical possibility that two DNA samples came from the same person if their fingerprints are the same in both samples. A similar strategy can be used to establish paternity in some cases.
DNA Fingerprint
- Professor Sir Alec Jeffreys invented DNA fingerprinting in 1984 after he realised that changes in human DNA, in the form of minisatellites, could be detected and identified.
- DNA fingerprinting is a technology that detects a large number of minisatellites in the genome at the same time, resulting in a pattern that is unique to each individual. This is an example of a DNA fingerprint.
- In the absence of identical twins, there is an extremely small chance of having two persons with the same DNA fingerprint who are not identical twins.
- Your DNA fingerprint is similar to your physical fingerprint in that it is something you are born with and is unique to you.
It is necessary to first get a sample of DNA-containing cells such as skin, hair, or blood cells before beginning the process of producing a DNA fingerprint. The DNA is taken from the cells and purified before being utilised. Based on restriction fragment length polymorphism (RFLP) technology, the DNA was then sliced at certain spots along the strand by proteins known as restriction enzymes, according to Jeffreys’s original approach. To sort the enzyme fragments, they were placed on a gel and subjected to an electric current (electrophoresis): the shorter the fragment, the more quickly it moved toward the positive pole of the electrophoresis gel (which was used to sort the enzyme fragments) (anode). Once the double-stranded DNA fragments had been sorted, they were exposed to a blotting procedure, in which they were separated into individual strands and transferred to a nylon sheet. The fragments were subjected to autoradiography, in which they were exposed to DNA probes, which were pieces of synthetic DNA that had been rendered radioactive and had been linked to the minisatellites. After that, a piece of X-ray film was exposed to the pieces, and a dark mark was generated at each spot where a radioactive probe had attached itself. The pattern of markings that resulted from this process might then be studied.
Polymerase Chain Reaction (PCR) based approaches and so-called microsatellites (also known as short tandem repeats, or STRs) have largely replaced the assay developed by Jeffreys. Microsatellites are repeat units that are shorter in length (typically 2 to 4 base pairs in length) than minisatellites and are therefore more sensitive (10 to more than 100 base pairs in length). PCR is a technique for amplifying a desired DNA fragment (for example, a certain STR) many times over, resulting in thousands of copies of the desired segment. It is an automated method that uses only minimal amounts of DNA as starting material and can be used with DNA that has been partially damaged or even destroyed. Once an adequate amount of DNA has been created by PCR, the exact sequence of nucleotide pairs in a section of DNA can be established by utilising one of several biomolecular sequencing methods, which are described in detail later in this chapter. As a result of advances in automated DNA sequencing technology, a wide range of new practical applications have become possible, including the identification of genetic disease-causing segments of genes, mapping of the human genome, engineering drought-tolerant crops, and the production of biological drugs from genetically altered bacteria.
First DNA Fingerprint Procedure
- DNA fingerprinting began with the extraction of DNA from a sample of human material, typically blood.
- DNA was sliced using molecular scissors called restriction enzymes. This resulted in hundreds of DNA fragments of varying lengths.
- These fragments of DNA were then sorted by size using a technique called gel electrophoresis?
DNA was placed into wells at one end of a porous gel that functioned similarly to a sieve.
A current was delivered to the gel, which drew the negatively charged DNA through.
The shorter DNA fragments passed through the gel the most easily and thus the fastest. Because longer bits of DNA have a harder time moving through the gel, they travel slower.
As a result, when the electric current was turned off, the DNA fragments were segregated according to their size. The smallest DNA molecules were located furthest away from the loading site for the original sample on the gel.
- After sorting the DNA, the fragments were transferred or ‘blotted’ from the delicate gel onto a durable nylon membrane and then ‘unzipped’ to obtain single strands of DNA.
- Following that, radioactive probes were incubated with the nylon membrane.
Small bits of minisatellite DNA is tagged with radioactive phosphorus to serve as probes.
The probes only bind to sections of DNA that are complementary to them – in this case, they bind to the genome’s minisatellites.
- The probes’ associated minisatellites were then visualised by exposing the nylon membrane to X-ray film.
When the film was subjected to radioactivity, a pattern of over 30 dark bands developed where the tagged DNA was located. This was the DNA fingerprint pattern.
To compare two or more distinct DNA fingerprints, the different DNA samples were electrophoretically separated side by side on the same gel.
Conclusion
The use of DNA fingerprinting in legal conflicts dates back to the 1980s when it was used to aid in the investigation of crimes and the determination of paternity. Since its inception, DNA fingerprinting has resulted in the conviction of a large number of criminals as well as the release from jail of a large number of individuals who were wrongfully imprisoned. Making scientific identification correlate exactly with legal proof, on the other hand, is sometimes a difficult task. Even a single allegation that there is a chance of error can be enough to persuade a jury not to condemn a defendant in certain circumstances. The contamination of samples, incorrect preparation techniques, and errors in the interpretation of results are all key sources of error in clinical trials. Aside from that, RFLP requires vast amounts of high-quality DNA, which restricts its application in forensics to specific situations. Forensic DNA samples are frequently deteriorated or taken after the death of the subject, resulting in lower-quality samples that are less likely to yield trustworthy results than samples received from a living individual. With the introduction of PCR- and STR-based techniques to DNA fingerprinting, some of the worries about DNA fingerprinting, and notably the use of RFLP, have been alleviated.