The variation in length of the minisatellites is the basis of DNA fingerprinting.
DNA fingerprinting is a way of identifying a person using the genetic material stored in the cells (particularly the nucleus within the cell where the DNA is found). DNA forensic analysis relies on one key characteristic of DNA: the configuration is the same in all cells of an individual. Altogether each person carries around 200,000 genes in each cell, comprising three billion “base pairs” -the chemical building blocks of DNA. Analysing all these base pairs and genes in order to identify an individual would be impossible, with current technology. This is why DNA markers have been selected for forensic analysis.
Apart from forensics minisatellites can be used as DNA fingerprinting markers for paternity testing, Monitoring bone marrow transplants, determination of zygosity in twins, genetic mapping, linkage analysis and human identity testing. (8), (4), (3) , (2).
In forensic science it is important to obtain a sample (containing cells) from the crime scene if a crime has been committed or a blood sample from persons wishing to clear up a paternity issue with the courts, or even for personal identification purposes. Cell samples from a crime scene can be obtained in the form of blood; dried blood, body fluid stains and hair with the root sheath intact seminal fluid (in a rape case).(1) Usually, the best sample of seminal fluid comes from the swabs, as long as they are preserved properly. Depending on sample size accuracy is effected. PCR is therefore used when only the sample obtained is small. For use of minisatellites within the samples DNA, the sample needs to be large. (3)
Once the sample has been obtained from the crime scene it is packaged into a plastic or paper bag. It is then sent off to a forensic laboratory for analysis. The type of analysis in question is DNA fingerprinting. There are three methods of DNA fingerprinting which I have listed below: (3), (2).
(1) VNTR DNA core sequence- Hybridisation to numerous repeats creating an individual fingerprint. This method means pairs of alleles cannot be identify.
(2) Single locus VNTR probes- Profiling is based on 4-10 VNTR’s of which the gene frequencies are known. This method allows calculations of probabilities.
(3) Southern blotting is also used. Southern blotting is a technique for identifying a specific form of DNA in cells. DNA is extracted from the cells. Restriction enzymes are used to cut it into small fragments. The fragments are separated and a gene probe known to match the DNA fragment being studied is used. The probe is a radioactively labelled cloned section of DNA that is used to detect identical sections of nucleic acid by means of pairing between complimentary bases. The probe can then be seen and compared to the genetic fingerprints taken of possible suspects. The probability of a DNA fingerprint belonging to a specific person needs to be reasonably high--especially in criminal cases, where the association helps establish a suspect's guilt or innocence.
Below I have given advantages and disadvantages of DNA fingerprinting: (5)
Advantages:
- Using certain rare VNTRs or combinations VNTRs to create a pattern increases the probability that the sample obtained from the crime scene and the sample from the suspect match.
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Specimens that are years old dating to a time when DNA technology was not available can be tested, resulting in overturned convictions and release of the innocent (7)
Disadvantages:
- Errors occur in hybridisation and the probing process. This effects probability by lowering it.
- The wrong DNA may be amplified by mistake leading to a false conviction.
DNA fingerprinting not 100% assured, faults in can leads to the conviction of the wrong people. It is therefore imperative that when the sample is collected from the crime scene, the Wright one should be collected without contaminants, and if any doubt occurs whether or not the sample has been tampered with on collection that it be rejected immediately.
Conclusion
In conclusion minisatellites are short tandem repeats used in forensic science in a technique known as DNA fingerprinting. DNA fingerprinting is a well-established technique and uses small samples of DNA, which sometimes have to amplified if they are too small by PCR. However this technique does pose problems as errors can occur. To minimise these errors safety and standards need to be kept to a high level. If successful the technique can be used as a means to identify a person, and in co-operation with the forensic scientists the police can deal with the individual accordingly.
References-
1)
2) .washington.edu/fingerprint/blot.html
3) .washington.edu/fingerprint/vntrs.html
4) .washington.edu/fingerprint/apps.html
5) .washington.edu/fingerprint/problems.html
6)
7)
8) Molecular Cell Biology, third edition. Lodish . H., Baltimore . D., Berk . A ., Zipursky S . L ., Matsudaira . P. and Darnell . J.
1996 pages 320,321,322
