DNA FINGERPRINTING AND ITS MEDICAL IMPORTANCE

A DNA fingerprint is the pattern of fragmented DNA molecules obtained from electrophoresis. Restriction enzymes are used to cleave DNA molecules into specific fragments that can easily be analyzed. Examples of restriction enzymes include EcoR1 , mst II ,Alu I ,Taq I and so on. DNA fingerprinting is also called DNA typing or DNA profiling and is based on sequence polymorphisms.




Variations that occur in the pattern of restriction fragments produced by the same restriction enzyme from DNA of different individuals of same species is termed restriction fragment length polymorphisms (RFLPs). RFLP analysis is therefore invaluable in Forensic Medicine and in screening for mutations and genetic diseases such as sickle cell anaemia.




In Forensic Medicine, restriction fragment length polymorphism analysis can distinguish one individual from millions of people. DNA obtained from congealed blood ,hair follicles and seminal fluid can be used to catch culprits in criminal investigations such as murder, rape, and theft respectively.
The use of techniques such as complementary DNA radiolabeled probes , southern transfer technique, and restriction enzymes are very essential in detecting and screening for mutations and genetic diseases.




The detection of RFLPs utilizes a specialized hybridization technique termed Southern Blotting or Southern Transfer. DNA fragments from digestion of genomic DNA by restriction endonucleases or enzymes by size (in kilobases) electrophoretically. These fragments are denatured by soaking the agarose gel in alkali, and then blotted onto a nylon membrane to reproduce the distribution of fragments in the gel. The membrane is then placed in a solution containing a radioactively labeled DNA probe. Autoradiography is then used to reveal the fragments to which the radiolabeled DNA probe hybridizes.




It is pertinent to note that the number of repeated units in these DNA regions differs in humans (except between identical twins.). Combining the use of several radiolabeled probes makes DNA fingerprinting so selective that it gives accurate results.




However ,the southern transfer technique requires relatively fresh DNA samples and larger amounts of DNA are generally present at a crime scene. RFLP analysis sensitivity is complemented by the use of polymerase chain reaction (PCR) which helps to amplify small amounts of DNA. Forensic investigators can easily obtain DNA from hair follicle, congealed blood ,seminal fluid ,vaginal secretions from victims and suspects to carry out their investigations. Accurate results can be obtained from samples that have stayed for months or even years.




Results obtained from DNA profiling have been useful in convicting criminals and acquitting suspects. Murder mysteries have been solved and paternity disputes have been amicably resolved. An interesting discovery that was achieved with DNA fingerprinting was the discovery and indentification of the bones of the last Russian Czar and his family who were assassinated in 1918 by Forensic investigators in 1996.