Bulletin Spring‧Summer Autumn‧Winter 1999

DNA Restriction enzyme DNA Fragments Gel electrophoresis Southern transfer Hybridization Exposure to X-ray film DNA Finger-print which may code for less essential features such as th e colour of the eyes. In such areas limited variations can be tolerated. Over 90 per cent of our genes are essential and therefore DNA sequences do not vary significantly from individual to individual. I n 1985, Alec Jeffreys discovered regions in the human DNA where certain sequences of base pairs repeat themselves over and over again. Because such sequences do not seem to be coded for anything essential, nature has allowed a higher degree of variation among individuals : the units of repeats within such DNA regions vary significantly in number and length from person to person. The name 'hypervariable tandem repeat' (VNTR) has thus been coined for such regions. By examining the length of the VNTRs, we can assess if a certain DNA sample belongs to a certain individual. Thi s is the molecular basis of DNA fingerprinting. In the laboratory, a technique called 'restriction fragment length polymorphism' (RFLP) is used to determine the length of a specific VNTR in the DNA. DNA is first extracted from a specimen by a standard procedure and is cut into short fragments by a selected restriction enzyme. The fragments are then rearranged according to length by electrophoresi s and transferred to a nylon filter by a procedure called Southern blotting. The filter is then immersed in a solution of a radioactively labelled geneprobe which can search and bind itself to the fragments carrying the particular VNTR. The positions where the probe interacts with the VNTR fragments will become visible on photographic film when it is placed onto the filter and ‘exposed’ by the radiation from the probe. The length of the restricted VNTR fragments can thus be easily obtained by comparison wit h known standard markers. Since different individuals will have different lengths of VNTR restriction fragments, it i s easy to determine if the DNA belongs to the same person just by comparing the photographic patterns. This procedure can be repeated by cutting the DNA with other restriction enzymes and reprobing with different VNTR probes. The likelihood of two individuals havin g a coincidental match becomes less and less as more patterns are compared. This is the underlying reason why DNA fingerprinting has been claimed to have such a high discriminating power. DNA fingerprinting is a very powerful identification technique in forensic work. To perform a DNA fingerprint, all one needs is a sample of livin g or dead cells from the suspect. With modem procedures in DNA amplification, the number of cells needed can be as small as ten. The DNA fingerprint of an individual remains the same no matter what type of tissue is used. Thus, the pattern obtained from a semen stain will be identical to that obtained from blood or saliva. Moreover, as the DNA structure remains constant throughout life, a DNA fingerprint obtained at birth will be the same as that obtained at death. There is n o way an individual can change his DNA pattern once he has been bom with it. 13