DNA is a chemical structure that forms chromosomes. A piece of a chromosome that dictates a particular trait is called a gene. Structurally, DNA is a double helix: two strands of genetic material spiralled around each other. Each strand contains a sequence of bases, also called nucleotides. A base is one of four chemicals, adenine, guanine, cytosine and thymine. The two strands of DNA are connected at each base and each base will only bond with one other base. Adenine (A) will only bond with thymine (T) and guanine (G) WILL ONLY BOND WITH CYTOSINE (C). If a one strand of DNA looks as follows: A-A-C-T-G-A-T-A-G-G-T-C-T-A-G. The DNA strand to this will look as follows: T-T-G-A-C-T-A-T-C-C-A-G-A-T-C. Together, the section of DNA would be represented as follows: T-T-G-A-C-T-A-T-C-C-A-G-A-T-C
A-A-C-T-G-A-T-A-G-G-T-C-T-A-G.
DNA strands are read in a particular direction, from the top to the bottom, in a double helix the strands go opposite ways.
The chemical structure of every ones DNA is the same. The only difference between people is the order of the base pairs. There are so many millions of base pairs in each persons DNA, that every person has a different sequence. Using these sequences, every person can be identified solely by the sequence of their base pairs. However, because there are so many millions of base pairs, the task would be time consuming. Instead, scientists are able to use a shorter method, because of repeating patterns in DNA. These patterns do not, however, give an individual “fingerprint,” but they are able to determine whether two DNA samples are from the same person, related people or non-related people.
Scientists use a small number of sequences of DNA that are known to vary among individuals and analyse those to get a certain probability match. Isolating the DNA in question from the rest of the cellular material in the nucleus is prepared. This can be prepared chemically, by using a detergent to wash the extra material from the DNA or repeatedly by applying a large amount of pressure to squeeze out the DNA. The pieces of DNA are then into a gel, such as agarose, and an electrical charge is applied to the gel, with negative charge at the top and the positive charge at the bottom. Because DNA has a slightly negative charge, the pieces of DNA will be attracted towards the bottom of the gel; the smaller pieces will be able to move more quickly towards the bottom than the larger pieces. The different sized pieces of the DNA will therefore be separated by size, smaller pieces towards the bottom and larger towards the top. Denaturing the DNA, so that the entire DNA is rendered single stranded, can be done either by heating or chemically treating the DNA in gel. Then the gel with size fractioned DNA is applied to a sheet of nitrocellulose paper, and then baked to permanently attaches the DNA to the sheet. This procedure is called “Blotting the DNA,” once completed the Southern Blot can now be analysed. In order to analysis a Southern Blot, a radioactive genetic probe is used in a hybridisation reaction with DNA. If an x-ray is taken of the Southern Blot after a radioactive probe has been allowed to bond with the denatured DNA on the paper, only the areas where the radioactive probe binds will show up on the film. This allows researchers to identify, in a particular person’s DNA, the occurrence and frequency of the particular gene pattern contained in the probe.
Every strand of DNA has pieces that contain genetic information, which informs an organism’s development (exons) and supplies no relevant genetic information at all (introns). Introns contain repeated sequences of base pairs called Variable Number Tandem Repeats (VNTRs) and contain from twenty to one hundred base pairs. Every human being has some VNTRs and a particular VNTR can be determined by performing a Southern Blot. The pattern that results from this process is what is referred to as a “DNA fingerprint”. Because VNTR patterns are inherited genetically, a person’s VNTR pattern is unique. The more VNTR probes used to analyse a person’s VNTR pattern, the more distinctive and individualized that pattern or DNA fingerprint will be.
Forensic Science is essentially the application of science to law, using highly developed technologies to uncover scientific evidence in a variety of fields. DNA is not going to change from one individual to another and point to the incorrect individual. If a sample is deficient, there will be no result at all. Whilst a conviction based exclusively on DNA evidence is rare, DNA testing has proven to be influential in freeing convicted prisoners, who have been wrongly convicted. In Britain, for example, 16 points of likeness have got to be found to confirm a match. This amount varies in different countries. As the quantity of fingerprint records has grown, a computer system has been developed which is able to store the records and compare more than 60,000 fingerprints per second.
Developments in DNA testing have pushed the boundaries of science and in addition have helped prove the guilt or innocence of alleged criminals. DNA fingerprinting is fast becoming the primary and most reliable method of identifying and distinguishing between individuals. The use of fingerprinting is also ideal for identifying runaways, rape victims or unidentified dead people. DNA does far more than certify our identity, and that of our ancestors, than it does to just catch criminals – even though the forensic arena is where the method is best utilized.
BIBLIOGRAPHY
Bender L, 1990, Through-The-Microscope Forensic Detection, Franklin Watts
Class Notes, DNA, 2003
Greenwood et al, 2002, Student Resource and Activity Manual 2002, BIOZONE
Lane B, 1998, Eye Witness Guide - Crime and Detection, Dorling Kindersley