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Biology 103
2002 Second Paper
On Serendip

DNA FIngerprinting in A Court Of Law

Kyla Ellis

As we dive head first into the new millennium, we are eager to embrace new "modern" technologies and ideas. One such idea is that of identification through the analysis of DNA, or genetic "fingerprinting." But, is this an idea we should rush into accepting? Should a shard of bone or fingernail be enough evidence to convict a person, to send them to the electric chair? And how does it work, anyway? Forensics and DNA have always been the areas of biology that interested me the most, so I decided that for this paper, I would explore the controversy as well as learn more about the process.

Deoxyribonucleic acid, or DNA, is made up of two strands of genetic material spiraled around each other. Each strand contains a sequence of bases (also called nucleotides). In DNA, there are four possible bases: chemicals called adenine, guanine, cytosine and thymine. The two strands of DNA are connected through chemical bonds at each base. Each base bonds with its complimentary base, as follows: adenine will only bond with thymine, and guanine will only bond with cytosine. DNA is a chemical structure that forms thin structures of tightly coiled DNA called chromosomes, which can be found in the cell nucleus of plants and animals. Chromosomes are normally found in pairs; human beings typically have 23 pairs of chromosomes in every cell. Pieces of chromosomes (or genes) dictate particular traits in human beings (1). There are millions of possible patterns, which gives rise to different physical appearances in humans. Every person's DNA also has repeating patterns, which allows scientists to determine whether two samples of DNA are from the same person.

To analyze the genetic patterns in ones DNA, scientists must go through extensive, meticulous steps. The first of such steps performed is called a Southern Blot. This is a brief outline:

The DNA must first be isolated, either by chemically "washing" it, or by applying pressure to "squeeze" the DNA from the cell. Next restriction enzymes cut the DNA into several pieces. The DNA pieces must then be sorted by size using electrophoresis, whereby the DNA is poured into wells of a gel, and then an electrical charge is applied to the gel. The positive charge is opposite the wells, and since DNA is slightly negatively charged, the piece of DNA is attracted toward the positive electric charge. The smaller pieces move more quickly than the larger pieces, and thus go farther than the larger pieces. The DNA is then heated so the DNA denatures (bases break apart) and render a single strand. The gel is then baked to a sheet of nitrocellulose paper to permanently attach the DNA to the sheet. This completes the Southern Blot, which is now ready to be analyzed.

To do this, 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 genetic pattern contained in the probe.
(For more details visit (8). )

Every strand of DNA has pieces that contain repeated sequences of base pairs, called Variable Number Tandem Repeats (VNTRs), which can contain anywhere from twenty to one hundred base pairs. Our bodies all contain some VNTRs. To determine if a person has a particular VNTR, a Southern Blot is carried out. The pattern that results from this process is known as a DNA fingerprint. VNTRs come from the genetic information donated by our parents; we can have VNTRs inherited from either our mother or father, or a combination of the two, but never a VNTR either of our parents do not have. Because these combinations are inherited, each person's DNA fingerprint is unique.

I notice that this is a very involved process, and having performed it myself, I can assure you it is difficult and time consuming. The possibilities for human error are definitely there. If we were to rule out human error, however, the accuracy of these tests far surpasses all such tests that we have so far. The closest thing we have to this is analyzing fingerprints, which can be smudged, or otherwise distorted. Fingerprint experts never give evidence unless they are 100% sure, meaning they had the whole fingerprint and found an exact match. One expert claims that if fingerprinting were introduced today, there would be a terrible time convincing people of its validity(2). However, since it has been going on for so long, it is widely accepted, and therefore more "valid" than DNA identification. People are comfortable with it.

DNA is also easily contaminated. Since the results are derived from microscopic elements, the slightest disturbance can be a factor. It is even possible that some of the expert's own genetic material could be mixed in with the sample, and no one would know. The relative "newness" of DNA fingerprinting is another factor; people don't understand it like they would fingerprints. In a court of law, lawyers can hold up the pictures of the two matching fingerprints, and the evidence is right in front of the jurors' faces. With DNA, the evidence is harder for people with no experience in forensic science to grasp, and they essentially have to take the scientist's word for what they are seeing(3). The last problem is that of DNA sample size and age. The smaller the sample, the more likely it is to have room for error in testing. Age of the specimen also matters, if it is old and small it is less likely that there will be an error free test.

Putting the doubts aside, DNA can be a valuable tool in criminal justice. So far, at least 10 people on death row have been pardoned due to DNA evidence examined after their initial trials(4). There was a case in 1999 of a man by the name of Clyde Charles who was convicted of aggravated rape and sentenced to life imprisonment. He served nineteen years he was finally proclaimed innocent due to DNA tests(5). This is a chilling reality that we have to face: have innocent people been convicted of horrendous crimes and put into jail, or even executed, while the guilty go free?

In my research, I also got the impression that part of the controversy surrounding DNA fingerprinting is the fact that courts of law do not want to admit they are wrong. As I mentioned above, convictions of innocent people and acquittals of guilty people, do not reflect well on our legal system. No one wants to admit making mistakes and therefore being possibly inept at doing their job, especially if their job is determines who is sent to death row. It is a bit of an embarrassment to admit our legal system could have such a huge glitch. One case that I got this impression from was that of Joseph Roger O'Dell, arrested for and convicted of murder, rape, and sodomy of a young woman. From death row, he made repeated pleas for a DNA test, but he was refused each time. After his death the last of the DNA evidence in his case was burned without any further testing(5).

In May of 2001, to date, more than 85 people in the United States had been set free through post-conviction DNA testing, and, as I said above, 10 of them had been on death row. The FBI has been analyzing DNA in rape and rape-homicide cases since 1989. When arrests were made on the basis of other evidence in such cases, biological specimens were sent to the FBI for DNA analysis. In 26 percent of the cases, the primary suspect was excluded by DNA evidence(6). The question is, how many of these would have been found not guilty without DNA evidence?

This country is committed to the idea of justice. If we are sending people that are not guilty to jail, that messes with our entire conception of our legal system. I believe that forensic science is a huge step in the right direction toward justice.


1)How Is DNA Fingerprinting Done,
2)Fingerprint Identification: Craft Or Science?,
3)You DNA ID Card,
4)How DNA Evidence Works,
5)The Case For Innocence,
6)How DNA Technology Is Reshaping Judicial Process and Outcome,
7)DNA files,
8)Southern Blot,

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