|What is DNA fingerprinting? Why is it so important, so useful, and so argued about in court cases?|
Can blood found at a crime scene really identify a criminal?
The O.J. Simpson trial has generated much interest in DNA. Formerly used only in research labs, DNA fingerprinting (called DNA profiling by scientists) has entered an intense public spotlight, where lawyers, crime investigators, and scientists constantly discuss its merits and pitfalls. Although usually used to establish paternity, DNA profiling is such good evidence that prosecutors are relying on it more and more to help convict suspects in criminal cases.
A DNA molecule resembles a long, twisted ladder. The supports of the ladder are the same for everyone, but the rungs are what make us all different. Each rung is made of a pair of organic molecules called nitrogen bases-adenine, thymine, cytosine, and guanine-usually symbolized as A, T, C, and G. The sequence of the rungs is important. The bases constitute a code for different proteins, much like the letters of an alphabet form words and sentences. Certain areas of the DNA molecule have no currently understood function, but they appear to vary widely among individuals. The most common form of DNA profiling, abbreviated RFLP, is a way of showing the unique patterns of bases in some of these areas.
Before the "fingerprint" analysis, the DNA must be sampled
and stored properly. Even blood or semen that has soaked into a rug or
dried in the sunlight can be a source for sample cells. An analysis can
be done from as few as 100 cells but requires several steps:
To eliminate any possibility of a mistaken identity, analysts use several different probes to look at several different DNA fragment patterns in a sample. More than one person might have a particular RFLP pattern, but it becomes less likely that multiple people will have two or more sequences in common. Statisticians call this the multiplication rule, because the individual probabilities of a mistaken identity for each pattern are multiplied together to find the overall probability.
1. Should an individual have the right to refuse to give a blood sample
for DNA analysis, or should the authorities have the right to take such
samples without permission?
2. Biotechnologists want to map the DNA of the entire human population. How would this help us? What problems could it create?
DNA genetic material contained in every cell
and unique to the individual
DNA profiling identifying people by visual representations of unique regions of their DNA
Frye standard of admissibility Scientific evidence may only be considered in court if the type of evidence is generally accepted by the scientific community.
gel electrophoresis technique used to separate pieces of DNA by size
multiplication rule method used to obtain likelihood of more than one event occurring simultaneously
restriction enzyme protein that cuts DNA at specific base sequences
The Tell-tale Band
Put your detective skills to the test by creating and comparing chromatograms.
You can separate different mixtures of chemicals by using chromatography, a technique similar to the electrophoresis that scientists employ to separate DNA fragments. These techniques depend on matching a standard mixture with the unknown mixture and producing a visual representation of the components. A band that appears at the same location for the sample and the standard is likely to be the same substance.
1. What other artificially colored food items could you analyze in this way?
2. If someone used a black felt-tipped pen to scrawl graffiti on a building, could you use chromatography to identify the ink? How would you design the experiment? What might interfere with the analysis?
Collect newspaper clippings and magazine articles on court cases that involved DNA profiling evidence. What do lawyers usually criticize about this kind of evidence? What do they usually try to prove with it? What other kinds of evidence do they also need to win a case?
Write a descriptive profile of each suspect from a Clue board game. What type of evidence might each suspect leave at a crime scene that would distinguish him or her from the other suspects? Pass out the weapon cards, one for each suspect. What evidence might the suspect leave at a crime scene using that particular weapon?
Collect fingerprints from the class by inking the thumb and index finger on a stamp pad and then rolling each firmly on an index card. Enlarge them on a photocopier. Do any two match exactly? If there is a set of twins in your school, are their fingerprints identical?
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Educational materials developed with the National Science Teachers Association.