What is DNA fingerprinting? Why is it so important, so useful, and so argued about in court cases?

DNA Fingerprinting
Can blood found at a crime scene really identify a criminal?
Brian gets caught red-handed as he investigates DNA evidence.
Segment length: 8:00

Insights

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:

  1. Scientists extract DNA from the cells.
  2. Restriction enzymes cut the DNA into pieces of various sizes.
  3. Gel electrophoresis separates pieces by size.
  4. Separated pieces are transferred to a nylon membrane, and some sequences are labeled with a radioactive substance called a probe.
  5. X-ray film is exposed by the radioactive emissions of only the labeled sequences. The developed film, called an autorad, shows the familiar track pattern of a DNA profile.

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.

Connections

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?

Key Words

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

Resources

  1. Fackelmann, K.A. (1994, Nov 5) Beyond the genome: The ethics of DNA testing. Science News, pp. 298-299.
  2. Glausiusz, J. (1994, Jan) Royal D-loops: Remains of Russian Czar Nicholas II and family proved authentic with DNA. Discover, p. 90.
  3. Horgan, J (1994, Oct) High profile: The Simpson case raises the issue of DNA reliability. Scientific American, pp. 33-36.
  4. McElfresh, K.C., Vining-Forde, D., & Balazs, I.C. (1993, Mar) DNA-based identity testing in forensic science. BioScience, pp. 149-157.
  5. Zurer, P. (1994, Oct 10) DNA profiling fast becoming accepted tool for identification. Chemical and Engineering News, pp. 8-15.

Additional resources

  1. Several catalogs sell complete DNA fingerprinting kits:
    Flinn Biological Catalog Reference Manual (800) 452-1261.
    Fisher Catalog-Educational Materials Division (800) 955-1177.
    Fotodyne Catalog-Educational Products Division (800) 362-4657.
    Ward's Catalog-Biology and Lab Supplies (800) 962-2660.
  2. America Online: Select in order: Reference. Access Excellence-Biotech. (Includes experiments, information, resources, and monographs on biotechnology, including DNA profiling.)

Main Activity

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.

Materials

  1. Pour about 1/4" of rubbing alcohol into one freezer box and cover it. Keep the lid on as much as possible, since the alcohol evaporates readily. Pour about 1/4" of water into the other freezer box and cover it.
  2. Use the toothpicks to make four spots of different food colorings evenly across a piece of the filter paper, in a horizontal row about 1/2" from the bottom. Repeat on a second piece of the paper. Allow the spots to dry.
  3. Make a fold in the filter papers about 1/2" from the top and place one in each freezer box. The top fold should hook over the side of the box, and the bottom 1/4" of the paper should be dipped in the liquid. Cover the box. Do not immerse the spots in the liquid.
  4. The alcohol and water will crawl up the papers, pulling the components of the food coloring along. When the liquid reaches an inch from the top of the paper, remove it and, with a pencil, mark how far the liquid traveled. Dry the papers. Did the secondary color separate into two primary colors? Did similar colors travel the same distance?

Questions
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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