This experiment uses the powerful new technique of polymerase chain reaction (PCR) to demonstrate the variable nature of human DNA. Although the DNA from one person to another is more alike than different, certain areas of our genome exhibit a great deal of diversity. Such variable sequences are termed polymorphic (meaning many forms) and provide the basis for paternity and forensic testing of DNA. Many DNA polymorhisms are found within the estimated 90% of the genome that does not code for protein. A special class of these polymorphisms is called an Alu insertion. Alu elements are short DNA sequences that have amplified to about 500,00 copies, distributed throughout primate genomes. A few of these Alu insertions have occurred within the last million years and are not fixed in the human genome. Students obtain samples of their own DNA using a simple mouthwash procedure (bloodless and noninvasive). PCR is used to amplify across an intron of the tissue plasminogen activator gene that may contain an Alu insertion. If the Alu sequence is present, then a 400-bp PCR product will result, while a 100-bp product is produced when the Alu sequence is absent. Following amplification, student samples are loaded into an agarose gel, electrophoresed, stained, and photographed. Each student will see one or two bands in their gel lane, indicating whether they are homozygous for the Alu insertion, homozygous for the absence of the insertion, or heterozygous for the insertion.
