How DNA is Used

DNA is used as an identification tool. It is used much like a fingerprint as evidence in many crimes. The advent of DNA technology has revolutionized crime fighting and has also led to exonorations of wrongly convicted citizens. There are two main types of DNA testing, Restriction Fragment Length Polymorphism and Short Tandem Repeat analyses. DFLP alalysis was one of the first methods used to analyze DNA. In this method, first DNA is dissolved and broken down into strands at specific points using restriction enzymes. The lengths of these strands are compared and by comparing these identification can be made. STR analysis was more recently developed, and is more prevalant today. STR analysis examines how often base pairs repeat in specific loci, or locatoins on a DNA strand. These can be in repetitions of two three, four, or five base pairs.

Sample A Sample B Sample C Steps in STR Analysis
  1. Extract DNA from the cells in sample
  2. Quantify the DNA
  3. Put DNA sample through a Polymerase Chain Reaction, which makes a greater quantity of identical DNA
  4. Use capillary electrophoresis to extract the amplified DNA
  5. Compare the repeated loci by separating them according to size on an electrophoretic gel. (See Below)
This gel will give results such as the example to the left. The black bars are called bands. Each band is made up of many identical-sized DNA molecules that are produced using PCR. The gel separates the smaller bands from the shorter ones. The bands near the lower end consist of the smaller molecules. Size increases as you progress up the ladder. If the bars match for two different samples, it is concluded that they came from the same person. In this example, samples A and C match up, so they would be from the same person.
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