DNA Fingerprinting Lesson Plan

DNA Analysis
Gel electrophoresis is a technique that scientists use to "measure" the size of DNA fragments. DNA
samples are first tagged with a dye, and then injected near one edge of an agarose gel. An electric
current is then applied, which pulls the DNA material through the gel. Smaller fragments travel farther
through the gel in a given time, which can then be detected thanks to the dye. A reference sample of
known material is always included, to set a scale for the results.
Figure 1 - Typical electrophoresis gel
Whenever a cell replicates itself, all of the contents of the cell must be duplicated, including the DNA
within the cell. The mechanism by which this happens starts with unwinding and then separating the
two strands of DNA from their initial double helix configuration. Each single strand then grows a
complementary mate through a polymerization reaction controlled by the biological chemical
polymerase. This polymerization reaction is termed the polymerase chain reaction, PCR. The
original strand serves as a template to ensure that only a specific matching strand of DNA gets created,
in an example of self-assembly.
Within the laboratory scientists can use the polymerase chain reaction in a slightly different form in
order to duplicate ( amplify ) a specifically chosen section of DNA for further analysis. First the DNA is
heated to completely unwind and separate the two strands. Then primers are added onto the DNA
strands in specific locations ( using self-assembly ) as starting points for the polymerization. Finally the
polymerase chain reaction is used to grow duplicate strands on each of the two original single strands.
This procedure is then repeated for 20 to 30 cycles, yielding millions to billions of copies through the
power of exponential increase. Careful selection of the original primers limits the duplication process to
only a short specific section of the original DNA. PCR can be applied to samples as small as a single
molecule, and even to degraded DNA that may have been broken down by time or harsh environments.
Short Tandem Repeats, STRs, are short sections of DNA which repeat themselves a certain number
of times. Where the repeat count differs among different individuals, it changes the length of the
encompassing DNA fragment, which can be isolated with PCR and analyzed with gel electrophoresis.
STR sections are frequently found within the 98% or so of "junk DNA" or "non-coding DNA" that does
not contain the codes for any known genetic traits. The probability of two random individuals having
identical STR counts at one particular DNA location ( loci ) is relatively large, but the chances of this
happening at thirteen different loci ( the number used in U.S. forensics ) are so vanishingly small as to
be nearly impossible. ( Identical twins, triplets, etc. have identical DNA, and the probability of
laboratory error is higher than the statistical probability of random DNA matching. )