DNA FINGERPRINTING LAB


A laptop has gone missing at the school. It happened yesterday sometime between 5 and 6PM. This time, the perpetrator broke through the glass in a teacher’s door, reached through the hole, and opened the door. Once inside, they took a laptop that was inside one of the cabinets in the back of the room where it was being used to monitor the rate of photosynthesis for a plant in the dark for someone’s science fair project. Only the laptop was taken.

The teacher has reason to believe that the perpetrator was a 9th grade student because he had only shown his biology students where this laptop was located. Although nobody reported seeing anyone in the building at this time, one clue was left behind—a bit of blood on the glass from when they reached through the broken window. The teacher was able to take a sample of that blood and do some DNA analysis on it and came up with the results as shown in the far left well of the gel electrophoresis chamber.

Your challenge is to take the sample of DNA that you have been given to determine if it matches that of the perpetrator. To do this, please follow these steps:

1) First, you must use a restriction enzyme to cut up the DNA into pieces. The pieces are called RFLPs (restriction fragment length polymorphisms). The particular restriction enzyme that we will be using always cuts the DNA when a “G” is followed by an “A”. Using a marker, go down the left hand side of your suspect’s DNA and draw a dark line between every “G—A” combination.
2) Now, count the number of bases on one side of each DNA fragment. For the sake of remembering, write that number of bases on each fragment.
3) Next, take a pair of scissors (the enzyme) and cut the DNA into fragments along each of the lines.
4) To actually make a fingerprint of the DNA we must put all our fragments into a single well in a gel electrophoresis chamber. We hook the chamber up to a socket and then run a current through the chamber. Current always runs from the negative pole to the positive pole. Because our DNA fragments are slightly polar (have an electric charge), they are carried with the current as it moves through the chamber. The longer fragments don’t move as far (because they are heavier) and therefore, end up closer to where the DNA was loaded into the chamber. Smaller fragments travel a long distance in the chamber and will end up near the bottom of the sheet. Look at each of your fragments and code in one row on the grid around the number of bases for each fragment (only underneath your particular well). For example, if your fragment was 18 bases long you would shade in one row of boxes underneath your well at about 1/2 way between 15 and 20. If you had another fragment that was 18, you would color in a row of boxes right above or right under the one that you did for the first 18. Although this is not exact, it is close enough that it will show the variety that exists between suspects.
5) Repeat step four for all of the people in your group. Compare each set of bands to the bands from the SUSPECT (far left of the chamber). If the bands match, there is a high probability that you have determined the perpetrator.


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