DNA Fingerprinting: Analysis of Crime Scene DNA

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DNA Fingerprinting

Just like no two people look exactly alike, no two people (except identical twins) have the exact same DNA. There are regions in our DNA that do not contain genes, and are especially variable. When you cut these areas of DNA with restriction enzymes, the pattern obtained will be different for each person. This is called DNA fingerprinting and can be used for identification. The police often use this technique to compare the DNA of a crime suspect to that of samples of blood, hair, and etc. found at the crime scene.

Once the DNA from a crime scene sample has been isolated and cut with restriction enzymes, it must be analyzed using a technique called electrophoresis.

Electrophoresis is a technique used to separate charged molecules in an electric field based on their sizes. If you had a solution of charged molecules and placed the anode of a battery (the + pole) at one end and the cathode (-) at the other, the + charged molecules would migrate to the cathode (-) and the – charged molecules would go to the anode (+). This alone doesn’t help us because the molecules are still in solution and we can’t see them or determine anything about their sizes or concentrations. To solve these problems, we use Agarose Gel Electrophoresis.

Agarose is a carbohydrate isolated from seaweed. When it is dissolved in a hot water solution (buffer, etc.), poured into a mold and then allowed to cool, it will harden into a wiggly gel very similar to Jell-O TM. This gel has tiny pores in it that will allow molecules to pass through.

In DNA Agarose Gel Electrophoresis, an agarose gel with small wells formed at one end is placed in an electrophoresis chamber and just covered with a buffer solution. The chamber has a + charged wire at one end and a – charged wire at the other. These wires are attached to a power supply that gives them their charge and allows control over the speed of the experiment. Since DNA is – charged, the DNA samples are loaded into the wells which are positioned at the opposite end of the gel from the + charged wire. When the power is turned on the DNA will move through the agarose gel pores toward the + charged wire. The DNA molecules are separated by size because the smaller molecules can move through the pores in the gel faster than larger ones.

If we perform this technique with a DNA specific dye added to the samples and the gel, we can stop the gel run before the DNA gets all of the way through the gel. Now we have a way to see the dyed bands formed by the DNA molecules of the same size that migrate together. We can also tell something about the concentration of DNA in each band because the dye binds in a defined amount to the DNA.

Due to the nature of the exercises, the file should be downloaded and opened with Microsoft Word.

DNA Fingerprinting: Analysis of Crime Scene DNA (Microsoft Word)
DNA Fingerprinting: Analysis of Crime Scene DNA (Web Format)