Gel Electrophoresis Worksheet - Morgan Park High School



Gel Electrophoresis

DNA. It's what makes you unique. Unless you have an identical twin, your DNA is different from that of every other person in the world. And that’s what makes DNA fingerprinting possible. Experts can use DNA fingerprints for everything from determining a biological mother or father to identifying the suspect of a crime. What, then, is a DNA fingerprint and how is it made?

Before DNA can be analyzed with gel electrophoresis, restriction enzymes must be applied. Restriction enzymes work like “molecular scissors” cutting the long DNA molecules at different locations. Where they cut depends on the code within the DNA molecule and the code within the enzymes. For example, one type of enzyme severs DNA wherever it encounters a sequence of A/AGCTT; it only severs between the two A (adenine) bases. The length of these fragments will vary from person to person because the code for every person’s DNA is different. Some fragments will be long, others short.

After DNA is cut using restriction enzymes, it is micropipetted into a well within an agarose gel. Agarose gel is a thick, porous, Jell-O like substance. It will act as a molecular strainer, allowing smaller pieces of DNA to move through it more easily than larger pieces.

DNA to be analyzed, now within wells in the agarose gel, is placed in an electrophoresis chamber. The DNA fragments have a slight negative charge so they move toward the tray’s positive end. (As with subatomic particles opposites attract.) Recall the gel acts like a strainer, thus, smaller DNA fragments travel further toward the tray’s opposite end than do larger DNA fragments. When electrophoresis is complete, the fragments are distributed in the gel according to their lengths.

After electrophoresis is complete, further analysis is still necessary. DNA probes are pieces of DNA that have been radioactively labeled. The probes attach themselves to the DNA only where their code encounter a certain sequence of code among the various fragments. One example of a probe would be a sequence of GTA. GTA would only attach to DNA where it encounters the complimentary code CAT. Excess probe – all material that does not attach itself – is washed away.

Answer all questions below in complete sentences on a separate sheet.

1. What do restriction enzymes do to the DNA?

2. What is agarose gel and how does it work?

3. Where is the DNA placed in the gel electrophoresis apparatus?

4. How does electrophoresis work?

5. What are probes? How do they work?

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