Name:______

WHERE’S THE CAT? A DNA Fingerprinting Simulation

THE SCENARIO

Mr. and Mrs. Dimsalp are not able to have their own children due to Mrs. Dimsalp’s infertility. They enlist the assistance of an egg donor/surrogate mother who, through the process of artificial insemination, carries and gives birth to the Dimsalp’s child. Conflict arises, however, when the surrogate decides to keep the child, claiming that the child’s biological father is her husband and not Mr. Dimsalp.

OBJECTIVE

After completing this lab, you will be able to describe how gel electrophoresis and DNA fingerprinting are used to determine relatedness.

MATERIALS

5 DNA samples (standard, surrogate, child, surrogate’s husband, or sperm donor)

GTA probes

Scissors

Tape

Whiteboard

Marker

Colored pencils

PRELAB

1. Set up your whiteboard as shown

2. Cut out the DNA strips. You will have two or three strips depending on your sample.

3. Tape the strips together in numerical order to form one long strand of DNA. The “2” from the end of the first strip should overlap the “2” from the beginning of the second strip. If necessary, you may cut off one or more of the numbers so that bases are not covered up.

4. Simulate the action of the restriction enzyme Hae III by cutting your DNA into fragments. Hae III recognizes the base sequence GGCC and makes the following cut:

GG / CC.

5. Use the colored pencils to identify the source of each fragment. Keep your fragments separated from the other samples in your group.

PROCEDURE

  1. Draw a “+” sign at the top of your simulation gel to represent the positive electrode and a “-“ at the bottom to represent the negative electrode.
  1. Moving left to right, label each well with its respective DNA sample:

#1 = standard

#2 = surrogate

#3 = child

#4 = surrogate’s husband

#5 = sperm donor (Mr. Dimsalp)

  1. “Electrophorese” the standard sample first. Using the numbers along the side of the gel as a guide, place each DNA fragment in its respective position, directly above the well. Be sure to follow the migration “rules” of electrophoresis.
  1. Repeat Step 3 with the other DNA samples. Fragments of similar length should form a single row across the gel. *GO TO QUESTION 6 BELOW
  1. “Probe” your DNA fragments by placing a “GTA” probe on each fragment that contains the complementary base sequence. Place the “GTA” directly on top of the target sequence.
  1. Remove all DNA fragments that do not have a probe attached to them.

QUESTIONS

  1. The following image represents a map of a piece of DNA, where each vertical line represents a recognition site for restriction enzyme BamHI. If this DNA was digested completely by BamHI, which of the following agarose gel results would you expect to see?

  1. What is the purpose of the standard set of DNA fragments? (Why is a control important?)
  1. Explain how the positive and negative electrodes cause the DNA to migrate.
  1. Why do DNA restriction fragments move at different rates?
  1. Why doesn’t the enzyme cut everyone’s DNA into the same sized fragments?
  1. Compare the DNA patterns formed during electrophoresis. Who is the child’s biological father? Justify your answer by using data from the gel.
  1. Describe why a real DNA or RNA probe “sticks” to the target DNA sequence. Use appropriate scientific vocabulary to support your answer.
  1. What was the purpose for completing steps 5 and 6? (Hint: Compare the results after Step 6 to those from Step 4.)
  1. A sample of DNA found at a crime scene is analyzed and found to have a very unusual sequence in a particular part of the genome. The sequence is AACGGGA.
  1. Design a DNA probe for this sequence ______
  1. Design a RNA probe for this sequence ______
  1. How could knowing about the existence of this unusual sequence be useful in identifying the suspect? Explain how this could be done with the use of a probe.
  1. Two DNA samples are analyzed with a probe and found to have the same STR sequence in a certain location. The probe in the first sample appears very dark, while the second is much lighter. Speaking generally, what do you think this tells you about the number of repeats present in sample one compared to the number of repeats in sample two?