Uploaded by lmchugh

Bloody Knife

Name: _______________________________
Date: _____________________
Gel Electrophoresis
At the DNA level, humans are very similar, and the genes that code for their proteins follow fairly
standard patterns. However, the segments of noncoding DNA found between the genes (referred to as “junk”
DNA) follow patterns that vary from one individual to another.
The patterns of noncoding DNA sequences give all individuals (except identical twins) a distinctive
“fingerprint.” The fingerprint is constructed by isolating DNA from a few cells, cutting it into fragments using
restriction enzymes, and sorting the fragments by size using gel electrophoresis. The shorter the DNA
fragment, the farther it will migrate through the gel. The result is a visual representation of an individual’s
DNA, or a DNA fingerprint. (See Figure 1 below)
A DNA fingerprint can be used to identify a child’s biological father. The identification can be made by
comparing the repetitive, noncoding sequence present in the child, but not found in the mother must have
been inherited from the father.
Examine the bands in the diagram of the DNA fingerprints shown below and on the next page. Each
band represents a fragment of DNA. The first vertical lane of bands represents standard markers of known
size, which are used as a reference. To the right of the standard markers is the lane of bands representing the
child’s DNA fingerprint. The next lane is the mother’s DNA, followed by the DNA fragments from two men
who might be the child’s father. Study the pattern of each lane of DNA bands. Then answer the following
Figure 1
Application Questions:
1. Which band location represents the longest DNA fragment?
a) Band A
b) Band F
c) Band B
d) The wells for injecting DNA
Which of the men, Father #1 or Father #2, is probably the biological father of the baby? ____________
Explain how you drew that conclusion using evidence from the gel electrophoresis below.
The term DNA “fingerprint” best refers to:
a) The part of DNA the codes for our unique print made by our finger.
b) The method used to create restrictive enzymes.
c) The distinct individual pattern made by non-coding DNA.
d) The standard used to compare band location
Solve the Case of the Bloody Knife!
Late one April night, government agents received an anonymous tip that the National Art
Museum was about to be robbed of a precious jewel collection. When they arrived at the museum,
they saw they were too late: the jewels were gone. Lying face down on the floor next to the empty
jewel case was the body of a man the chief inspector recognized as the international jewel thief John
Milhouse. Milhouse had been shot in the chest at close range: his clothes were saturated with blood.
Underneath the body, the inspectors found a bloody knife.
At the airport the next day, police arrested Woody Swick, the murdered thief’s occasional
partner in crime. Swick denied all knowledge of the murder and the theft, but had a suspicious cut on
his hand. His explanation of the wound was that he had an accident in the kitchen that morning.
DNA analysis was ordered on the victim (Milhouse), the blood on the victim’s clothes, the blood
on the knife found under the victim and Swick. Police laboratory technicians used gel electrophoresis
to compare the DNA fragments. The test results are shown in the figure below.
1. Compare the fragments: Whose DNA is on the clothes? ___________________________________
2. Compare the fragments: Whose DNA is on the knife? _____________________________________
3. Why was DNA analysis ordered on the victim (Milhouse)? _________________________________
4. If Milhouse was dead from a gunshot wound, where is the gun? ____________________________
5. Why was there a knife under him with Swick’s DNA on it? _________________________________