Using Biology to Solve a Crime

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Using Biology to Solve a Crime

• Who killed Mrs. Brogan??

Disclaimer Mrs. Brogan is actually alive and
well, this scenario is intended for educational
purposes only.
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Case Background

• Mrs. Brogan was found dead in the prep room.
• Stab wound to the chest.
• Blood stains found on the cabinet (Blood 1) and
  on the floor (Blood 2).
• Manalapan police suspect this class.
• Blood samples were sent to the AP biology class
  for analysis.

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Goal

• Determine whose blood was left on the wall and on
  the cabinet.
• Use DNA fingerprinting to help solve the crime!

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Step 1Polymerase Chain Reaction

• Used to increase the amount of DNA collected at a
  scene.

Primers
Nucelotides
Sample
Millions of copies in a few hours
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Step 2Restriction Enzymes

• Cut isolated DNA at specific locations to create
  bands of differing lengths.

TATTAACGCAACGCGCTCAGGCCAAACTTGTAT
Cuts after AAC
TATTAACGCAACGCGCTCAGGCCAAACTTGTAT
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Step 3Electrophoresis

• Uses electrical charge to separate DNA fragments
  by size
• DNA is negatively charged, runs toward pos.
• Gel is a matrix
• Smaller fragments travel further
• Dying the gel allows us to see the different
  bands of DNA

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Run DNA on a gel
TATTAACGCAACGCGCTCAGGCCAAACTTGTAT
Fragments Size TATTAAC
7bp GCAAC 5bp GCGCTCAGGCCAAAC 15bp
20
15
10
5
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Activity

• 1 Draw lines on your DNA strand to represent
  where the restriction enzyme would cut (after
  AAC).
• 2 Count how many bases are in each new segment.
• 3 Draw lines on the gel to demonstrate what the
  banding pattern would look like.

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Which suspects blood was on the scene?
Blood 1
SuspectA
SuspectB
Blood 2
Brogan
15,10,9,8
13,11,7,5
15
10
5
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Conclusions

• Can we be 100 certain that Suspect B killed
  Mrs. Brogan??
• Why or why not?
• Other applications
• Paternity testing
• HIV conformation (western blot)
• Determining evolutionary relationships