1. DNA Fingerprinting Exercise:
Blood found at a murder scene is thought to be that of the attacker (there was a vicious fight and the attacker did not got away unscathed!). We have three suspects. Carry out a DNA fingerprint on blood samples from each suspect and on blood found at the scene. We are going to use restriction endonucleases and for simplicity we will only test two loci (we aren’t really bothered about catching the real murderer, just so long as we get a conviction!). Solve the crime!
1. Restrict each DNA sample with the following restriction endonucleases:
ECORI —G—A—A—T—T—C— and Hind III —A—A—G—C—T—T—
—C—T—T—A—A—G— —T—T—C—G—A—A—
And calculate the fragment sizes in bp.
Suspect one has been done for you already:
29 bp fragment
-GGCTACTGGAAGCTTAATGTCGGTCGGTCGGTCGGTCGTTCGAATTGCATGGGCG-
-CCGATGACCTTCCAATTACAGCCAGCCAGCCAGCCAGCAAGCTTAACGTACCCGC-
-GGCTATGGAATTCGGGTATAGTATAGTATAGCCAAGCTTGCTATTGCCCTGAGGCC-
-CCGATACCTTAAGC CCATATCATATCATATCGG TTCGAACGATAACGGGACTCCGG-
26 bp fragment
Suspect 2:
-CTGGAAGCTTGGCCGGCCGGCCGGCCGGCCGGCCGGCCGGCCGGCCTTCGAAT-
-GACCTTCGAACCGGCCGGCCGGCCGGCCGGCCGGCCGGCCGGCCGGAAGCTTA-
-TTGAATTCATCGATTAATTAATTACCAAGCTTGCTATTGCCC TGAGGCCGCTATGCT-
-AACTTAAGTAGCTAATTAATTAATGGTTCGAACGATAACGGGACTCCGGCGATACGA-
Suspect 3:
-ATTGATTGATGGCTACTGGAAGCTTAACTAACTAACTGCTTCGAATTGCATCGGCG-
-TAACTAACTACCGATGACCTTCGAATTGATTGATTGACGAAGCTTAACGTAGCCGC-
-TACCTTAAGCATACATACATACATACATACATACATACATACATACATACAATTCGAA-
-ATGGAATTCGTATGTATGTATGTATGTATGTATGTATGTATGTATGTATGTTAAGCTT-

2. Base Pairs (markers) Crime scene Suspect 1 Suspect 2 Suspect 3
2. Plot the position the fragments would take on the following electrophoretic gel. The blood found at the scene has already been done for you:
Base Pairs (markers)
50bp
40bp
30bp
20bp
10bp
Crime scene
Suspect 1
Suspect 2
Suspect 3
3. Which suspect do you think committed the murder?
Uses of DNA fingerprinting:
To test whether or not parents carry a disease-causing mutation that may be passed on to their offspring, e.g. sickle-cell disease, cystic fibrosis.
To determine the real father of a child (paternal testing).
To test crime suspects to see if their DNA matches that found at the crime scene.
Problems with DNA fingerprinting:
False Negatives: mistakenly declaring that there is no match.
False Positives: mistakenly declaring that there is a match. Is this an acceptable error?
Although the probability of two people coincidentally having matching DNA fingerprints is extremely small, the process of obtaining a DNA fingerprint by gel electrophoresis is prone to experimental error. The same DNA sample will never give two identical gels!