After this activity, students should be able to:

• Describe the organization of DNA into repeating nucleotide base pair sequences
• Explain how DNA profiling is used to link people to crime and paternity cases.
• Describe the role of biomedical engineering in DNA profiling.

A robbery takes place at a bank. As the thief escapes the building, a security guard grabs one of the bank robber's gloves. The bank robber leaves the scene in a phone service van. The phone company identifies three employees who may have been in the vicinity of the bank at the time of the robbery. All employees deny robbing the bank. Can you think of some way, besides witness testimony, that the bank robber could be identified from among the three individuals?

DNA evidence is more reliable than fingerprints at identifying people. click for copyright

DNA can identify people — even better than fingerprints. DNA is found in all of our cells: hair, teeth, bones, blood, skin. Though all humans share 99.9% of their genes, our DNA differs from everyone else's by three million nucleotide base pairs.

Our DNA is organized in 23 chromosomes in the nucleus in each of our cells. Regions in each chromosome contain what are called "junk DNA," which does not contain genes. But often, this junk DNA contains repeating nucleotide base pair sequences that can be used for matching purposes. (Show students Figure 1 or the same image in the attached CODIS Visual Aid.) In this example, you can see chromosome locations where the FBI looks for repeating sequences of DNA. They're called CODIS sites, which stands for the FBI's Combined DNA Index System.

Figure 1. The 23 human chromosomes and 13 chromosomal locations at which the FBI looks for repeating DNA sequences. For this activity, note the TPOX region on chromosome 2. (X and Y count as one chromosome pair. The AMELs are not CODIS sites.) click for copyright

In our case, the police found a hair in the bank robber's glove. Remember that we have 23 pairs of chromosomes, each pair containing one chromosome from our father, the other from our mother. A DNA analysis shows that the hair in the robber's glove contains the following nucleotide base pair sequences in the TPOX region (show students Figure 2 or the same image in the attached CODIS Visual Aid).

Figure 2. TPOX region of chromosome 2 of hair found in bank robber's glove. click for copyright

Note that the GAAT sequence is repeated twice in the father's side and three times in the mother's side (the sides of each chromosome are often not the same length). Equivalently, we could say that the CTTA sequence is repeated. Why is that? (G always pairs with C, and A always pairs with T).

So now let's compare the TPOX regions of the DNA found in the bank robber's glove with the TPOX regions of the DNA of two suspects. Note that we are just looking at the one side of the DNA with the GAAT repeating sequence. This simplifies the comparison. (Show students Figure 3 or the same image in the attached CODIS Visual Aid.)

Figure 3. Comparison of the TPOX region of chromosome 2 of the unknown bank robber and two suspects. click for copyright

Suspect 1 matches the GAATGAAT sequence of the hair found in the glove on one chromosome, but the other chromosome does not match. Both chromosomes must match to show that the hair in the glove came from a specific suspect. Thus, from just one CODIS site we already know that the hair in the bank robber's glove cannot belong to suspect 1.

Suspect 2 matches the GAATGAAT sequence on one chromosome and the GAATGAATGAAT sequence on the other chromosome, so you can say that suspect 2 matches at the TPOX location. To confirm that the hair belongs to suspect 2, the other 12 chromosome locations (see Figure 1) must also match. If all 13 CODIS locations match, then the hair in the bank robber's glove belongs to suspect 2.

The random probability that one of your CODIS sites matches with someone else's is about one in 10 (1/10). Therefore, the probability of two CODIS sites matching is 1/10*1/10 = 1/100 (one in 100). The chance of three CODIS sites matching randomly is 1/10*1/10*1/10 = (1/10)³ = 1/1000 (one in 1,000). The random chance that all 13 CODIS sites match is (1/10)¹³ = one in 10,000,000,000,000. The chance of being struck by lightning in your lifetime is, roughly, one in 1,000,000. So you are 10 million times more likely to be struck by lighting than you are to have the same 13 CODIS sequences as another person. This is what makes DNA profiling so certain.

Engineers can be involved in many aspects of crime scene investigation. They might use their knowledge of CAD (computer-aided drawing) to create a reconstruction of the crime scene. First they might develop a model of the room, and then determine the path of bullets and analyze the blood splatter patterns to determine the position of victims and their killers at the time of the crime. Biomedical engineers create the tools, equipment and processes to accurately collect and examine DNA evidence for crime and paternity cases. Biomedical engineers also help investigators with the analysis of the gene sequencing for DNA profiling.

Before the Activity

With the Students

• Make sure students use care with scissors.

Pre-Activity Assessment

Activity Embedded Assessment

Post-Activity Assessment

• For lower grades, use fewer CODIS sites and suspects.
• For upper grades, use more CODIS sites and suspects.