What are the base bonds in DNA?
- Adenine and Thymine/Guanine and Cytosine
- Adenine and Cytosine/Guanine and Thymine
- Adenine and Guanine/Thymine and Cytosine
Can you think of some way that a bank robber could be identified if no one saw who he or she was?
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.
Here's the situation:
Can you think of some way, besides witness testimony, that the bank robber could be identified from among the three individuals?
DNA can identify people — even better than fingerprints. DNA is found in all of our cells: hair, teeth, bones, blood and skin. Though all humans share 99.9% of their genes, our DNA differs from everyone else's by three million nucleotide base pairs.
Do you remember how many chromosomes a human has?
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.
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.)
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.
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?
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 lightning 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.