Scientists working in forensic labs are often asked to perform DNA profiling or “fingerprinting” to analyze evidence in law enforcement, mass disasters, and paternity cases. In this laboratory activity, you will enter into the role of a forensic scientist who has been called upon to help solve a crime. You will use forensic techniques, and the first steps will be to gather DNA found at the “crime scene” and obtain DNA samples from five “suspects”. The DNA will be digested with a fixed set of restriction enzymes, separated on a gel by gel electrophoresis, and then analyzed for patterns of similarity with the crime scene sample. From these results, you will make recommendation to identify the perpetrator.
Restriction enzymes are a special class of proteins that cut DNA at specific sites and have become an indispensable tool in molecular biology. Restriction enzymes, also known as endonucleases, recognize specific sequences of DNA base pairs and cut, or chemically separate, DNA at that specific arrangement of base pairs. The specific sequence of DNA is called a restriction site.
These unique enzymes occur naturally in some bacteria and act to protect them from invading viruses. Viruses called bacteriophages, phages for short, attack bacteria by inserting their genetic material into the bacterial cell. The phage commandeers the bacterial cell, replicating rapidly until the bacterial cell lyses and releases more phages to carry out the same infection process in neighboring cells. However, if the bacterial strain has restriction enzymes that recognize restriction sites on the invading phage nucleic acid, then enzymes can destroy the invading genetic material by digesting and inactivating the phage genes. Bacterial cells protect their own DNA from being self-digested by modifying certain nitrogen bases along their genome, this prevents their restriction enzymes from recognizing and digesting their own sequences..
In this activity, you will
- Digest DNA found at the “crime scene” and the DNA of five “suspects” with two restriction enzymes.
- Perform agarose gel electrophoresis on DNA samples.
- Stain the gel to visualize the DNA bands.
- Document and examine gel results with an imaging system.
- Evaluate who cannot be excluded from the investigation by constructing a standard curve and determining the size of the DNA fragments from the gel using Logger Pro.