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Notes for the Instructor

It is impossible to talk about forensic science or crime scenes and not talk about DNA or deoxyribonucleic acid. Most of your students will have already heard about DNA and know that it can be used to convict and exonerate suspects in a crime, but few may have a grasp on what it actually is.

DNA is a material found in all living things. It is a body’s biological instruction manual to develop, survive and reproduce.

DNA is made up of four different building blocks called nucleotides: adenine (A), thymine (T), guanine (G), cytosine (C). The order that these nucleotides appear on a strand of DNA determines what instructions are given. This is known as a genetic code. Each living thing has its own unique genetic code. It is amazing that changing the order of these four nucleotides results in all of the diversity in life, from the simplest single-celled creature to a sunflower to a giant blue whale.

Most humans have the exact same genetic code; in fact we are all 99.9% alike. It is the remaining 0.1% difference between all of us that results in the wide variety we see in people – variation in eye, hair, skin color, height, etc. Except for identical twins, every person has his/her own unique genetic code. It is this uniqueness that makes DNA testing useful at a crime scene.

DNA testing is the process of using skin, hair, blood or other bodily fluids to identify patterns in a person’s genetic code and compare them to patterns found at a crime scene. This type of test was first designed in 1985, and first used to convict someone of a crime in 1987.

During DNA testing, scientists focus on the part of the human genetic code that is different from person to person – the 0.1%. Enzymes are special proteins that can cut DNA into sections at particular spots. By comparing the size of these cuts, scientists can determine how similar or different two genetic codes are.

Investigators can feel confident with the results when DNA does not match the suspect’s sample. However, there is always a little bit of ambiguity when using a positive DNA match to connect someone to a crime. This is partially because there are many steps to collecting and analyzing DNA that can result in errors. Additionally there is no way to know exactly how a DNA sample arrived at a crime scene. Because of how easily we shed skin and hair, and how it can be transferred to other objects that also travel, our DNA has the potential to end up in places we may have never been! This is why the more DNA that can be found to link a suspect to a crime scene, the better. It is also why DNA evidence must be considered against all of the other forensic evidence gathered during an investigation.

In this lesson students will experience how forensic scientists use DNA first hand. In part one of the lesson students will be challenged to extract DNA from a strawberry. Students will make an extraction liquid out of dish soap, salt and water. All of these are household ingredients that are safe to handle and dispose of in the trash. However, it is a great time to instill lab safety rules in your students.

  1. Never eat anything you are using in a science lab.
  2. Follow procedures exactly to ensure results.
  3. Properly dispose of all materials at the end of the lab.

In part two, students will examine how enzymes can be used to cut DNA into identifiable patterns. Then they will perform the task of forensic scientists by comparing the patterns in the DNA of suspects to the DNA found at the cookie jar crime scene.

The activities in this lesson address Next Generation Science Standards practices of Planning and Carrying Out Investigations and Analyzing and Interpreting Data. In addition, they address Common Core Learning Standards. See the appendix on page 105 for more details.