Summary

In this lesson, students will extract DNA from strawberries and analyze evidence to figure out who perpetrated a petty crime. For many students, one of the most fascinating applications of chemistry is crime investigation, so they will compare sets of DNA, test pH, and design and conduct interviews to crack the case. This lesson is designed to engage students in chemistry processes and applications.

Grade Level

Elementary school

Standards

NGSS and Cross-Disciplinary Extensions addressed in this lesson

Objectives

By the end of this lesson students will be able to:

• Describe how to extract DNA from strawberries.
• Compare the pH of different liquids.
• Compare patterns in mock samples of DNA.
• Design and conduct an interview.
• Make and support a claim about a crime scene based on analysis of evidence.

Chemistry Topics

This lesson supports students’ understanding of the following topics in chemistry:

• States of matter
• Chemical changes
• Solutions
• Acids and bases
• Observations
• Inferences

Time

Teacher Preparation

Strawberry DNA extraction activity: 15 minutes
CSI activity: 20 minutes

Lesson

Strawberry DNA extraction activity: 30 minutes
CSI activity: 70 minutes

Materials

Crime Scene

• Water in a plastic drinking cup, colored with food coloring
• Straw to place in the cup of water
• A comb or brush with hair in it
• Baseball, toy top, or some other object representing a hobby
• Note written in black felt-tipped pen: I have your mascot!!!!!! (or similar)
• Crime tape or similar

DNA Extraction

For each group:

• Salt
• Water
• Dishwashing soap
• Clear cups (2)
• Small funnel
• Coffee filter
• Strawberries
• Sealable baggie
• Isopropyl alcohol (refrigerated overnight)
• Toothpick
• Stirring rod or spoons
• Measuring spoon
• Pipet or medicine dropper
• Safety goggles
• DNA Extraction Student Activity Sheet or Science Journal

Crime Scene Investigation

• 4-6 “Persons of Interest” to participate in the interviews (could be school staff or students from other classes)
• CSI Teacher Resource Interview Questions & Responses document

For each group:

• CSI Crime Scene Observations Handout
• CSI Liquid Analysis Handout
• CSI DNA Analyses
• Science journal
• Litmus paper or pH meter
• Milk
• Orange juice
• Gatorade that is similar color to the crime scene liquid
• Tap water
• Seltzer water
• Liquid from the crime scene (tap water with food coloring)

Safety

• Remind students not to eat or drink anything in the lab or put their hands in their mouths.
• Isopropyl alcohol can sting if it touches the skin or eyes.
• Always wear safety goggles when working with chemicals.

Vocabulary Terms

• DNA
• pH
• evidence
• physical evidence
• chemical evidence
• chemical reaction

Keywords

DNA, CSI, suspect, pH, evidence, criminal, crime

Teacher Notes

Science Background

DNA Extraction

DNA, or deoxyribonucleaic acid, is a molecule found in the cells of living organisms. DNA contains the information needed for an organism to live and grow. DNA is like a blueprint that tells the body how to grow. A gene is a segment of DNA, and a chromosome is a little packet of genes. Therefore analysis of DNA allows scientists to analyze the chromosomes and genes in an organism.

Almost all of the cells in an organism contain DNA. Because DNA is such a long and complex molecule, the DNA of almost every individual living thing on Earth has a unique composition. An exception is organisms that are clones, which are genetically identical to each other. Because every person has unique DNA, analysis of DNA found at a crime scene can be used to identify the person it came from.

What is actually happening during the DNA extraction?

• Squishing the strawberries: breaks the cells, releases the cell nuclei
• Adding the extraction mixture:
  • Soap: breaks down the fatty membranes of the cells, breaks down the nuclear membrane, and releases the DNA
  • Salt: causes DNA molecules to stick together and to separate from the proteins in the cell
• Coffee filter: separates the extraction mixture with the DNA dissolved in it from the pulp, or leftover cell material
• Adding alcohol: causes the DNA to precipitate out of solution
• More about DNA extraction
  

For more about DNA, see

• Introduction: What Is DNA?
• deoxyribonucleic acid / DNA
• DNA, Genes, Chromosomes
• Create a DNA Fingerprint
• Challenges in DNA Testing and Forensic Analysis of Hair Samples

Tips

• A crime scenario has been provided, and all descriptions and explanations of procedures are based on this scenario. However, it should be adapted as appropriate for your class.
• The DNA extraction and the CSI portions of the lesson can be done separately.
• All activities can be made more engaging by involving students in the process of designing the tests.

Download the Teacher Guide to view the rest of this lesson.