Grade: Grade 7 Subject: Science Unit: Genetics Lesson: 3 of 6 ACT: Science

Investigation Lab: Extracting DNA

Learning Objectives

In this hands-on investigation, you will:

  • Extract DNA from plant cells using household materials
  • Observe the physical properties of DNA
  • Understand why each step of the extraction process is necessary
  • Connect the structure of DNA to its function in cells

Background Information

DNA (deoxyribonucleic acid) is found in the nucleus of every living cell. Although DNA is microscopic, when you extract it from millions of cells at once, you can see it with the naked eye as a white, stringy substance.

In this lab, you will break open plant cells to release their DNA. The extraction process involves three main steps:

  1. Cell lysis: Breaking open the cell membrane and nuclear membrane using detergent
  2. Protein removal: Using salt to separate proteins from the DNA
  3. DNA precipitation: Using cold alcohol to make the DNA visible and collectible

Lab Procedure

Materials Needed

  • 1/2 cup of split peas, strawberries, or banana
  • 1/8 teaspoon table salt
  • 1 cup cold water
  • 1 tablespoon liquid dish soap
  • Blender or mortar and pestle
  • Strainer or coffee filter
  • Test tube or tall narrow glass
  • Ice-cold rubbing alcohol (91% isopropyl)
  • Wooden skewer or toothpick

Step-by-Step Instructions

  1. Prepare the extraction liquid: Mix 1 cup of water, 1/8 teaspoon salt, and 1 tablespoon dish soap. Stir gently to avoid bubbles.
  2. Blend the plant material: Add 1/2 cup of your plant material to the blender with 1 cup of water. Blend for 15-20 seconds until smooth.
  3. Strain the mixture: Pour the blended mixture through a strainer into a clean container. Collect about 2 tablespoons of the filtered liquid.
  4. Add extraction liquid: Mix the filtered plant liquid with 2 tablespoons of your extraction liquid. Stir gently for 2-3 minutes.
  5. Transfer to test tube: Pour the mixture into a test tube, filling it about 1/3 full.
  6. Add cold alcohol: Slowly pour ice-cold alcohol down the side of the tube until you have an equal amount of alcohol as mixture. Do not mix!
  7. Wait and observe: Let the tube sit undisturbed for 2-3 minutes. Watch for white, stringy material forming at the boundary between the layers.
  8. Collect the DNA: Use a wooden skewer to gently spool the DNA strands by twirling the stick in the white material.

Practice Questions

Answer these questions based on the lab procedure and your understanding of DNA extraction.

1. Why is dish soap used in the DNA extraction process?

Think about what cell membranes are made of and how soap interacts with those materials.

2. What is the purpose of blending the plant material?

Consider what physical changes occur when you blend fruits or vegetables.

3. Why must the rubbing alcohol be ice-cold?

Think about how temperature affects the solubility of substances.

4. A student forgot to add salt to their extraction liquid. Predict how this might affect their results.

Consider the role of salt in separating DNA from proteins.

5. Why does DNA appear white and stringy when extracted, even though individual DNA molecules are invisible?

Think about what happens when millions of tiny things clump together.

6. Compare extracting DNA from strawberries versus peas. Which might yield more visible DNA and why?

Research fact: Strawberries are octoploid, meaning they have 8 copies of each chromosome.

7. In the extraction process, the DNA rises to the top where the alcohol layer is. What does this tell you about the density of DNA compared to the water-based mixture below?

Think about what happens when less dense substances are placed in denser liquids.

8. A scientist wants to extract DNA from human cheek cells instead of plant cells. Would the same procedure work? Explain what modifications might be needed.

Consider the differences between plant and animal cells.

9. After completing the extraction, a student notices very little DNA was collected. List three possible reasons for this result.

Think about what could go wrong at each step of the procedure.

10. Design a simple experiment to test whether the amount of dish soap affects the amount of DNA extracted. Include your hypothesis, variables, and basic procedure.

Remember to identify your independent variable, dependent variable, and controls.

Check Your Understanding

Key Concept Check: Put the following steps of DNA extraction in the correct order:

  • Add cold alcohol to precipitate DNA
  • Break open cells with detergent
  • Blend plant material to separate cells
  • Use salt to remove proteins
  • Spool DNA with a wooden stick

Connection Question: How does this lab help you understand where DNA is located in cells and why it is protected there?

Next Steps

  • Complete your lab report with observations and analysis
  • If you performed this lab, take photos of your results to document your work
  • Research how scientists extract and purify DNA for genetic testing
  • Continue to the next lesson: Data and Graphs in Genetics