To develop understanding of DNA by modeling the process of DNA extraction.
In middle school, students learn about genes and traits. By high school, students may have yet to delve into a broad understanding of how these things relate to DNA. High-school students have enough biology background to understand where DNA is located as well as how it affects each individual.
Students need to learn about DNA in order to understand why there are "similarities and differences between parents and offspring, hereditary diseases, and the evolution of new species." Their knowledge of DNA will also help them understand how scientists manipulate genes in order to create new traits and varieties of organisms.
This lesson will introduce DNA, genes, chromosomes, the chemicals that make up DNA, etc. After the basic information, students will do an experiment in which they will separate out DNA from peas. Knowing that DNA can be separated will give them a base of understanding for future lessons in biology, evolution, biotechnology, and health technology.
Note: The Motivation and Part 1 of the Development were created for ninth graders. This lesson may also be appropriate for advanced students in seventh and eighth grade, or older students that need a quick review of the basics. High school students will benefit from Part 2 of the Development, the experiment in which DNA is "floated" in a pea soup mixture.
Go to Genetic Science Learning Center and read the instructions for setting up the experiment How to Extract DNA From Anything Living, which students will do in Part 2 of this lesson. You can either have students go to the website to follow the instructions for the experiment, or print the experiment for your students ahead of time.
Begin by having students explore DNA: The Instruction Manual for All Life, from the Tech Museum of Innovation. (They should keep pressing the "next" button at the top of the page to work through the exhibit.)
We don't recommend that students play the game toward the end of the exhibit, since it is time consuming and adds little to student understanding at this point in the lesson. Tell students to stop once they get to the Finding a Sequence in a Genetic Haystack page.
After students have explored the resource, ask the following:
- What is DNA? (Answers will be simple for now and probably consist of "directions for your body to grow.")
- What are some things DNA determines? (Color of hair, height, etc. Lead students to realize that DNA determines everything about our bodies.)
- Where is DNA? (The nucleus of almost every cell.)
- What are chromosomes made of? (They are formed from DNA and contain the information for building more cells.)
- What makes up DNA? (Discuss the four chemicals and their endless combinations.)
- How do scientists do sequencing? (Using x-ray films. Each dark band on the X ray matches a colored base and each row of bands corresponds to one of the four bases.)
This is a two-part lesson. In the first part, students will explore Internet resources to develop their knowledge of DNA. In the second part, the class will do an experiment that demonstrates DNA extraction.
Students will do the first part of the lesson independently. Send them to the following page: Tour of the Basics.
Have students read through the following topics on that page:
- What is DNA?
- What is a gene?
- What is a chromosome?
- What is inheritance?
- What is a protein?
These pages contain links to exercises and activities. Advise students to skip most of these for now and just read the information on the pages. Once students have read these pages, have them go to the Build a DNA Molecule activity. They should do this activity.
To help students avoid confusion regarding how chromosomes, genes, and DNA are related, have them take notes on these three things. Also let them know that they will be responsible for producing a poster at the end of the lesson. They may want to collect artwork from the websites they explore, or draw their own. If they want to draw their own, they should sketch and label illustrations that they find interesting. (See assignment below.)
After students have explored the resources and made notes or copied illustrations, discuss the following questions:
- What do chromosomes look like and how are pairs identified?
- When you built the DNA strand in the What is DNA? section, what did you notice?
- How do we inherit characteristics from our parents?
- Describe transcription. (Making a copy of the DNA sequence in the form of RNA.)
- Describe translation. (When the information in RNA is used to produce a string of amino acids.)
- What roll do proteins play in an organism?
Note: Make the point that anything living has DNA, this includes even insects, like fireflies, and plants and peas. Tell students they will now do an experiment to extract DNA from peas.
Part 2 (Extracting DNA Experiment)
While students read, you can set up for the experiment How to Extract DNA From Anything Living. As mentioned in the Planning Ahead section, you can provide copies of this experiment to your students or have them go to the website and the follow the experiment online.
The best way to do this experiment is to blend the mixture yourself, and let students do the rest of the extraction. To save time, you could set up the stations with appropriate supplies at each.
- Divide students into groups of four and give each group a glass jar with the mixture. Tell students that the blending separated the cells.
- As you give each group detergent, ask if they know what the detergent will do. Perhaps lead them to realize that the nucleus needs to be broken open for the DNA to "fall out." The detergent breaks open the sack.
- Ask what else needs to be done to separate out the DNA. Remind students that the cell and nuclear membranes have been broken apart. What is left? The proteins, carbohydrates, and DNA. The meat tenderizer (an enzyme) cuts the proteins away from the DNA.
- Ask students what they think the rubbing alcohol will do. Lead them to understand that the rubbing alcohol floats on the mixture, the protein and grease sink, and the DNA floats to the top. Students should see white stringy DNA floating in their glass containers.
- Discuss how scientists are capable of extracting DNA from anything with the right tools.
To assess students' knowledge of the content of this lesson, assign for homework a poster that students can use to teach someone else about DNA.
The poster should address the following:
- What is DNA? (It is the molecule that has the instructions our cells follow to build a living organism. DNA is like a blueprint for building different parts of a cell.)
- Where is DNA? (The nucleus of almost every cell.)
- What are chromosomes made of? (They are compact units of DNA.)
- What makes up DNA? (DNA is made of long strands of nucleotides.)
- What do chromosomes look like and how are pairs identified? (They look like pairs of worms that match. Every human has 46 pairs of chromosomes, two sets of 23. There are two sex chromosomes. Males have an x and y chromosome, and females have two x chromosomes [no y].)
- How do we inherit characteristics from our parents? (We get half of our chromosomes from each of our parents.)
- Describe transcription. (Trasncription is the process of creating a complementary RNA copy of a sequence of DNA.)
- Describe translation. (Translation is when the information in RNA is used to produce a string of amino acids.)
- What roll do proteins play in an organism? (Proteins carry out all the functions in a living cell.)
Student directions for this assignment can be found on the Explaining DNA student sheet. In the directions, students are told that the poster can rely heavily on artwork and/or diagrams. This is okay as long as the poster clearly answers the questions listed above.
Another related experiment from the Genetic Science Learning Center is DNA Extraction From Wheat Germ. You may want to consider combining this with the pea experiment, or having half the class do one, and half the class do the other.
Also of interest from the Genetic Science Learning Center is What are Genetic Disorders? It includes general information as well as hands-on experiments.