This lesson introduces students to the relationships between chromosomes, genes, and DNA molecules. Using the example of a strawberry, it also provides activities that clearly show how changes in the DNA of an organism, either naturally or artificially, can cause changes. Grades 9-12

• Design 'Yer' Genes Part 1 Lab Sheet
• Design 'Yer' Genes Part 2 Lab Sheet
• Design 'Yer' Genes Part 3 Lab Sheet & DNA Cut-out Models
• Where Do Genes Come From? chart

chromosome: a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

codon: a series of three specific nucleotides of a base that specify the cell to make a particular amino acid

gene: a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring

genotype: the genetic makeup of an organism

mutation: changes in the genetic sequence that lead to new traits and diversity in offspring; often in a single generation

phenotype: the set of observable characteristics of an organism resulting from the interaction of its genotype with the environment

Prior to this activity students should have a basic knowledge of phosphate groups, adenine, guanine, cytosine, thymine, genes, chromosomes, codons, and DNA. Students should understand that their traits (and the traits of all organisms) are controlled by genes on chromosomes. These genes are made up of a set of molecules that are the same for all living things - sugars, phosphates, and bases. See the lesson DNA: Expressions in Agriculture for a basic lesson on DNA.

This lesson is hypothetical and very simplistic. The goals are for students to understand the general structure of DNA, the natural changes that occur in a DNA strand, and then the concept of genetic engineering. The lab activity itself is broken into three parts.

Part 1 has the students create a model of a small portion of a strawberry chromosome, complete with 3 genes. This activity illustrates the components of DNA molecules and shows how they hook together to make genes and chromosomes. The students will use this model to develop their understanding of DNA mutations and genetic engineering.

Part 2 requires the students to model a naturally occurring mutation. They will remove a segment of their DNA model (four base pairs) and replace it with a new piece (gene) inserted where the old one had been. This change will cause a different trait to appear in the strawberry's phenotype. (Real genes can be hundreds of base pairs in length, but for the sake of model size, the genes here will be four base pairs.) This activity shows students how a change in the genetic code (mutation) can result in an altered phenotype of the organism. The strawberry will be used as an example. Students will remove one of their strawberry genes and replace it with another. In nature, this is a random event and rarely provides an immediate benefit to the organism.

In Part 3, students work as "genetic engineers" and alter the strawberry's DNA. This part of the lesson provides students with a basic understanding of "real" genetic engineering that occurs in the laboratory. Your students will need some basic information on how genes are taken from one organism and then inserted into the genetic code of another organism. Some additional facts you may find useful are listed below.

• In Part 2 of this lab, students created a natural mutation by removing one gene and replacing it with another. Until recently, this could not be done purposely in the laboratory. These "removal and insertion" changes occur most often in natural situations.
• The desired gene can come from any organism-a dog, cat, tree, bacterium, etc. The trick is to make the organism accept the gene from another organism.
• Genes are genes. For example, a gene for the production of a protein like insulin is the same for all organisms and can theoretically be inserted to make insulin in any organism if the gene is accepted into the DNA molecule.
• In most commonly used genetic modification processes, genes are added, not removed or replaced. It is technically much easier to insert genes into a chromosome than it is to remove or replace them.
• Inserting a gene does not guarantee that the desired trait will be expressed in the new organism. There are many factors controlling gene expression, and in many cases successful gene transfer is a process of trial and error.

1. Inform your students that you will be giving them a series of clues and that you'd like them to guess the item you are describing. Instruct students to raise their hand when they think they know what the item is.
2. Give the following clues:
   • They have 8 chromosomes, each containing thousands of genes.
   • Breeding programs use traditional hybridization methods to continue improving them.
   • California produces 80% of this U.S. crop annually and is the world's largest producer.
   • Eight of these contain 140% of the U.S. RDA for Vitamin C.
   • Many research articles have been written about the origin of the name for the fruit, but none seem clearly definitive. Here are some theories on how it got its name:
     • Historically, straw was placed under the fruit to prevent bruising.
     • Early cultivators noticed the vines grew all over the place or were "strewed" or "strawed."
     • English children threaded the berries onto straw and offered them for sale.
     • The plant's runners resembled straw.
     • The ancient Latin word stragum means fragrant.
3. What is it? A strawberry! Ask your students to identify what distinguishes a strawberry from a raspberry or other fruit. Students may offer many different answers. Use guided questions to introduce the lesson and to help them identify that the DNA and genetic makeup of a strawberry distinguish it from other fruits.

Part I: Building a Strawberry DNA Molecule

1. Review and complete the entire lesson yourself so you can get a feel for the concepts and sequence. Jot down notes that will help the lesson flow smoothly with your students. Save your completed model of the strawberry DNA to use as a visual example for your students.
2. Begin this activity by referring to Darwin's finch experiences or the peppered moths of England. Let the students know that this lesson will shed light on how a living species, such as the birds in the Galapagos, can evolve into a different species due to the changes in their genetic blueprint.
3. Distribute a fresh strawberry to each student. Before they eat it, brainstorm a list of observations about the berries. Have the students observe some of the strawberry's characteristics such as size, shape, and color. Have students compare their strawberries with their neighbor's strawberry. Allow students to eat their strawberry. (As always, be aware of student food allergies before having a student eat the fruit.) 
4. Distribute the Design Yer Genes Part 1 lab sheets to your students. Discuss and clarify the problem the students are trying to solve. Explain to your students that they are going to build a simple model of a strawberry DNA molecule to better understand genetics.
5. Review the procedure for building the strawberry DNA with your students. Pair your students together into working teams of two. Monitor their work continuously. Provide plastic bags or envelopes for your students to organize and store their work. Allow the students to figure out how the base pairs should match with the sugar units by trial and error. Display your DNA model. Remind students that sugar units alternate with phosphate units and that base pairings must be A-T and C-G. Do not have your students tape or glue their models together until they "dry-fit" their model and get it approved by you! Refer the students to the strawberry Gene Key (found in the handout) at the appropriate time.  
   • Note: As the students complete their models, check them for accuracy. You want your students to be successful in this portion of the activity so they will be encouraged to learn the science concepts rather than get bogged down with the coloring and cutting activity. Coloring only the left or right side of the sugar-phosphate links may be an option for student groups who are having difficulty.
6. Discuss the answers of the Part 1 questions. Assign these questions for homework if they do not finish them in class.
7. During and after DNA model completion, check your students for understanding regarding traits and genes. Have the students twist their completed models into the classic double helix and discuss how X-ray diffraction led Watson and Crick to the discovery of DNA's shape. Stress that DNA and the manipulations of DNA done by geneticists are much more complicated than their models suggest.
8. Prepare the students for Part 2 of this lesson by asking for their opinions on how the strawberry could be improved. You might also want to inquire about their knowledge regarding gene splicing or genetic engineering. Consider having students do research on how farmers have "changed" certain produce items through selective crossing or hybridization. Examples include the production of seedless watermelons and grapes, strains of corn and wheat that are disease resistant, dwarf trees, and the production of tangelos and broccoflowers. Refer to the attached chart, Where Do Genes Come From?

Part 2: Genetic Mutations

1. Explain to your students that the DNA model they developed in Part 1 of this lesson is just that-a model. Briefly discuss how DNA molecules are reproduced and how easy it is to make a slight error during DNA replication. You might even discuss how errors were made in the production of the student DNA models. An error in DNA replication is called a mutation. Genetic engineering is when a DNA molecule is purposely altered. Explain to the students that they will act as geneticists and purposely alter a strawberry DNA molecule by removing a gene and inserting another.
2. Review the problem and procedure of Design Yer Genes-Part 2 Working in the same groups as they did in Part 1, have students carefully follow the described procedure. Extra cut-out sheets may be needed. Again, your prepared model will help students visualize what it is they are to do. As with Part 1 of this activity, remind students that they are modeling a very complex procedure-DNA (genetic) replication. It is much more complicated than can be represented by this model.
3. After the students have completed altering their strawberry DNA model, have each group explain to the class (or in writing) what characteristic they altered.
4. Have the students answer the Design Yer Genes-Part 2 Questions.  These questions can be an in-class assessment or a homework assignment. Previewing and discussing the questions will be helpful to the students. Here are some points to discuss with your students prior to their work on the questions:
   • Question 4 could benefit from references to the Galapagos finches and the peppered moths. Negative effects of DNA mutation can be discussed by referring to the many human genetic disorders such as Huntington's Disease, Cystic Fibrosis and Sickle Cell Anemia. It is important to stress that there may also be positive effects to what we call negative genetic disorders. For example, people who carry the Sickle Cell gene (Ss) but do not express the trait (ss) are resistant to Malaria.
   • You might wish to assign question 6 as a research paper rather than as one of the regular questions. Again, the students will gain more insight for this question if you guide them in a discussion of the positive and potentially negative implications of genetic engineering. Enlighten your students as to how the scientific and political communities are dealing with public concerns.

Part 3: Genetic Engineering

1. Review the problem and procedure of the worksheet, Design Yer Genes-Part 3 with your students. Discuss that what makes this lab more like genetic engineering than the Part 2 activity is that one gene is not removed or altered from the DNA; rather a new gene is added.
2. The students will need four gene cut-out sheets to choose from. Gene A, Gene B, Gene C and Gene D. Explain that each gene codes for or controls a specific trait, which you will reveal after the students have chosen and added one or more of the traits to their strawberry DNA model.
3. Have the students complete the activity. Remind the students that when adding their new gene, they can insert the new gene anywhere in the molecule as long as the three previous genes are not destroyed. They can insert the new gene between two other genes, at the end of one gene, etc.
4. After the students have completed their genetic manipulations, reveal what the hypothetical new genes do:
   • Gene A comes from a bacterium and causes an increase in sugar production in the strawberry for a super sweet berry.
   • Gene B comes from red algae and causes an increase in beta-carotene pigment production for very red berries.
   • Gene C comes from a banana and causes the strawberry to have a banana taste.
   • Gene D comes from a virus and causes the strawberry to become resistant to a certain bacteria that makes strawberries rot. Therefore, this altered strawberry resists rotting.
5. Have the students complete the "Questions" section of this lab.
6. Discuss the implications of some of the hypothetical genes mentioned above. For example, if a strawberry plant does not produce sweet berries, Gene A might do wonders for the strawberry industry. If Gene B is added to a light pink strawberry, it might make the berry more appealing to the consumer. However, if Gene B is added to an already red strawberry, the increase in red color may cause the berry to be so red it could appear brown or black. Gene C may or may not affect the saleability of the strawberries while Gene D could reduce the need for pesticides.
7. Discuss that some unwanted side affects may result from genetically modifying the strawberry plant. For example, a gene may insert itself into the blooming mechanism of the plant and produce sterile flowers or no flowers at all. If this is the case, the redder color or change in taste may not work because strawberries would not even be produced to show the new trait. This is one reason why the process of transgenics is so complex and time consuming.
8. Review the fact that the students' models are only simplified versions. A strawberry plant has eight chromosomes, each made of thousands of genes. Each gene is made of thousands of base pairs!
9. Emphasize that the study of genetics is very complex and that if the students like this activity they may want to pursue taking more classes in genetics.

Although strawberries are used as an example crop in this lesson, it's important to note that strawberries have not been genetically altered (GMOs) using biotechnology. All strawberry varieties on the market were developed through traditional cross-breeding methods to obtain desired characteristics.

Variations

• Have students create edible. DNA models out of marshmallows, gum drops, etc., and then have an "Eat Your Genes" party in class.
• Make "Gene D" a funny or unusual trait, such as a "skunky" smell. This may add humor as well as show that genetic engineering does not always produce desired results.

After conducting these activities, review and summarize the following concepts:

• Strawberries are an important food source grown by farmers and distributed through the U.S. and even the world.
• Science, such as our knowledge of DNA and genetics is used to improve the production of our food.
• More in-depth science such as biotechnology and genetic engineering provide further ability to produce higher quantities of healthy food such as strawberries. Using these technologies requires careful scrutiny.

This lesson was funded in 1995 by Calgene, Incorporated and the California Farm Bureau Federation. To meet the needs of California educators, From Genes to Jeans was revised to support the Curriculum Content Standards for California Public Schools. Funding from the California Farm Bureau Federation and private donations was used to make this revision possible.

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