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.
A gene is a sequence of DNA, which serves as a blueprint for the production of proteins in all living things. Thousands of genes make up chromosomes. DNA is found in the nuclei of cells with the exception of bacteria and viruses. Bacteria have their DNA in nuclear areas called nucleoids; viruses have their DNA coiled up in the cytoplasm of cells. DNA is made of sugars, phosphates, and four nitrogen-containing bases: adenosine, cytosine, guanine, and thymine. A gene codes for a specific protein or has an assigned function.
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.
