THE EDIBLE OPERON David O. Freier Lynchburg College [BIOL 220W Cellular Diversity]

Transcription

1 THE EDIBLE OPERON David O. Freier Lynchburg College [BIOL 220W Cellular Diversity] You have the following resources available to you: Short bread cookies = DNA / genetic elements Fudge Mint cookies = RNA polymerase Large Sprinkles = monosaccharides Fruit slices = Repressor protein Icing tubes (and Edible markers) Your task. Create an edible version of the lac operon. You must include the following components; the LacI, LacZ, LacY and LacA genes, the promoter, the operator, the repressor protein, RNA polymerase and lactose/allolactose. You must demonstrate both repression and induction of the operon. Take photographs of your creation(s). Use the images to provide a written explanation of how the Lac operon works. This should be a short paper of 2-4 pages in length, double spaced. Be sure any assertions you make can be verified by reference to Karp or a similar level of authority. Use a standard 12 point font (i.e. times new roman) with 1 margins and citations as appropriate. Rubric (9 points) Explanation of the structure & operation of the Lac operon [4 points] Successful Moderately Successful Needs Development Unsuccessful Effective use of the images in presenting the Lac operon [3 points] Successful Moderately Successful Needs Development Unsuccessful Writing clarity and accuracy including proper citation of sources (2 points) Clear/Accurate Mostly Clear/Accurate Unclear/Incorrect EXTRA: Correct inclusion of Glucose / camp (1 point) Correct Partially Correct Incorrect

2 ABOUT THE LAC OPERON Please answer the following questions to the best of your ability and print clearly. 1. Briefly, define the purpose of the lac operon. 2. How many structural genes does this operon have? 3. What specific protein acts to regulate the expression of the structural genes? 4. What monomeric sugars are produced by action of the lac operon? 5. Have you ever taken a general microbiology course? [Yes or No] 6. Have you ever taken genetics? [Yes or No] PLEASE INITIAL HERE

3 Kandace Carter BIOL 220 February 25, 2013 The Edible Operon Bacteria have sections of DNA that encode for the enzymes necessary for metabolism, these sections are known as operons. Through the study of the bacteria Escherichia coli a better understanding of operons has been gained. The Lac operon has been the most specifically studied operon; it contains the genes necessary to produce the enzymes that are needed to break down lactose within the bacterial cell. As an inducible operon, these genes are only expressed when lactose is present in the cell, which saves valuable energy that would be otherwise wasted if the enzymes were produced at all times. (Karp) For the purpose of understanding and explaining the function of the Lac operon, a model was constructed. The materials used to make this model were all edible, consisting of shortbread cookies, a chocolate cookie, green icing, blue icing, gummy candy, and candy dots. The shortbread cookies lined with blue icing are representative of the genes in the Lac operon. The cookie separate to the far left is a model of the Lac 1 gene and the space represents the hundreds of base pairs that separate this gene from the rest of the operon. The smaller cookie that is solid blue is representative if the promoter site where the RNA polymerase attaches to the DNA. The smaller cookie outlined in green is the operator site where the repressor protein binds. The next three cookies are representative (in order) of the Lac z, Lac y, and Lac a genes. The gummy candy is a model of the repressor protein and the chocolate cookie is a model of RNA polymerase. The candy dots represent lactose molecules. Pictures of the models are referenced throughout the explanation and are located on pages 5 and 6.

4 Figure 1 depicts the Lac operon in its repressed state (p 5). In the repressed state, the genes necessary for the metabolism of lactose are not transcribed and the result is that the enzymes are not produced by the cell. The repressed state is achieved by the attachment of a protein known as a repressor protein to the section of DNA known as the operator site. The operon in its entirety consists of four specific genes as well as a promoter site for RNA attachment and an operator site for repressor protein attachment. The promoter site and the operator site are not completely separate, they overlap each other. When lactose is not present in the cell, the Lac 1 gene, which is located several hundred base pairs prior to the promoter site of the operon, transcribes the repressor protein. The repressor protein binds to the operator site that is located adjacent to the other Lac genes. With the repressor protein attached, RNA polymerase is not able to attach to the promoter site and therefore is unable to transcribe the DNA in order to make the proteins used for breaking down lactose. In this way, the repressor serves as a sort of road block to the creation of the proteins. (Karp) Figure 2 depicts the Lac operon in an induced state (p 6). When lactose is present inside of the bacterial cell it binds to the repressor protein. This binding causes a change in conformation and the protein detaches from the operator site. With the repressor protein gone, the RNA polymerase is able to attach to the promoter site and begin transcribing the genes in the Lac operon. The first gene it comes to is the Lac z gene, which encodes for the β- galactosidase enzyme. The next gene transcribes is the Lac y gene, which encodes for the galactoside permease enzyme. The final gene transcribed from the Lac operon is the Lac a gene which encodes for thiogalactoside transacetylase. Each of these proteins has an important role in the metabolism of lactose. Once all of the lactose has been broken down the repressor protein is no longer bound to lactose molecules and is able to return to its original conformation. Once returned to its original

5 conformation it again binds to the operator site and effectively shuts off the production of lactose metabolizing enzymes. (Karp) The above explanation of the Lac operon is a simplified version of an incredibly complex genetic mechanism. The Lac operon is still not entirely understood. Many details remain to be discovered; such as the function of thiogalactoside transacetylase whose purpose is still not clear. Additionally, the relationship between glucose, cyclic AMP and the function of operons add complexity to the basic system of operons. The process described above explains gene regulation through negative control. However, the Lac operon is also controlled in another manner which is considered positive through the relationship of camp and glucose. When both glucose and lactose are present in the cell, the Lac operon remains in a repressed state and does not begin synthesizing lactose metabolizing enzymes until the glucose has been completely broken down. The glucose inhibits the amount of camp present in the cell. As the glucose decreases, the camp increases and binds to the camp receptor protein. This complex binds to the DNA and allows the RNA polymerase to effectively transcribe the Lac operon. (Karp) This process is just one of many other factors that may be involved in the function of the Lac operon. Additionally, the Lac operon is only one operon. Bacterial DNA has multiple operons that have yet to be studied or possibly even discovered. As with most scientific concepts, discovery often only brings more questions and possibilities. However, for now, the edible operon provides a simple and understandable explanation for the inducible Lac operon. Literature Cited Karp G Cell and molecular biology. 6 th ed. Hoboken: John Wiley and Sons.

6 Figure 1 Lac operon in repressed state. Shortbread cookies lined in blue (from left to right) Lac 1 gene, Lac z gene, Lac y gene and Lac a gene. Solid blue cookie is the promoter site and green lined cookie is the operator site. The red gummy candy is the repressor protein. Figure 2 Lac operon in induced state. Shortbread cookies lined in blue (from left to right) Lac 1 gene, Lac z gene, Lac y gene and Lac a gene. Solid blue cookie is the promoter site and the green lined cookie is the operator site. The red gummy candy is the repressor protein, the candy dots are lactose molecules. The chocolate cookie is RNA polymerase.