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Evolutionconnection lactase
- 1. Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids Learning goals: Students will understand that 1) our evolutionary history has affected our genes and proteins, 2) the availability of particular carbohydrates in the environment has shaped the evolutionary history of different human populations, and 3) our evolutionary history matters in our everyday lives. For the instructor: This short slide set weaves together basic information about carbohydrates, proteins, and nucleic acids into one evolutionary story that relates to students’ everyday lives. To integrate it best, use these slides immediately after you’ve discussed the structure of these biological molecules. You may wish to prime students for this story by using lactose and lactase as examples when discussing carbohydrates and proteins. Additional details about the translation of nucleic acids into proteins could be incorporated into the last slide of this series, to the degree that you cover this material in your discussion of the structure of nucleic acids. You may also wish to return to this story as an example when you cover Mendelian genetics. The European version of the lactase persistence gene is inherited as a dominant Mendelian trait. Each of the following slides comes with a sample script for the instructor. To review this script, download the PowerPoint file and view the Notes associated with each slide.
- 2. Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids Digesting lactose with an enzyme Lactose Galactose Glucose
- 3. Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids Lactase to the rescue! Galactose Glucose Lactose Lactase image is from BioMolecular Explorer 3D and is used under the conditions of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.
- 4. Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids Gene for lactase
- 5. Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids Photo of highland cattle is by the Scottish government and is used under the conditions of a Creative Commons license. Photo of Masai with cattle is by Oxfam International and is used under the conditions of a Creative Lactose tolerance arises several times and increases in frequency. Lactose tolerance arises and increases in frequency. 10,000 years ago: 100% lactose intolerant
- 6. Milkshake photo by Svadilfari and used under a Creative Commons license. 1. Ancestral lactose control gene on chromosome 2 lactose intolerance 2. C nucleotide replaced by a T nucleotide lactose tolerance 3. One of three single nucleotide changes lactose tolerance Evolution connection: proteins, carbohydrates, and nucleic acidsEvolution connection: proteins, carbohydrates, and nucleic acids
- 7. Reference:Reference: Tishkoff, S. A., Reed, F. A., Ranciaro, A., Voight, B. F., Babbitt, C. C., Silverman, J. S. . . . Deloukas, P. (2006). Convergent adaptation of human lactase persistence in Africa and Europe. Nature Genetics. 39: 31-10. Evolution Connection slideshows are provided by Understanding Evolution (understandingevolution.org) and are copyright 2011 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California. Feel free to use and modify this presentation for educational purposes.
Notas do Editor
- We’ve just learned about the many different functions that proteins serve—including catalyzing chemical reactions. Proteins that speed up chemical reactions without being used up themselves in the process are called enzymes. For example . . . Lactose is the primary sugar in milk. It is a disaccharide made up of two sugar monomers—glucose and galactose. Animal cells have transporter molecules that can carry monosaccharides across the cell membrane, but we don’t have any transporter molecules that can carry lactose (a disaccharide) across. In order to get nutrition (i.e., calories) from lactose, we must break it down into monosaccharides so it can get across the cell membrane. How? With an enzyme! (Note to instructor: An animal sucrose transporter was recently discovered in Drosophila, but so far this is the only disaccharide transporter known from animals.)
- Lactase is an enzyme—a protein. What do you notice about the structure of lactase in comparison to lactose? The enzyme lactase has a very different structure from a sugar like lactose. It has a complicated 3-D structure caused by three levels of folding of its amino acid chain, as we discussed before. (Note: Lactase is made of a single polypeptide, so it does not have a quaternary structure.) (CLICK) Lactase can cleave lactose into its constituent monosaccharides, so we can absorb them. Lactase is normally released in the small intestines, where this step of digestion takes place. What happens if you eat lactose, but don’t produce any lactase? Lactose intolerance! What are the symptoms of this? Bloating, gas, cramps, etc. But we could all digest milk when we were infants. What happened in the mean time?
- We all carry the gene for lactase. (CLICK) When we are infants, this gene is turned on and we can all drink milk. However, as adults, it is different. (Click) In some of us, this gene keeps producing lactase into adulthood; in others of us, it does not. Why?
- The reason some of us are lactose tolerant and some not has to do with our evolutionary history. 10,000 years ago, all human populations were lactose intolerant. We all carried the gene to produce lactase, but this gene was naturally turned off after weaning. So no one could digest milk as adults. Around 7500-9000 years ago, everything changed. (CLICK) Cattle were domesticated in Africa and in the Middle East—(CLICK) but only a few human cultures began herding cattle as a way of life, some in Europe and some in Africa. In these populations, being able to drink milk directly would have been advantageous. Any mutation that happened to have the effect of keeping the lactase gene turned on into adulthood would have been favored and increased in frequency. (CLICK) This happened once in European populations and (CLICK) several times in African populations.
- Depending on the evolutionary history of your ancestors, you probably carry different versions of the control gene that turns the lactase gene on and off. Those of us whose deep evolutionary history traces to West Africa, Asia, or indigenous American populations are likely to have the ancestral version of the gene that controls when lactase is turned on and off. This gene is located on human chromosome 2. The ancestral version keeps the lactase gene turned off in adulthood and, hence, causes lactose intolerance. (CLICK) Those of us whose deep evolutionary history traces to Northern Europe (CLICK) are likely to carry a version of this gene that is just a tiny bit different—just one nucleotide different. At position 13,910 of the gene, a C nucleotide was replaced by a T. This tiny difference means that lactase can be produced as an adult and results in lactose tolerance. (CLICK) Those of us whose deep evolutionary history traces to East African populations (CLICK) are likely to carry one of three versions of this gene—each just a tiny bit different from the ancestral version: A G replaced by a C at position 14010, a T replaced by a G at position 13915, or a C replaced by a G at position 13907. Each of these changes is associated with the persistence of lactose tolerance into adulthood—and the ability to enjoy a milkshake! (CLICK)
- Depending on the evolutionary history of your ancestors, you probably carry different versions of the control gene that turns the lactase gene on and off. Those of us whose deep evolutionary history traces to West Africa, Asia, or indigenous American populations are likely to have the ancestral version of the gene that controls when lactase is turned on and off. This gene is located on human chromosome 2. The ancestral version keeps the lactase gene turned off in adulthood and, hence, causes lactose intolerance. Those of us whose deep evolutionary history traces to Northern Europe are likely to carry a version of this gene that is just a tiny bit different—just one nucleotide different. At position 13,910 of the gene, a C nucleotide was replaced by a T. This tiny difference means that lactase can be produced as an adult and results in lactose tolerance. Those of us whose deep evolutionary history traces to East African populations are likely to carry one of three versions of this gene—each just a tiny bit different from the ancestral version: A G replaced by a C at position 14010, a T replaced by a G at position 13915, or a C replaced by a G at position 13907. Each of these changes is associated with the persistence of lactose tolerance into adulthood—and the ability to enjoy a milkshake!