A Simply Fruity DNA Extraction

Transcription

1 A Simply Fruity DNA Extraction Category: Biology Type: Experiment (60 min class) Rough Parts List: 1 Strawberry 1 Plastic freezer bag 100 mls Water 100 mls Soap 1 Tbsp Salt 15 mls Ice cold 91% isopropyl alcohol 4 Layers of cheese cloth 1 Rubber band 1 Large plastic cup 1 Small clear plastic cup 1 Toothpick Video: How To: To break open the cells of the strawberry we need a lysis buffer. Prepare the lysis buffer in a cup as follows: mix 100ml of water, 100ml of soap, and 1 tablespoon of salt. Remove the leaves from the strawberry, place the berry in the freezer bag and mash it up really well using your hands.

2 Add lysis buffer to the bag, close and squish for a few minutes. Mix carefully, the fewer bubbles created the better the results. Cover the plastic cup with the cheese cloth and push the strawberry/detergent mix through the muslin little by little. Throw away the cloth. Pour the alcohol into the cup and wait 1-2 minutes for the DNA to start precipitating out. You should see a white fluffy cloud form, this is the DNA. View of the precipitated DNA from above. Use the toothpick to lift the DNA out of the alcohol. Voila! Strawberry DNA. Fine Points: The lysis buffer can be made in class by the students or made beforehand by the teacher. For the soap you can use dish liquid, hand soap or shampoo.

3 Don t use a sandwich bag as these are thinner than freezer bags and can break during strawberry mashing. Other fruits can be used but strawberries are easy to mash. They are also octoploid, which means that they contain a lot of DNA. You can isolate human DNA but you will need to get someone to fill a shot glass with spit! 100% isopropyl alcohol or 95% ethanol can also be used to precipitate DNA. The alcohol needs to be ice- cold so leave the bottle in the freezer overnight before the class. The precipitation normally begins immediately but the DNA can take a couple of minutes to condense. Objectives: During this experiment, students will: 1. Extract DNA from a Strawberry and understand that DNA is in the food they eat. 2. Understand why each step of the experiment is peformed. 3. See DNA with their own eyes. Concepts Involved: Deoxyribonucleic acid (DNA) is the molecule that controls everything that happens in the cell. It contains instructions that direct the activities of cells. DNA may be extracted with a simple method using household items. Each step is necessary because of the complex way that DNA is packaged in cells: Soap is used to break down the cell membrane, a lipid bilayer. Salt helps to remove proteins bound to the DNA. It also helps to keep the proteins soluble in the water layer (detergent) and stops them from precipitating with the DNA into the alcohol. The alcohol causes the DNA to precipitate i.e. to separate in solid form from a solution. Focus Questions: 1. Why would we want to extract DNA from things? 2. Can you think of something you ve seen before that looks like the DNA? 3. Compare the DNA you ve extracted to the size of the strawberry originally. How much of the strawberry was DNA? 10%? 1%? Less than 1%? How could you figure that out for sure? Elaboration: All organisms are made up of cells, from simple single- cell bacteria to multi- cell humans and plants. It doesn t matter if you are human, a bacteria or a strawberry, every cell contains deoxyribonucleic acid (DNA). DNA is the blueprint that determines everything that happens in a cell. It contains instructions that are used in its development, function and reproduction. Removing DNA from a cell is called DNA extraction. Extraction of DNA from cells and its purification are techniques that are very important to scientists. Isolating DNA allows scientists to identify and study genetic diseases, and if they experiment further with this DNA they can discover new treatments/medicines. Scientists can also genetically engineer organisms in the lab to produce things that are beneficial to humans, such as insulin. In forensic science DNA is used to answer questions such as maternity/paternity, or whether a person was present at a crime scene. These tests take advantage of the fact that every person on the planet has a unique DNA. In this experiment we make our own DNA extraction kit from household materials and use it to purify DNA from strawberries. We are using strawberries to extract DNA because their cells each have eight copies of the genome in every cell. This means that it has a lot more DNA per cell than an organism that

4 only has one or two copies. It also means that we have a better chance at having a successful DNA preparation! During the DNA extraction, we use physical force (mashing the strawberry) and the lysis buffer to break open the cell and release the DNA into solution. DNA is highly soluble in water because the phosphate group of each nucleotide carries a negative charge. Positively charged sodium ions from the salt in the lysis buffer are attracted to the phosphate groups on the DNA. This neutralizes DNA's electric charge, and allows the DNA to clump together. The DNA precipitates out in the presence of the alcohol because it is not soluble in alcohol. How can we be sure that we have isolated DNA? This class can be extended using experiments to confirm that we have DNA. Place a drop of distilled water on a microscope slide. Use a toothpick to remove a tiny piece of the DNA from the toothpick. Put a coverslip over the DNA and add a drop of methylene blue at the edge of the cover slip and use the corner of some paper towel to draw the stain under. Look at the DNA under the microscope and write down what you see. It should be noted that the white stringy stuff that we see is a mixture of DNA and RNA, and that the protocol that we have used is a nucleic acid isolation. Using other chemicals in a proper lab setting, a very very pure sample of DNA could be isolated. Links to k- 12 CA Content Standards: Grades k- 8 Standard Set Investigation and Experimentation: Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other strands, students should develop their own questions and perform investigations. Grades k- 12 Mathematical Reasoning: 1.0 Students make decisions about how to approach problems: 1.1 Analyze problems by identifying relationships, distinguishing relevant from irrelevant information, sequencing and prioritizing information, and observing patterns. 1.2 Determine when and how to break a problem into simpler parts. 2.0 Students use strategies, skills, and concepts in finding solutions: 1.1 Use estimation to verify the reasonableness of calculated results Apply strategies and results from simpler problems to more complex problems. 1.3 Use a variety of methods, such as words, numbers, symbols, charts, graphs, tables, diagrams, and models, to explain mathematical reasoning. 2.5 Indicate the relative advantages of exact and approximate solutions to problems and give answers to a specified degree of accuracy. 3.0 Students move beyond a particular problem by generalizing to other situations: 3.1 Evaluate the reasonableness of the solution in the context of the original situation. 3.2 Note the method of deriving the solution and demonstrate a conceptual understanding of the derivation by solving similar problems. 3.3 Develop generalizations of the results obtained and apply them in other circumstances. Grade 7 Standard Set 1. Cell Biology. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept: 7.a. Students know cells function similarly in all living organisms. 7.b. Students know the characteristics that distinguish plant cells from animal cells, including

5 chloroplasts and cell walls. 7.c. Students know the nucleus is the repository for genetic information in plant and animal cells. Grade 7 Standard Set 2. Genetics. A typical cell of any organism contains genetic instructions that specify its traits. Those traits may be modified by environmental influences. As a basis for understanding this concept: 2.d. Students know plant and animal cells contain many thousands of different genes and typically have two copies of every gene. The two copies (or alleles) of the gene may or may not be identical, and one may be dominant in determining the phenotype while the other is recessive. 2.e. Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell. Grade 8 Standard Set 6. Chemistry of Living Things (Life Sciences) Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept: 8. c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.