Lesson 6: DNA Extraction lesson

Introduction:

This activity is designed to allow students to isolate and observe actual DNA as well as review structures in the cell responsible for keeping the DNA in tact. By the end of the activity, all students will have a sample of DNA that they may take home with them.

Learning outcomes:

Students will be able to describe where in the environment and in the cell that DNA is able to be found. They will be able to list the cellular components which protect genetic information.

Curriculum alignment:

Biology Goal 2 – Describe the structure and function of cell organelles, particularly: cell wall, cell membrane as well as the function of proteins and nucleic a

cids

Classroom time required:

45 minutes: 25 for procedure, 20 for answering questions

Materials needed:

1 copy of “Banana DNA extraction” per student, 1 banana per class, 1 package of cheese cloth, blender, 1 container of meat tenderizer, 1 small container of table salt, 95% ethanol (Everclear works if you don’t want to order from a catalogue), water, liquid dish detergent (preferably clear), flat bottomed centrifuge tubes (1 per student), rubber bands (1 per student), large clear plastic cup, measuring spoons, plastic bulb pipettes (1 per student)

Technology resources:

None required

Pre-activities:

Review with students the function of DNA and where in the cell DNA is located. Explain to students that they will be removing DNA from a banana which will involve performing a few chemical and physical reactions. One reaction will require a buffer; review with students the meaning of this word. The final reaction will involve dehydration of the mixture. When discussing the formation of polymers, such as nucleic acids, students should have encountered the term “dehydration synthesis,” review this concept with students. Also, review with students that biotechnology involves using DNA from organisms to solve problems. One problem that it is being used to solve is the American Chestnut blight (or Panama disease in bananas). In order to insert the gene for blight resistance, scientist must first study the DNA. Therefore they must extract the DNA from the organism in order to study it. Copy “Banana DNA extraction” per student and gather the remaining materials listed above. Be sure to place the ethanol in a freezer or on ice well in advance of the activity.

Activity:

1. Review items mentioned in the pre-activities section.
2. Hand out “Banana DNA extraction” copies to each student.
3. Have a student read the opening paragraph for the class. Emphasize the need for removing the cell wall and membrane layers as well as the histones. Remind students again about the need for a buffer and that the alcohol will dehydrate or precipitate the DNA in the final step.
4. Perform the first 4 steps of the procedure as a demonstration for the class, being sure to explain that the blender helps to break open the cell wall and cell membrane, the straining through the cheese cloth removes large solid parts of the cell (the nucleus is small), the dish liquid acts to attract lipids – such as those in the phospholipid bilayers of cell and nuclear membranes, and the meat tenderizer acts to break up proteins such as histones that bind DNA.
5. Students will perform the final dehydration step on their own. Explain that it will take a few minutes for the DNA to fully precipitate out of solution and they should try to keep the mixture still during that time. By using flat bottomed centrifuge tubes, the mixture can sit on their desk undisturbed and they students can take the DNA with them when they are done for the day.
6. Direct students to answer follow up questions. (This may be completed for homework if time is short.)

Assessment:

Once activity is complete, check student’s understanding by asking the class the following questions:

• What is DNA?
• Where in the cell is DNA found?
• Why is it important that DNA is protected?
• What layers in the cell protect the DNA?
• Why may a scientist want to remove DNA from an organism?
• Also, check the responses in the follow up questions for accuracy.

Modifications:

It may be helpful for students with reading and/or comprehension difficulty to work with another student on the follow up questions. This would give them the opportunity to discuss the questions and check their own understanding.

Supplemental information:

Once genetic markers have been located on the chromosome, scientists have to extract DNA for further study. The method outlined in the worksheet does not allow students to isolate specific chromosomes or sections of chromosomes, but it does let them see DNA and that it can be separated from the rest of the cell. DNA is often extracted by grinding up the sample, adding an “extraction buffer”, then , spinning the solution in a centrifuge until a pellet of nuclei is produced. The nuclei are then put into solution so that the DNA may be used in PCR with the appropriate primers to produce the desired segments of DNA. A good website that illustrates centrifugation is: http://fig.cox.miami.edu/~cmallery/255/255tech/255techniques.htm An alternative method for the 1st four steps of the procedure is to have students mash the banana (about a 2” piece) themselves in a Ziploc bag. This requires having a premade buffer solution consisting of 20 mL dish washing liquid, 10 g salt, 10 g meat tenderizer, and 380 mL of distilled water. Once the banana is mashed, have them add 10 mL of buffer to the Ziploc and mix again. After this they would strain with cheese cloth and continue the procedure at the student steps.

Critical vocabulary:

• Chromatin: a complex of nucleic acid in eukaryotic cells that is usually dispersed in the interphase nucleus
• Histone: simple proteins that are rich in the basic amino acids lysine and arginine and are complexed with DNA in the nucleosomes of eukaryotic chromatin
• Buffer: a substance capable in solution of neutralizing both acids and bases and thereby maintaining the original acidity or basicity of the solution
• Dehydration synthesis: a chemical reaction that involves the loss of water from the reacting molecule
• Precipitation reaction: the formation of a solid in a solution during a chemical reaction
• Phospholipid bilayer: a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends facing outward
• Biotechnology: the manipulation of living organisms or their components to produce useful usually commercial products or to solve environmental problems

Comments:

Bananas were chosen for this activity as a follow up to the lesson: “Bananas, a modern example.”