Isolation of DNA (Deoxyribonucleic Acid) from Bananas

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Purpose

This laboratory exercise is designed to show how DNA can easily be extracted from bananas using kitchen reagents.

Materials/Group

• thumb-size pieces of fresh banana
• non-iodized salt in plastic tube (labeled)
• 10 ml graduated cylinder
• fresh meat tenderizer in plastic tube (labeled)
• 1 plastic reaction tube (labeled)
• mortar and pestle
• 2 bulb-tip pipets
• paper boat for weighing
• ICE COLD 95% ethanol (teacher has it in freezer)
• paper clip, bent
• liquid detergent in beaker
• beaker of distilled water

Procedure

Each person needs to read and carefully do their part. If one person does not do his/her part, then the experiment will not work.

Person #1

1. Weigh the "boat" (shiny colored piece of cardstock paper to weigh salt and meat tenderizer without having to put them directly onto the balance pan) on triple beam balance.
2. Write down the mass of boat in grams here: __________________ (to the nearest 1/10th, or .1 of a gram).
3. Subtract the mass of the boat from the rest of the dry reagents that you weigh! If you add 1.0 gram of salt and the boat weighs 1.1 gram then the balance will read 2.1 grams!
4. Weigh out 1.0 gram of salt on the triple beam balance on "boat." Give the salt on boat to Person #3.
5. Weigh out 2.0 grams of meat tenderizer on the triple beam balance on the "boat." Give the meat tenderizer to Person #3.

NOTE: Meat tenderizer contains enzymes that will destroy proteins that stick to the DNA and may contaminate the extraction.

Person #2

1. Measure 9 ml of distilled water in the 10ml graduated cylinder and give it to person #3.
2. Use bulb pipet to add approximately 1 ml (soap to the bottom of the bulb) of liquid detergent soap (in plastic bottles-share with another table) into the mortar. Be careful the soap is very messy!
3. Dispose of bulb pipet into the lined trash can, without dripping!
4. When water and soap have been added to mortar, let person #3 continue with his/her job. The detergent/salt solution breaks down the lipid walls of the cells to release the cytoplasm and the cell contents. The salt shields the negative ends of the phosphates of the cell membrane.

Person #3

1. Get a thumb-sized piece of banana from your teacher.
2. Place banana in the mortar and pestle. Macerate (mash it up) the banana in the mortar. The macerating action further breaks apart the cell membranes and cell walls of the banana.
3. Try not to macerate too hard as you will get foam and bubbles, making it harder to extract the DNA from it.
4. As team members #1 and #3 hand you reagents, carefully pour all the reagents into the mortar and continue to macerate while mixing all the reagents.
5. The resulting mix should be a runny thick paste, called a lysate, once all of the reagents are added. If it is not a runny paste, add a couple more milliliters of water, measuring the water in the graduated cylinder.

Person #4

1. Carefully pour the lysate from the mortar into a clean plastic reaction tube. Wipe off lysate from the sides of the reaction tube so all the lysate is in the bottom of the tube.
2. If you have any foam or bubbles on top of your lysate, in the bottom of the reaction tube, try to wipe some of them out with a paper towel. Dispose of paper towels in lined trash can.
3. Carefully, using squeeze bottle, squirt 15-20 ml (use side of reaction tube as your measurement) of ice cold ethanol down the side of the test tube to form a visible alcohol layer on top of the lysate. Do not touch the squirt bottle tip to the reaction tube nor do not squirt directly into the lysate. The 95% ethanol must be ICE COLD. It should be left in a plastic container in the freezer overnight. The teacher has already put the 95% ethanol in the freezer. DNA, in the presence of alcohol, will form into a mucous-appearing solid, called a precipitate.
4. You should immediately begin to see bubbles forming in the alcohol layer. DNA will form a mucous looking solid with small bubbles, causing it to rise from lysate layer to the alcohol layer. You can gently twirl your test tube to allow more DNA to come up into the alcohol layer.
5. Taking turns, use a clean bulb pipet to suck the DNA out of the alcohol layer and put it into a small DNA test tube that the teacher will provide at the end of the lab. Person #4, make sure that each person gets a small amount of DNA in his/her test tube with a small amount of ethanol in the tube as well.
6. If the DNA is too thick for the bulb pipet, use a paperclip with a bent tip to "fish" DNA out of the alcohol layer.

Congratulations! You have extracted DNA!!

Person #4: After you have extracted DNA, dump alcohol and lysate into lined trash can.

Cleaning Duties

Person #1—Wipe down pan of balance with damp paper towel, help Person #2.

Person #2—With Person #1, wipe down table with wet paper towels.

Person #3—Wipe off items in the metal pan, including the pan itself with damp paper towels.

Person #4—Rinse mortar and pestel well in sink. Rinse and clean out the reaction tube.

Conclusion

Draw a comic strip of each step of the procedure, with explanations as to why we added each reagent. Include yourself in the comic strip. The comic strip must be neat and use colors.