Title: DNA Extraction Genetics Molecular Biology

Title: DNA Extraction Genetics Molecular Biology

Name: ______Date:______Period:______

Title: DNA Extraction – Genetics Molecular Biology

Standards:

Grade 7 – Genetics2. 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:

Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell. Chromosomes are located in the nucleus of eukaryotic cells like animal, plants, fungi, or protists and in the cytoplasm of prokaryotic cells like bacteria.

High School Biology 5. The genetic composition of cells can be altered by incorporation of exogenous DNA into the cells. As a basis for understanding this concept: Students know the general structures and functions of DNA, RNA, and protein.

HS Chemistry. 10. The bonding characteristics of carbon allow the formation of many different organic molecules of varied sizes, shapes, and chemical properties and provide the biochemical basis of life. As a basis for understanding this concept: a. Students know large molecules (polymers), such as proteins, nucleic acids, and starch, are formed by repetitive combinations of simple subunits.

Summary:

What is DNA? What does it look like? In this activity, you will extract DNA from strawberries using diluted dish soap and alcohol. Suddenly this mysterious secret of life can be seen materializing out of strawberry juice right in front of your eyes. The long tangled DNA strands that ultimately form may be collected using a bamboo skewer.

Objectives:

  • Can extract DNA.
  • Can recognize that DNA is found in all cells.
  • Can explain the steps needed to isolate DNA from a cell.
  • Can begin to describe the structure of DNA – that it is a long, invisibly thin polymer.

Time:40 minutes

Materials

Each student group needs:

  • 1 frozen strawberries
  • 1 ziplock bag
  • 2 clear glass test tubes
  • 1 piece of cheesecloth
  • 10 ml extraction buffer (see recipe)
  • 1 bamboo skewer

For the whole class to share:

  • 90% ice cold rubbing alcohol
  • droppers and graduated cylinders for dispensing solutions

Student Background

This activity is essential to demonstrate that DNA exists and that it can be extracted from any cell. Strawberries are used in this activity because they are octaploid, meaning they have 8 copies of every gene rather than the usual 2 (diploid); thus providing large quantities of DNA to extract.

The DNA molecule is an invisibly thin, very long strand. DNA is a nucleic acid polymer made of repeating subunits (monomers) of nucleotides. Each DNA nucleotide has a 5-carbon sugar (deoxyribose), phosphate group (negatively charged) and one of four nitrogen bases (adenine, thymine, cytosine, or guanine). The DNA found in each human cell is almost 2 meters long. If all the DNA in a human adult (that’s 100 trillion cells) were laid end to end, the DNA would stretch 113 billion miles. That would take you to the sun and back 610 times. Even though DNA is invisible to the naked eye, no microscopes are needed! The reason is that you release so many DNA strands that they tangle together into a thick cable, visible without magnification. For example, it would be the same as if you took a thin piece of thread and held it up on the far end of the hallway. You probably wouldn’t be able to see the thread from that distance. However, if you took the thread and tangled it up with a hundred thousand other threads, you would be able to see the tangled clump from far away because there is so much of it.

The process itself is fairly straightforward. First the cell walls are broken open by crushing the strawberries in a ziplock bag. Next, detergent is used to dissolve the cell and nuclear membranes. The membranes are made of phosholipids (fat) and the detergent will cut through the membrane just like it cuts through grease on a dirty plate when washing dishes.

No longer confined inside nuclear membranes, the DNA goes into solution because DNA highly soluble in water because the phosphate group of each nucleotide carries a negative charge. Some salt is present in the detergent solution in order to match the salt concentration of the cells and also to help remove proteins that are bound to DNA and to keep the proteins dissolved in the aqueous layer so they don’t precipitate in the alcohol along with the DNA.Additionally, the positively charged sodium ions from the salt (NaCl) in the extraction solution are attracted to the negatively charged phosphate groups on the DNA backbone, effectively neutralizing the DNA's electric charge. This neutralization allows the DNA molecules to aggregate with one another.

Now you have a big mixture of smashed cell walls, dissolved membranes, loose DNA and random other cell parts. This mixture is filtered through cheesecloth. Finally, you take advantage of the fact that DNA is soluble in water but not in alcohol. In fact, alcohol makes DNA clump together and precipate at the water/alcohol interface. Thus a layer of alcohol laid on top of the filtrate. Any DNA that contacts the alcohol will clump together, pulling the rest of the DNA strand along behind it. Soon you should see gossamer white strands of DNA bubbling their way up from the red strawberry extract.

The DNA may be collected by twirling a bamboo skewer. The DNA will spool itself around the skewer and can be pulled out of the solution.

Procedure

  1. Put the strawberry in the zip lock bag, squeeze out most of the air and seal the bag. The strawberry can then be crushed into juice and pulp. Try to squish all of the chunks into an even, smooth puree. Warning: do not to pound the strawberry on the table or risk the bag bursting and getting strawberry pulp all over yourself and the classroom. If a problem is created at this time, your group will not be continuing with the experiment.
  2. Open the bag and add 10 ml of extraction buffer. Seal the bag again (remove as much air as possible) and gently mix the strawberry juice with the extraction buffer. Warning:do not to mix too vigorously or it will generate a lot of bubbles and can’t be filtered effectively. Use a gentle tilting back and forth motion while lightly squeezing the bag.
  3. Set up a filtration system bylaying the cheesecloth across the mouth of the 50 mL beaker, forming a well into which the strawberry juice can be poured. Don’t make bubbles
  4. Carefully pour the extract into the well in the cheesecloth. Allow the juice to filter through the towel into the container below. Let it drip for 5-10 minutes. Do not squeeze the towel or you will create lots of bubbles, disrupting the interface needed in the next step.
  5. The cheesecloth can be thrown away and the ziplock bag rinsed.
  6. Carefully transfer liquid from the 15 ml tube into the clear test tube until the test tube is about a third full.
  7. Slowly add 3 ml (3 eye droppers full) of ice cold alcohol to the test tube. The alcohol should be added so that it trickles down the side of the tube before pooling on top of the strawberry extract. You should end up with a red bottom layer and a clear top layer.
  8. Make observations of anything going on in the clear alcohol layer.
  9. After 2-3 minutes, a skewer or stirring rod can be inserted into the tube and gently swirled around. This will spool the DNA around the stick. The DNA can be pulled out of the tube. You may safely touch the DNA although the DNA MUST NOT be tasted under any circumstances.
  10. Clean up your work area.

Assessment Answer summary questions about the extraction using complete sentences.

  1. Describe what DNA is and what it does?
  1. Draw a diagram showing DNA (as a long tangled thread) within the nucleus of a cell. Label the DNA, nucleus, cell membrane, and cell wall.
  1. Why is it necessary to mash the strawberries?
  1. What is the purpose of the detergent?
  1. What is the purpose of the salt?
  1. Name a liquid that DNA is not soluble in (does not dissolve).
  1. Is the DNA that you extracted pure? What else might be attached to the DNA?
  1. What did you observe?
  1. Why might some groups get more DNA than others?
  1. Can you see a single strand of DNA without a microscope? Explain how you were able to see the DNA in this experiment without magnification.

Adapted from: A MyScienceBox Lesson Plan by Irene Salter ( This work is licensed under the Creative Commons Attribution-NonCommercial License. To view a copy of this license, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California94305, USA. (1/6/2011, M.Elizabeth: