Lab: DNA Extraction from Human Cheek Cells

Introduction

DNA…you hear about it all the time. DNA is used every day by scientists and lawyers to help in criminal investigation, paternity suits, cloning, etc. Your DNA is your “genetic fingerprint”—this means that your DNA is like no one else’s in the world! The procedure that we will use to see your DNA includes the same basic processes that researchers use to isolate, analyze, and manipulate DNA in a laboratory setting (although the DNA isolated here is not nearly as “pure” as the research lab version).

DNA is a nucleic acid, made of the elements: carbon, hydrogen, oxygen, nitrogen, and phosphorous. DNA stands for deoxyribonucleic acid. Watson and Crick were the two scientists that were credited with the discovery of its structure. DNA is made up of several nucleotides. Each nucleotide consists of a sugar, a phosphate and a nitrogenous base. There are four different nitrogenous bases. They are adenine, guanine, thymine, and cytosine. Each base of a nucleotide attaches to a corresponding base of another nucleotide by a hydrogen bond, Adenine will only bond with thymine and cytosine will only bond with guanine. As nucleotide chains are formed, the DNA takes the shape of a twisted ladder.

DNA can be considered the hereditary “code of life” because it possesses the information that determines an organism’s characteristic and is transmitted from one generation to the next. You receive half of your genes from your mother and half from your father. Day to day, DNA’s job is to direct the functioning within the cells of your body.

DNA is in the nucleus of almost every cell in your body. The length of DNA per cell is about 100,000 times as long as the cell itself. However, DNA only takes up about 10% of the cell’s volume. This is because DNA is specially packaged through a series of events to fit easily in the cell’s nucleus. The structure of DNA, the double helix, is wrapped around proteins, folded back onto itself, and coiled into a compact chromosome.

Individual chromosomes can be studied using microscopes, but the double helix of a chromosome is so thin that it can only be detected through innovative, high-tech procedures. Chromosomal DNA from a single cell is not visible to the naked eye. However, when chromosomal DNA is extracted from multiple cells, the amassed quantity can easily be seen and looks like strands of mucous-like, translucent cotton.

We will first collect cheek cells by swishing a sports drink in our mouths and using our teeth to gently scrape cells off our cheeks. (The more vigorous and the longer that you swish, the more cells are removed, and the more materials you’ll have from which to extract DNA.) Then, we will lyse the cell membranes by adding a detergent based cell lysis solution, which allows the DNA to be freed. DNA is soluble in water, but much less soluble in alcohol. Thus, alcohol will be slowly added, and DNA will precipitate to the sports drink/alcohol solution, and you will be able to see your own DNA! The white, stringy material is thousands of DNA molecules stuck together (with some proteins too).

Experiment: Materials and Methods

  1. Label your test tube with a piece of tape and your initials.
  2. Obtain a small cup of sports drink and swish it around in your mouth for 1 full minute. As you swish, gently and continuously scrape the sides of your cheeks with your teeth to help release your cheek cells.
  1. Spit the drink (with your collected cheek cells) back into the small cup.
  1. Pour the contents of the cup into your labeled test tube (discard the cup).
  1. Holding the test tube at an angle, use the provided plastic pipette to add 2mL (1 pipette full) of cell lysis solution to your collected cheek cells.
  1. Cap your test tube with your finger, and invert it 5-8 times. (This mixes the lysis solution with the cheek cells.)
  1. Place your test tube in the warm water bath for 2 to 5 minutes. Begin answering your lab discussion questions. After 2 to 5 minutes, carefully remove your test tube.
  1. Go toMrs. Pangburn to get the alcohol, add 9mLof the cold alcohol by letting it run gently run down the side of the test tube (hold the test tube at an angle). You should have 2 distinct layers. DO NOT mix the cheek cell solution with the alcohol!!!
  1. Watch as wispy strands of translucent DNA begin to clump together where the alcohol layer meets the cheek cell solution. (It kind of looks like cobwebs extending upward.)
  1. Place your test tube in a test tube rack and let it stand undisturbed for 5-10 minutes. During this time the DNA will continue to precipitate out.
  1. Return to your desk and answer your LAB Discussion Questions.
  1. Remove your test tube from the rack and observe your DNA. Take a pipette and remove some of the DNA. Place the DNA on a slide and cover it with a plastic cover slip.
  1. Go to a microscope and observe your DNA.
  1. Complete your microscope drawing, discussion questions, and turn in your complete lab.

Clean your lab area.

Remove the tape from your test tube and place it in the disinfecting solution.

Name: ______

Discussion Questions

  1. Where is DNA found in a eukaryotic cell? In a prokaryotic cell?
  1. What does DNA stand for?
  1. What 2 scientists discovered the structure of DNA?
  1. DNA is made up of several nucleotides. What three components make up a nucleotide?
  1. What are the four bases that make up the rungs of the DNA?
  1. How is a base from one nucleotide attached to a base of a different nucleotide? (think “bond”)
  1. How did you get your full set of DNA?
  1. What is the function of DNA from day to day?
  1. Why is DNA referred to as your genetic fingerprint?
  1. What are two examples of how DNA can be used in society?

What do you see under the microscope? (draw & describe)

TEACHERS' NOTES

In order to understand what you are doing in this activity, it is

important that you know the "big picture" behind the methods we will be

using:

  • Cells may be physically and chemically treated to break open the outer cell membrane and inner nuclear membrane.
  • The portion of the cell mixture containing DNA (the watery portion) will be separated from the cell membranes and organelles (the gloppy portion).
  • The solution containing dissolved DNA will be chemically altered so that the DNA can precipitate out of the solution in its solid, string-like state.

Additional Notes

  • The recipe for the Salt/detergent mixture is: 2 L distilled water, 100 mL detergent (we use Palmolive dishwashing detergent), 15 g salt.
  • The ethanol needs to be ice cold--keep in freezer until the time it is needed.
  • There is about a 15-minute wait time--plan something DNAish during this time.