All living organisms are made up of cells. Inside nearly every cell is a special code that makes every organism unique. The code is called DNA, or deoxyribonucleic acid, which is the hereditary material that carries the instructions for life and allows cells and organisms to function. Most DNA is located in the cell nucleus (nuclear DNA), while a smaller amount can also be found in mitochondria (mitochondrial DNA or mtDNA).
In this activity using a few simple materials, and some easy steps, you will extract DNA from cells and see the long, stringy fibers that carry the instructions of life.
Science Involved
Structure of DNA
The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). These bases pair up in a specific way – A with T and C with G – to form units called base pairs. Each base is attached to a sugar and a phosphate molecule, forming a nucleotide. Nucleotides are arranged in two long strands that twist around each other to form a double helix, a structure that looks like a twisted ladder. The base pairs form the rungs, while the sugar and phosphate molecules form the sidepieces.
The discovery of the structure of DNA is one of the greatest achievements in science. In 1953, James Watson and Francis Crick proposed that DNA has a double helix shape. Their discovery revealed how DNA carries genetic information, changing the course of biology and medicine forever. This discovery was made possible through the X-ray diffraction images taken by Rosalind Franklin and Maurice Wilkins, which provided key clues about DNA’s structure. For this groundbreaking work, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Physiology or Medicine in 1962.
Image Credit: "The Structure and Function of DNA", Campbell - Essential Biology 7e, Pearson. 2019.
Human DNA consists of about 3 billion bases, and more than 99 percent of them are the same in every person. What makes each individual unique is the order, or sequence, of these bases – like how letters arranged in different ways form different words and sentences. Except for identical twins, no two people share the exact same DNA sequence.
DNA technology has many real-world applications, such as solving crimes through forensic science, developing vaccines, advancing gene therapy and genetic engineering, and even determining family relationships through paternity tests.
Activity
Requirements
Strawberry or banana, dishwashing liquid, salt, water, fresh papaya or pineapple juice (optional), chilled ethanol or iso-propyl alcohol, tea filter, small plate, metal spoon, two beakers, test tube, glass rod or skewer stick.
Procedure
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Take 4-5 strawberries or peeled half banana, and mash it on a plate with the help of a spoon till there are no large chunks left. The goal is to break down the cell walls.
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In a beaker, mix 100 mL of water, 1 teaspoon of salt, and 1 teaspoon of dish washing liquid. Stir the solution gently to dissolve the salt. This is called a “lysis solution”. Cell membranes and nuclear membranes consist primarily of lipids. Soap breaks up clumps of lipids and thus helps to break down the cell membrane, while the salt helps to release the DNA by neutralizing its charge.
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Add the mashed paste to the lysis solution and gently mix the contents for about 2-3 minutes without creating bubbles.
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(Optional) DNA is tightly bound to protein molecules called histones. Add 2-3 teaspoons of fresh papaya or pineapple juice to the mixture. Stir the mixture gently for 2-3 minutes, allowing the papain enzymes in the papaya juice to break down the proteins and release the DNA from the proteins.
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Place a tea filter over the other beaker and pour the mixture through the filter, allowing the liquid to drain through into the container below.
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Carefully pour about 5 mL of the liquid into a test tube.
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Using a dropper, slowly add about 5 mL of chilled alcohol down the side of the test tube so it forms a layer on top of the soapy solution. Chilled alcohol reduces the solubility of DNA. When chilled alcohol is poured on top of the solution, the DNA precipitates out into the alcohol layer, while the lipids and proteins stay in the solution.
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Do not mix or shake the test tube for at least 5 minutes. DNA molecules will clump together where the soapy water below meets the chilled alcohol as white strands.
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Use a glass rod or skewer stick to spool and lift the DNA out of the solution.
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(Optional) Stain the extracted DNA using methylene blue and observe it under the microscope.
Reflect and Discuss
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Why is it important to mash the strawberry or banana thoroughly before adding the lysis solution?
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How might the DNA extraction differ if you used a different fruit?
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Why does alcohol form a layer on top of the DNA containing soapy solution?