Key concepts
Biology
Life
DNA
Double helix
Introduction
Ever wondered how DNA, the genetic blueprint of a life-form, can encode and pass on the information on how to grow and maintain that life-form? Just like a cookbook contains a recipe for a dish, DNA stores the recipe for the life of an organism. Unlike various recipes, however, very different organisms’ DNA such as in a fungus, plant or animal all look very similar. Although each human has a unique DNA sequence, the DNA in all of us is about 99.9 percent identical! In this activity you will make a model for a short section of DNA—enough to get a sense of what it is like and how it encodes life.
Background
Plants, fungi and animals might seem very different from one another but they are all made up of tiny building blocks called cells, and—with very few exceptions—each of these cells has in its center a molecule containing the organism’s blueprint. This molecule is called DNA: deoxyribonucleic acid. Although the blueprints differ across life-forms—after all, plants, fungi and animals are very different organisms—the way it is encoded in DNA is identical.
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The DNA molecule encodes all information using four chemical bases: cytosine (C), guanine (G), adenine (A) and thymine (T). It has two complementary strands, each with a long sugar–phosphate backbone to which the four chemicals attach. The sequence or order of these chemicals contains the data to develop, maintain and grow the organism. In DNA these four chemicals always link together the same way to form pairs: A pairs with T; C pairs with G. In this very specific way the two complementary strands link together to form DNA: a long molecule that looks a little like a rope ladder—only about 200,000,000 times smaller! Give the “ladder” a clockwise twist, and you can see why DNA is also called the “double helix.”
When organisms grow, their cells divide and in almost all cases each cell receives a duplicate of the DNA molecule. DNA's ingenious structure allows for easy replication: Each strand of the double helix contains all the information needed to create a new DNA molecule. If the pairs let go of each other, each backbone with its sequence of four chemicals can be the bases of a new DNA molecule. As A and T always pair up and C and G also always go together, one strand is enough to re-create the molecule.
Materials
Tape, about two centimeters wide and 1.5 meters long (such as painter's tape)
Card stock or sturdy paper
Red, yellow, blue and green markers
Ruler
Pencil
Scissors
A partner
Blanket or sheets of paper
Rope or thick string about one meter long (optional)
Preparation
Cut 40 strips out of paper, four centimeters by one centimeter each. Divide each strip with a pencil line so it consists of two rectangles that are two centimeters by one centimeter each. These rectangles represent the bases or code chemicals of your DNA.
There are only four different bases or code chemicals in DNA. We will use color to indicate each one: red, yellow, blue and green. These code chemicals are very particular—red only combines with yellow, and blue only with green.* Color the strips on either side of the dividing line so you have red–yellow and blue–green strips. Color them identically front and back. (Note that in your body these base pairs are tiny and not colorful! We have made them large and colorful so the model is beautiful and easier to understand. Your DNA has a length of about three billion base pairs, so you will only model a piece of DNA—not the whole sequence!) This activity makes one model of a piece of DNA.
DNA looks like a twisted rope ladder. For your model you will first make the “backbone” sides of the ladder and then add the “base pair” rungs. Start by cutting two pieces of tape, each 65 centimeters long. Lay them parallel to each other on the table, sticky side up, with about two centimeters of space in between. Use short pieces of tape to keep the two backbones in place while you add the base pairs.
Procedure
Pick a prepared strip of paper (a base pair) and stick one end to one backbone and the other end to the other parallel backbone. The strip should make a nice bridge from one backbone to the other and cross the backbones at a right angle, just like the rungs on a rope ladder. Note your strips will not reach across the full width of both strips of tape, but start and end near the middle of the pieces of tape.
Stick more strips to the backbones so they make parallel rungs. Leave about one centimeter of space between rungs. Do this until your backbones are connected by base pairs from one end to the other.
Remove the pieces of tape securing the backbones to the table and fold each line of tape over lengthwise in the middle. The tape will fold over the ends of the base pairs, fixing them between the folded pieces of tape.
Your model is almost finished! One detail is missing: DNA is twisted. Hold one end of your ladder and have your partner hold the other end. Twist your end clockwise a few times. What happens to the length of your DNA piece when you twist it?Do you see why DNA is called a double helix?
DNA can duplicate itself using the information contained in either strand. Do you think you can do the same with your DNA molecule?
To test whether you can, untwist your DNA model and lay it flat on a table or the ground. Hide one strand with paper or a blanket. Your job is to use the knowledge you gained while making the DNA molecule to complete the molecule. For each visible color (code chemical)can you tell which color (code chemical) is hidden?How could this help you duplicate your DNA molecule?
Extra: Cut each base pair just where the two colors—each representing one code chemical—meet. Now, use each strand to duplicate the original DNA model. Do you get two identical molecules?
Extra: Count the number of base pairs in your DNA section.Human DNA consists of three million base pairs. Can you estimate how long your model would be if you modeled all three million?
Extra:Take a piece of rope about one meter long. Twist the rope and keep on twisting. Do you see how a long strand can twist and fold into a much more compact space? How do you think this helps inside our bodies?
Observations and results
You were most likely able to tell what the hidden colors were—no matter which strand you chose to hide. This is because once you know one side of a pair you know its partner, because these chemicals always pair with the same partner: red with blue; green with yellow.
The four code chemicals in real DNA are usually represented by the letters T, A, C and G. They are not colorful, but they are as particular: T and A always pair together, as do G and C. The sequence along one backbone of the DNA molecule contains all the information to re-create the molecule.
You probably have about 30 base pairs in your model. You would need to make it 100 million times longer to model all three billion base pairs of human DNA. Your model would be about 60,000 kilometers, or 37,300 miles, long—about 1.5 times around the world! Your DNA molecule is probably about four centimeters wide. Real DNA is about two nanometers, or two millionths of a millimeter wide. This means your model is about 20 million times wider than real DNA. This long string of DNA is coiled and folded into the center of almost every cell of the human body!
More to explore DNA: Definition, Structure and Discovery, from LiveScience Find the DNA in a Banana, from Scientific American Squishy Science: Extract DNA from Smashed Strawberries, from Scientific American Identical Twins Genes Are Not Identical, from Scientific American Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
*Editor’s Note (7/10/20): This sentence was added after posting to correct the description of the color combinations.