Summary
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 complete recipe for a dish, DNA stores the recipe for the life of an organism. Although each human has a unique DNA sequence, the DNA in all of us is about 99.9% identical! In this activity, you will make a model for a short section of DNA—enough to get a sense of what DNA is like and how it encodes life.
Materials
- Tape, about 2 cm wide and 1.5 m long
- Cardstock or sturdy paper
- Red, yellow, blue, and green markers
- Ruler
- Pencil
- Scissors
- A partner
- Blanket or sheets of paper
Prep Work
Instructions
- DNA encodes the genetic blueprint of a life form using four chemicals. It is a long molecule that looks a little like a rope ladder, only about 200,000,000 times smaller! Give the long "ladder" a clockwise twist, and you can see why DNA is also called the "double helix." The instructions below make a model of a piece of DNA. It will clarify what a DNA is and looks like!
- DNA uses four chemicals (abbreviated by the letters A, T, C and G) to encode the data to maintain and grow the organism. We will use color to indicate each of these chemicals: red, blue, yellow, and green. These code chemicals are very particular, they always pair up in specific ways: A pairs with T and C pairs with G. In your model, red only combines with blue, and yellow only with green. Color the 4 cm by 1 cm strips on either side of the dividing line so you have red-blue strips and yellow-green strips. No other combinations of colors are allowed. Color them identical front and back. Note that in your body, these pairs are tiny and not colorful! We have made them large and colorful so that the model is beautiful and easier to understand. Your DNA has a length of about 3 billion pairs, so you will only model a piece of DNA—not the whole sequence!
- DNA looks like a twisted rope ladder. For your model, you will first make the "backbone" sides of the ladder and then add the "pair" rungs. Start by cutting two pieces of tape, each 65 cm long. Lay them parallel to each other on the table, sticky side up, with about 2 cm of space in between. Use two extra pieces of tape to keep the two backbones in place while you add the pairs.
- To link the pairs of code chemicals to your DNA backbones, pick a prepared strip of paper (a pair) and stick one end to one backbone and the other end to the other parallel backbone as shown in the figure.
- Stick more strips (pairs) to the backbones so they make parallel rungs. Leave about one centimeter of space between rungs. Do this until your backbones are connected by pairs from one end to the other.
- Remove the extra pieces of tape securing the backbones to the table and fold each line of tape over lengthwise in the middle. Tape will fold over the ends of the pairs, fixing them between the folded pieces of tape as shown for the bottom backbone in the figure.
- 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, and you can too. To test, untwist your DNA model and lay it flat on a table or the ground. Hide one strand (a backbone with one side of each pair) with paper or a blanket as shown in the figure. Your job is to use the knowledge you gained while making the DNA model to complete the section.
For each visible color (code chemical), can you tell which color (code chemical) is hidden? (Hint: look back at the second step of the procedure for help.) How could this help you duplicate your DNA molecule?
- To get an idea of how long human DNA is, count the number of pairs in your DNA section. Human DNA consists of three billion pairs.
Can you estimate how long your model would be if you modeled all the three billion pairs?
- Take your ruler and measure how wide your DNA molecule is when untwisted. A real DNA molecule is about two nanometers or two millionths of a millimeter (2÷1,000,000 mm) wide.
How many times wider is your model than a real DNA molecule?
What Happened?
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 as these chemicals always pair with the same partner: red with blue; green with yellow.
You probably have about 30 pairs in your model. You would need to make it 100,000,000 times longer to model all three billion pairs of human DNA. Your model would be about 60,000 km (37,000 miles) long, which is about 1.5 times around the world!
Your DNA molecule is probably about 4 cm wide. Real DNA is about 2 nanometers or 2 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!
Digging Deeper
Plants, fungi, and humans might seem very different from each other, 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 blueprint of the organism. This molecule is called DNA: deoxyribonucleic acid. Although the blueprint is different—after all, plants, fungi, and humans are very different organisms—the way it is encoded in DNA is identical.
The DNA molecule encodes all information using four chemicals: 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 maintain and grow the organism. In DNA, these four chemicals always link together to form pairs: A pairs with T and 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 and twisted.
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 recreate the molecule.
Ask an Expert
For Further Exploration
- This model is made from paper and tape. Can you create an edible model?
- Cut each pair in your model 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?
- Take a piece of rope about one meter long. Twist the rope and keep on twisting. Do you see how a long string can twist and fold into a much more compact space? In a similar way, the DNA molecule twists and folds into a more compact entity.
Related Resources
Project Ideas
Activities
Links
- Blog Post: Celebrating DNA and the History of the Double Helix
- Blog Post: DNA-Based Crime Prevention
- Scientific American: Identical Twins' Genes Are Not Identical
- Genome News Network: What's a Genome
