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Modeling DNA Structure

Students explore the structure of the DNA molecule and begin to understand how chromosomes, genes, and the base pairs, sugars, and phosphates of the DNA molecule are related.

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Lesson Summary

Overview

In this activity, students explore the structure of the DNA molecule and begin to understand how chromosomes, genes, and the base pairs, sugars, and phosphates of the DNA molecule are related. Students view and discuss video segments that describe the role of various genetic units. They also build models of DNA molecules -- using gumdrops, licorice, and toothpicks. At the end of the lesson, they join their model molecules together to form one large strand of DNA.

Objectives

  • Recognize the relationship between base pairs, DNA molecules, genes, and chromosomes
  • Understand the following terms: DNA molecule, nucleotide, base pairs, genes, and chromosomes
  • Build a model of a DNA molecule and identify the different parts and their functions
  • Understand that genes provide the code for an organism's traits

Suggested Time

  • Two class periods

Resources

Materials

  • Gumdrops of four different colors: red, orange, green, yellow
  • Licorice sticks
  • Toothpicks
  • Schematic picture of a nucleotide, identifying the base, phosphate, and sugar portions of the molecule (from biology text book).

Before the Lesson

  • Review the videos and read the background essay that accompanies each resource to gain information that will help you facilitate class discussion.
  • Gather together the following items for each team of three to four students: Two dozen gumdrops (assorted red, orange, green, and yellow), 4 strands of licorice, and 10 toothpicks (each broken into 3 pieces, for a total of 30 toothpick pieces).
  • Make one copy of the nucleotide schematic for each team.

The Lesson

Part I: DNA's Structure

1. Have students explore the Journey into DNA in pairs. When pairs are finished, discuss the meaning of each section as well as any questions students have during the activity. When finished, discuss the following:

  • Inside what structure are the cell's chromosomes found?
  • How many chromosomes are there in nearly every one of your body's cells?
  • What is the shape of the DNA molecule?
  • How many different types of bases are there in a molecule of DNA?

2. Have students watch the Human Genome Project video and discuss the following:

  • What is DNA?
  • Where is DNA found?
  • In what way does DNA provide the code of life?
  • What are the four letters in a DNA molecule, and what do they represent?

3. Divide the class into groups of three or four students. Give each group a copy of the nucleotide schematic and the licorice, toothpicks, and gumdrops. Tell each group to study the picture, making note of the following:

  • the parts of a nucleotide
  • that a DNA molecule is made up of thousands and thousands of nucleotides
  • how bases pair up: adenine with thymine (AT, TA), and guanine with cytosine (CG, GC)

4. Tell students that they are going to build models of the DNA molecule, each five nucleotides long, using the materials you have given them. In other words, they're building the equivalent of genes. Remind students that, while the order of nucleotides could be critically important in a real gene, they should not be concerned about the order of their nucleotides, as long as the bases are paired correctly (see above). Write the following key on the board. Ask students to refer to the key when constructing their models.

Key

  • Licorice = phosphate "backbone" of the DNA molecule
  • Short sections of toothpicks = sugar that connects the phosphate to the base
  • Gumdrops = bases (red = A; orange = T; green = C; yellow = G

5. Have the groups begin producing their models, as follows: Each model should consist of two strands of licorice, with five gumdrops attached to each strand. Tell students to use short sections of toothpick to attach the gumdrops to the strands at equal intervals. Because each color gumdrop represents a different base, the gumdrops on one strand must correspond to the appropriate gumdrops on the opposite strand. For example, a red gumdrop always pairs with an orange gumdrop, while a green gumdrop always pairs with a yellow gumdrop. Instruct students to attach the gumdrop base pairs with pieces of toothpick. The resulting model can be handled carefully without it falling apart.

6. Have students write down the base pairs of their model molecules. Each group will probably have a different sequence of base pairs for each molecule. Some examples are listed below.

Molecule 1

Left Strand Right Strand Base Pairs
yellow gumdrop - G green gumdrop - C GC
red - A orange - T AT
red - A orange - T AT
green - C yellow - G CG
yellow - G green - C GC

Molecule 2

Left Strand Right Strand Base Pairs
orange gumdrop - T red gumdrop - A TA
red - A orange - T AT
red - A orange - T AT
orange - T red - A TA
green - C yellow - G CG

7. Write each group's base-pair sequences on the board. When finished, you will have the sequence for the entire class. Remind students that the class sequence is only a tiny fraction of most real DNA molecule sequences.

8. Have groups connect their DNA segments to form a giant model DNA molecule.

Part II: DNA's Function

9. Have students watch the Gene Control and Genetic Tool Kit videos and discuss the following:

  • What types of things do genes control in a developing organism?
  • What are master control genes and how do they function?
  • What does the master control gene called the "eyeless gene" control? What happens when it is removed or damaged?
  • What happened when the eyeless gene was transferred from a mouse to a fly?
  • Where does an individual organism get the genes it has?
  • Can an individual organism change the sequence of its genes during its lifetime?
  • How are the traits of an organism related to its DNA sequence?
  • What would happen if there was a "mistake" in the DNA sequence?

Check for Understanding

Ask students to discuss the following:

  • Where is the genome (complete genetic code) of an organism found?
  • Describe the basic structure of DNA.
  • What are genes? How are they related to chromosomes? How are genes related to strands of DNA that make up an organism's genetic code?
  • What is the relationship between an organism's genes and its traits?
  • Is there a one-to-one relationship between genes and traits? In other words, does one gene code for one trait?

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