A GUMMY BEAR MODEL OF DNA
Advanced Preparation
The teacher needs to purchase the gummy bears and dinosaurs. The time needed to assemble the materials is ten minutes per class
Introduction
Molecular models enable us to visualize the structure of chemical compounds. This allows us to analyze the molecules to determine the bond angle between the atoms and calculate the bond strengths. The structure of a DNA molecule is very complex, as it contains a repeating series of base pairs, sugars and phosphate groups. In this lab, the students will build a model of a DNA molecule; the gummy bears will represent the various atoms in the bases, while the dinosaur gummys will represent the sugar and phosphate groups in the DNA molecule. Each lab group will first construct one of the nucleotide bases; Adenine, Guanine, Cytosine or Thymine. (Adenine and Guanine are purines while Cytosine and Thymine are pyrimidines). The lab groups will then pair up to form base pairs, either Adenine - Thymine or Cytosine - Guanine. The students should realize that the base pairing is always the same. If Adenine and Cytosine pair, or Guanine and Thymine pair, a mutation has occurred. After the base pairs are built, the student groups will assemble a DNA molecule.
The students need to study the diagram of the DNA molecule to observe that the orientation of the base pairs in the molecule varies, since the purine or the pyrimidine molecules may bond on either side. By studying the diagram, the students should also note the random sequencing of the base pairs in the DNA molecule.
Student Objectives
The students will:
* construct a molecular model of a nucleotide base ( Adenine, Guanine, Cytosine or Thymine) using gummy bears.
* combine molecules with one other lab group to form either the adenine - thymine base pair
or the guanine - cytosine base pair.
* combine all the molecules (base pairs, sugars and phosphates) to build a DNA molecule.
* sketch the DNA molecule .
* analyze the data from the various molecules to determine the bond angles, bond types and
bond strengths.
Materials (per class)
1. ten pounds of gummy bears
2. two pounds of gummy dinosaurs
3. protractors
4. toothpicks
Procedure:
1. Have students look at the picture of the bases below. Instruct each lab group to build one of the bases.
2. Assign gummy bear colors to represent the elements (N, O, H, C, ) and fill in the data chart..
Remember there is a carbon at each corner of the pentagons and hexagons.
Gummy Bear Colors N, O, Bases
H, C
Adenine
Guanine
Cytosine
Thymine
Dinosaur Gummy Sugar, phosphate
3. Use toothpicks to represent bonds
Adenine and Thymine form two hydrogen bonds while Guanine and Cytosine form three bonds
Notice the the change in orientation for the bonding
4. Once the groups are finished, they will attach their base to the complement base being built by one of the other groups. All of the groups will work together to construct a DNA molecule.
5. After the students have attached the bases, phosphates and sugars to build the DNA molecule,
the teacher needs to check the molecule. See diagram 1.
6. Have the students trace the DNA molecule on butcher paper, then label the bond angles.
If the model is correct, the students may then eat the molecules.
Analysis
1. What type of bonds are present in nucleotide base molecules/
Non polar covalent ( N-H, N-O, N-C, C-O, C-C) and polar covalent ( O-H) bonds.
2
. What type of bonds hold the base-pairs together?
Hydrogen bonds are formed between a hydrogen atom in one molecule and a highly electronegative atom (N, O ) in another molecule. The attraction is chiefly electrostatic.
3. Which bonds have the greatest strength.
The more covalent the bond, the greater the bond strength.
4. Which bonds have the least amount of strength and why might that be important.
The hydrogen bonds between the base pairs are primarily electrostatic, those bonds are the first to break, creating the RNA molecules.
5. Determine the electronegativity difference between the following bonds in the DNA molecule.
A. P-O BOND
P = 2.2, O = 3.5, difference in electronegativities = +1.3, polar covalent bond
B. H-H BOND
H = 2.2, difference in electronegativities = 0, non polar covalent bond
6. What is the molecular mass of each of the bases?
Adenine (C - 5 = 60) ( N - 5 = 70) (H - 5 = 5) total 135 g
Guanine (C - 5 = 60) ( N - 5 = 70) ((H - 5 = 5) (O - 1 = 16) total 155 g
Cytosine (C - 4 = 48) ( N - 3 = 42) (H - 5 =5) (O - 1 = 16) total 111g
Thymine (C - 5 = 60) (N - 2 = 28) (H - 6 = 6) (O - 2 = 32) total 126g
7. What is the mass of deoxyribose?
C - 5, O -4, H - 9 ( 5 x 12 = 60) (4 x 16 = 64) (9 x 1 = 9) total = 133 grams per molecule
