A readily available resource to create a model for the study of DNA is the human hand. Students can recognize how structure and function of nucleotides determine structure and function of the DNA molecule by labeling parts of a gloved hand with the parts of a DNA molecule.

Introduction

Modeling activities are at the forefront of science education in American schools today (Bryce et al., 2016). Modeling enables students to create, analyze, and demonstrate the dynamic inner workings of a concept or object, thereby offering evidence of a thorough understanding of it (Louca & Zacharia, 2012). Many teachers at the middle and high school levels devote a sizable portion of their curriculum toward construction of models to exhibit structure and function of DNA (Robertson, 2016). I have developed a DNA model that can supplement some of the DNA modeling materials like marshmallows, toothpicks, and construction paper. It's a “Handy DNA Nucleotide Model,” using the human hand.

Handy DNA Nucleotide Model

A portion of a DNA molecule (Figure 1) can be broken down into individual DNA nucleotides (Figure 2) for analysis. A DNA nucleotide is drawn on a glove with markers to serve as a model (Figure 3). Students will mark seven locations on the top and bottom surfaces of the gloved hand to represent the parts of a DNA nucleotide, excluding the nitrogen base. Five of the locations are comprised of an oxygen atom and four carbon atoms of the deoxyribose ring structure on the palm of the glove and one carbon bonded to the exterior of the ring located on the thumb at the sixth location. The last location is a phosphate ion, an encircled P, bonded to the fifth carbon atom. The phophate ion enables bonding to a carbon on an adjacent nucleotide. These phosphate-to-carbon bonds up and down the length of the molecule contribute to making the two “DNA backbones” of the DNA molecule. The pyrimidine nitrogen bases are cytosine (C) and thymine (T), and the purine nitrogen bases are adenine (A) and guanine (G).

Figure 1.

Portion of a DNA molecule model.

Figure 1.

Portion of a DNA molecule model.

Figure 2.

DNA nucleotide model.

Figure 2.

DNA nucleotide model.

Figure 3.

Location of oxygen and carbon atoms and phosphate ion in DNA nucleotide glove model.

Figure 3.

Location of oxygen and carbon atoms and phosphate ion in DNA nucleotide glove model.

In Figure 4, the forefinger is a model for a pyrimidine base, either C or T, because the pyrimidine bases are shorter one-ringed molecules like the shorter forefinger. The second finger represents a purine base, either A or G, because the purines are longer two-ringed molecules like the longer second finger. (Compare the sizes of nitrogen bases in Figure 1.)

Figure 4.

The pyrimidine base on the forefinger and purine base on the second finger.

Figure 4.

The pyrimidine base on the forefinger and purine base on the second finger.

Separate the class into two groups. Instruct the groups to mark their gloves as follows:

1st group:

  • Mark the forefinger of the left glove with a C and second finger with an A.

  • Mark the forefinger of the right glove with a T and the second finger with a G.

2nd group:

  • Mark the forefinger of the left glove with a T and the second finger with a G.

  • Mark the forefinger of the right glove with a C and the second finger with an A.

Modeling DNA Nucleotides & Molecules

Using both gloved hands, one palm up and the other inverted (palm down) and allowing the tip of the forefinger or the second finger of one gloved hand to touch the tip of the second finger or forefinger, respectively, of the other gloved hand, a model for the bonding pattern of the DNA nucleotides is created. For example, with one palm up and the other palm down, the student could touch the forefinger of the left glove with the second finger of the right glove, thus modeling a C-G or T-A bond (Figure 5). The thumbs of both gloves should be extended in opposite directions. Or students could touch the second finger of the left glove with the forefinger of the right glove, modeling A-T or G-C bonding. Students can switch fingers of each glove to continue to model the activity of the bonding of nitrogen bases. Other fingers not being used in the model are folded back into the palm of the glove. If students practice this activity, they will begin to see how nitrogen base sizes affect the bonding patterns of the nucleotides and shape of the DNA molecule. With the help of other students, a portion of a DNA molecule can be modeled by students sitting on opposite sides of a long table and reaching across the table until their hands meet. However, this is not the intent of the model. The model is designed to be a quick reference for individual students to supplement their understanding of DNA nucleotide structure and function.

Figure 5.

DNA nucleotide bonding of cytosine and guanine.

Figure 5.

DNA nucleotide bonding of cytosine and guanine.

DNA Nucleotide Bonding Pattern

James Watson and Francis Crick were aided by the data of Rosalind Franklin (Franklin & Gosling, 1953) and Erwin Chargaff (Chargaff et al., 1950) in determining the structure of the DNA molecule. Franklin's data showed that the DNA was a double helix and water molecules were on the outside of the molecule, not on the inside as Watson and Crick had originally thought. Chargaff's data established that the concentration of adenine to thymine was equal in all samples of DNA, as was the concentration of cytosine to guanine. This data helped Watson and Crick determine the bonding pattern of the nitrogen bases to be A-T and C-G. Additionally, the diagrammatic model (Figure 6) shows how this bonding pattern establishes an equal distance across the width of the molecule along its entire length. That pattern is created by a longer purine base bonding to a shorter pyrimidine base, and no matter where that occurs, the distance across the molecule is the same.

Figure 6.

DNA nucleotide bonding patterns (© https://britannica.com/science/DNA). By courtesy of Encyclopaedia Britannica, Inc., copyright 1998; used with permission.

Figure 6.

DNA nucleotide bonding patterns (© https://britannica.com/science/DNA). By courtesy of Encyclopaedia Britannica, Inc., copyright 1998; used with permission.

Hydrogen Bonding of Nucleotides

For an in-depth understanding of nucleotide bonding, students can add dots to represent sites of weak bonds called hydrogen bonds that bond the nucleotides to each other. C (pyrimidine) and G (purine) each have three sites for hydrogen bonding. This results in the bonding of C to G. Similarly, adenine (purine base) and thymine (pyrimidine base) each has two sites for hydrogen bonding. This results in the bonding of A to T. Students will place three horizontal dots on the tips of glove fingers that represent C or G for the three hydrogen bonds participating in the C-to-G bonds (Figure 7). Students will mark the tips of the glove fingers representing A or T nitrogen bases with two horizontal dots, representing the two hydrogen bonds of A and T that allow them to bond to each other. This illustrates that only C can bond with G, and only A can bond with T, due to the matching up of the hydrogen bonds. This was a key factor that contributed to the description of the structure of the DNA molecule. At this point, you can have students construct a grid to show the relationships of the DNA nitrogen bases (Table 1).

Figure 7.

DNA nucleotide bonding of cytosine to adenine, showing three hydrogen bonds.

Figure 7.

DNA nucleotide bonding of cytosine to adenine, showing three hydrogen bonds.

Table 1.
DNA nitrogen base grid.
Nitrogen BaseKind of BaseBonds withNumber of RingsRelative Size of MoleculeNumber of H Bonds
Purine Long 
Purine Long 
Pyrimidine Short 
Pyrimidine Short 
Nitrogen BaseKind of BaseBonds withNumber of RingsRelative Size of MoleculeNumber of H Bonds
Purine Long 
Purine Long 
Pyrimidine Short 
Pyrimidine Short 

Conclusion

Models in biology can aid in translating a difficult concept into an everyday context. By constructing, analyzing, and explaining models, students can achieve a better understanding of a concept they may not otherwise thoroughly understand (Roberts et al., 2005). The enhanced understanding results in higher retention of the learning, and it is retention of learning and its applicability for which we need to strive. Modeling activities continue to pave the way.

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