John A. Anderson
Texas Tech University Lubbock, 79409
Construction of a Framework Model of DNA A class proiect
The freshman chemistry course for nonscience majors at Texas Tech University includes general chemistry in the fall semester and organic and biochemistry in the spring. During a 3-hr laboratory period in the spring semester a framework model of the DNA double helix was constructed as a class project. The necessary amounts of plastic tubing (12 cm) and connections for this framework model were obtained from Prentice-Hall, Englewood Cliffs, N. J. The aluminum support, which consisted of small aluminum rods radiating from a larger central rod, was constructed in the departmental shop. Bond lengths in angstroms1 weIe converted to cm (1 A = 3 cm). Students were provided with a diagram of types of bonds (trigonal or tetrahedral), bond lengths, and color of plastic tubing to be used. One group of students constructed one nucleotide pair. The nucleotide pair was brought to the DNA chain and connected with the help of the teaching assistant. The 3'-carbon of the group's nucleotide pair was attached to the phosphate oxygen of the pair below it, and the phosphate oxygen of the other nucleotide was attached to the 3'-carbon below it. The nucleotide pair was positioned so that the angle between it and the pair below was approximately 36" (10 nucleotide pairs per com@ete turn) and the distance between pairs was 3.4 A (10.2 om). The nucleotide pair was secured by attaching the inner H bond t o the central rod with cellophane tape and attaching fine copper wires between the nucleotide pair and the horizontal rods. The teaching assistants had half the students rotate the nucleotide pair 180' around the two-fold axis before positioning on the double helix, so that one chain contains all four bases. Some dimensions of the helix are shown in Figure 1. A photograph of the model is shown in Figure 2. The height with 18 residues is 184 cm, 72.5 in. The laboratory assistants assembled the first two nucleotide pairs and started the double helix before the 'LANGRIDGE, R., SEEDS,W. E., WILSON, H. R.,HOOFER, C . W., WILKINS,M. H. F., AND HAMILTON, L. D., J. B i o p h y ~Biochcm. M., Ada Cryst., 12, 59 (1959). Cylol., 3, 767 (1957). SPENCER, ARNOTT,S., WILKINS,M. H. F., HAMILTON, L. D., AND LANGRIDGE, R.,J . M d . Biol., 11,391 (1965). WATSON, J. D., "The Double Helix," Atheneum, New York, 1968.
first laboratory. The first and succeeding laboratories then added nucleotide pairs until the model was completed. With four students per group, 56 students plus two teaching assistants constructed the complete model with 18 nucleotide pairs. To accommodate different numbers of students, the size of the groups or the scale of the model may be changed. The model constructed by the students, although obviously less precisely constructed, is similar in appearance and construction to the model of DNA made by Watson and C r i ~ k . ~ The students were thus able to refer to their own experience when the dramatic progress toward the final model was described in class. The DNA model has been used in biochemistry and freshman chemistry to illustrate the conformation of DNA. The model was also displayed at an open house with a poster explaining the scale of the model, the base pairing, and the three-base code.
Figure 1.
Dimensions
of the DNA double helix. Scole: 1 A = 3 sm. A, distance 0
between nucleotider along helix oxis. B, dirtanse for one complete turn 11 0 nucleot i d e d C, D, dirtonce from one choin to the next. E, thickners of helix.
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Figure 2.
Completed DNA model
Volume 49, Number 5, May 1972
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