Modelling the DNA double helix using recycled materials
Submitted by sis on 25 July 2006
Molecular structure of DNA
Each of the three constituents of the nucleotide were represented with 3D objects (see Table 1) which were connected to form a double helix with ten steps (base pairs). See below.
Our choice of materials reflected their abundance in the school recycling bins.
First, each of the three nucleotide constituents (deoxyribose, phosphate and base) are modelled, reflecting the geometry of the molecule as far as possible. Next, the components are assembled to form nucleotides and the DNA helix is constructed.
Puncture the aluminium cans and the bottle caps with the same nail. Heating the nail will enable the caps to be more easily pierced. Choose a suitable thickness of nail to enable the plastic drinking straws to pass through the holes and fit firmly, creating a stable link between the structural elements.
Complementary base pairs
Next, plastic bottles representing the bases are modelled so that they can only be connected to their complementary base (adenine to thymine, and guanine to cytosine).
To construct two complementary base pairs, cut two Fanta bottles and three Coca Cola bottles in cross-section, using the scalpel and scissors. Take care!
Constructing the bases
Place green cellophane in a Coca Cola bottle without a base.
To the base of a Fanta bottle, attach the neck of a Coca Cola bottle. Place blue cellophane inside both parts.
Thymine (T), represented by the colour green, is connected by two hydrogen bonds to adenine (A), represented by the colour blue. To model this, push the blue neck firmly into the green bottle without the base.
Place red cellophane in a Fanta bottle.
Place yellow cellophane in a Coca Cola bottle without a base. Firmly attach the base of another Coca Cola bottle, upside down.
Guanine (G), represented by the colour red, is connected by three hydrogen bonds to cytosine (C), represented by the colour yellow. To model this, open the base of the yellow bottle (cytosine) along the incisions to allow the base of the red bottle (guanine) to enter and lock firmly.
For symmetry and the scale of the model, the two pairs of linked complementary bases should be 42 cm long. Each coloured bottle is screwed into the bottle cap (carbon) at position 1’ of a deoxyribose molecule, forming four different nucleotides (see below).
This representation of the hydrogen bonds enables the easy connection and detachment of complementary bases. This, in turn, facilitates not only the separation of the DNA strands but also the change in position of bases for teaching purposes.
Complementary base pairing
Constructing the DNA molecule
Having constructed 20 nucleotides, we can build a double helix with 10 steps – two strands of 10 nucleotides each. Because the distance from the end of the Coca Cola can (phosphate group) to the orange cap (hydroxide linked with the next phosphate group) is 23 cm, the strand of 10 nucleotides will be 2.3 m long.
Attach the telephone cable to approximately 3 m of the thin rope and use the stiff cable to pass the rope through the straws of the nucleotides to form two strands of molecules, which are hung vertically 2 m high and 65 cm apart. The two strands of the DNA molecule are read in the direction 5’ to 3’ and are anti-parallel. In the model, the direction in which we read the word Coca Cola coincides with the direction 5’ to 3’. Thus, in one of the strands, the words Coca Cola can be read from top to bottom and in the other strand, from bottom to top. Our model DNA strands are thus also anti-parallel.
We must also make sure that the bases on one strand are complementary to those on the opposite strand. Adenine should be opposite thymine and cytosine opposite guanine.
If these criteria have been met, tie a paper roll at the end of each strand so that a thin bar can be passed through the roll and used to twist the linked strands clockwise, 360 degrees (see below).
The model represents a DNA molecule at a scale of 320 000 000:1, that is, 320 million times bigger than it really is. If we tried to represent an entire human DNA molecule with our model, we would need a double helix 640 000 km long, capable of wrapping around Earth’s equator 16 times.
Students learn much more quickly and easily when they are actively involved in the lesson. Teaching the structure of DNA is made much easier if a 3D representation of the molecule is used. Jigsaw puzzle-type activities give a 2D picture, but it is difficult to visualise the shape of the molecule. This ingenious project describes how a scale model of DNA can be made using cans and bottles. It would be easy to collect the materials required to make this model, as students could recycle cans and bottles.
It may be a good idea for a technician or teacher to do some of the preparation work; this would decrease the amount of time needed in the lesson as well as addressing safety considerations with the use of a hot nail to puncture the bottle tops and a sharp implement to cut the plastic bottles. Alternatively, the model could be made in a design-technology class and then used in biology lessons. Group work could be designed so that teams race each other to prepare a DNA model. The model could be used as a teaching tool to demonstrate DNA replication, either in mitosis or in the polymerase chain reaction. The fact that the model is to scale will help the students appreciate the spatial relationship of the components of the DNA molecule. I feel that students will enjoy learning about DNA using this idea, which means that the lesson will be both understood and remembered.