Deoxyribonucleic acid, or DNA, was discovered in 1953 by James Watson, Francis Crick and Rosalind Franklin. This molecule is regarded as the fundamental basis for life, as it contains the information for building proteins and structures required in all organisms. Every human being's DNA is unique in terms of the sequence of its thousands of individual nitrogenous base pairs, just as every book contains words but no two books contain the same sentences or the same ordering of words. But all DNA takes the form of a simple structure, a double helix, consisting of a repeating series of phosphate groups, five-carbon sugars and nitrogenous bases, represented schematically as A, C, G and T.
Models of DNA can be constructed from a variety of everyday, readily available items. Such models serve as valuable tools to communicate the essentials of this elegant work of nature.
The Basic Structure of DNA
A double helix can be conceived as a very long, flexible ladder, with the sides of the ladder twisted in opposite directions from both ends, with the result being a spiral shape. The "rungs" are the hydrogen bonds between adjacent base pairs, with A (adenine) bonding only to T (thymine) and C (cytosine) bonding only to G (guanine). Each base binds to a five-carbon sugar (S) opposite its hydrogen bond, and these sugars bond to each other along the sides of the "ladder" via a phosphate group (P) between them.
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The degree of the twist is important to visualize for purposes of making models of the DNA molecule. The double helix makes one complete "twist" about every five to six base pairs. But any correct model need only have the essentials right: the sugars, phosphates, and bases must all be in their proper positions with respect to each other.
Middle-School Models: Recycled Items
A spirit of environmental conservation can show up in the building of DNA models. After consulting a diagram detailing the basic structure of the molecule, consider how many different kinds of unique objects are needed to represent a length of DNA. (The answer is six: one each for A, C, G, T, S and P.) Working alone or in groups, come up with lists of items in school or home recycling bins that might plausibly fit together to create a model of the molecule.
The items selected must be similarly sized, and not overly large, to create an accurate model. For example, a different type of soda can for each of the four bases could be combined with the use of portions of egg cartons for the sugars and popsicle sticks for the phosphate groups.
High-School Models: Digging Deeper Into DNA
When making more elaborate DNA models, one challenge is to explain why A might pair with, and only with, T and similarly for C and G. (The answer is that at the level of their three-dimensional conformation in space, A tends to fit with T in the manner of, say, jigsaw puzzle pieces.) A clay model with flexible wire forming the spine of the "rungs" and the "sides" is an ideal way to represent this. Use different colors of clay for the four base types, and come up with different plausible shapes for each; they need only be consistent and meet the "puzzle pieces fitting" criteria.
For extra credit, form hypotheses about the reason that DNA twists itself into a double helix rather than remaining in a basic ladder shape. (Answer: the positive and negative charges on the different molecules attract and repel each other in such a way to ensure that the double helix is the only way for the molecule to exist in a stable form.)