Source: The Science Teacher 56 (No. 3): 76-78 (1989).
A CLASS EXERCISE FOR TEACHING THE GENETIC CODE
Gregor Mendel published his results in a scientific journal, lectured about them to fellow scientists, and discussed them in his extensive correspondence with the famous botanist Karl Nageli. Yet, much grass had grown on Mendel's cheeks before his work came to be appreciated by the scientific community. According to some historians, lack of mathematical sophistication on the part of his audience contributed to this long and curious delay.
When many liberal arts college students, or advance placement high school students, are introduced to the genetic code, their plight is similar to that of Mendel's distinguished audience. For them, the biochemistry of protein synthesis is hard enough; combined with the unfamiliar idea of a code, it is Greek.
Nature's problem is this: How can a linear sequence of four bases specify a given linear sequence of twenty amino acids? Nature's solution is simple enough: a redundant code in which one or more base triplets serves as a code for one specific amino acid. Unfortunately, upon encountering this solution for the first time, students must assimilate within a short period numerous biochemical and genetic terms and ideas, along with the concepts, purposes, and limitations embodied in the coding aspect of this process. To make the learners' task easier, it seems reasonable to introduce them first to coding problems in terms far more familiar than the terms biochemists use to describe the building blocks of nucleic acids and proteins.
I have so far attempted a few approaches to this instructional problem, including the use of Morse Code and computer languages. But Morse Code is unfamiliar to most students, employs codes of varying lengths, and only relies upon two symbols. Likewise, many students found binary language even more difficult to understand than the genetic code I tried to explain it with. Similar criticisms could be raised against other approaches to this pedagogical problem. Only recently I have stumbled across an idea which facilitates teaching of the genetic code.
Evolution's way of translating the language of nucleic acids into the language of proteins is analogous to the problem of translating a sequence of four colors into English. Somehow, 26 letters, several punctuation marks, and a few other English symbols need to be expressed through a sequence of four colors. To anyone familiar with the genetic code, one solution to this immediately comes to mind: constructing a triplet color code for the 35 or so symbols of written English.
One example should suffice to clarify this. Since it would be cumbersome to use actual colors here, let the letter P stand for the color pink, W for white, Y for yellow, and B for blue. (Owing to the high incidence of red‑green color blindness, I prefer to avoid red and green in this exercise). We can then arbitrarily create a code for all the essential symbols of the English language. Thus, if in this code the triplet PPP serves as the code for the letter "s," PPW for "a," PPB for "m," PPY for "n," and BBY as the code for the capitalization of the triplet it precedes (like the shift key on a typewriter), then, in this code, "Sam" can be expressed by the color sequence BBYPPPPPWPPB and "man" by PPBPPWPPY.
In the classroom, I try to combine this approach with self‑discovery and hands‑on experiences. Before the first session, two students are given copies of the same color code (which is deliberately constructed to closely resemble the format of their text's genetic code) and instructed in its use. In class, the subject is introduced by means of "a conversation in four colors" between these two student‑actors. The class is divided into two parts. One part agrees upon a brief message which is then conveyed to one actor while the other actor is outside hearing range. While ostensibly consulting the color code he holds, the first actor then converts the English message he received to a color‑coded message on the board. The other actor then uses his color code to decipher and read the message aloud, and then colors the reply which has been secretly decided upon by the other half of the class in the absence of the first actor. The process continues until the class is entirely convinced that the actors indeed use colors to "talk" to each other.
At this point, most students guess that the solution to the problem lies in the paper the actors diligently consulted throughout their "performance," and not in magic or trickery. We then have a class discussion in which we try to figure out what it is that a piece of paper might have which would make the meaningful information exchange they have just witnessed possible. At some point during this discussion, students receive their own copy of the color code. As a take‑home assignment, they are asked to use this code to convey a brief message.
By next class, they are ready to tackle a few conceptual problems. In particular, I usually discuss such issues as the insufficiency of a code based on doublets alone, the inefficiency of a quadruplet code, and the redundancy afforded by the 64 possible permutations of a triplet code.
Needless to say, this approach lends itself to many variations. It can, for instance, be used as a laboratory exercise in the two weeks preceding lectures about the genetic code. Instead of four colors, any four symbols will do (I chose four colors because colored chalks and crayons are readily available). And, depending on one's students, imagination, and inclinations, the "dramatic" element can be enhanced, played down, or altogether eliminated.
To sum up. This method employs familiar symbols and a simple idea to teach the genetic code. It thereby renders the student's task of understanding this key biological concept a bit easier and more enjoyable than it is when traditional instructional approaches are employed.