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.