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.