Gödel, Escher, Bach An Eternal Golden Braid by Douglas R. Hofstadter

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the Prelude, Ant Fugue. The global structure-consisting of the letters 'M'

and 'U'-is its quaternary structure; then each of those two parts has a tertiary

structure, consisting of "HOLISM" or "REDUCTIONISM"; and then the

opposite word exists on the secondary level, and at bottom, the primary

structure is once again the word "MU", over and over again.

Polyribosomes and Two-Tiered Canons

Now we come to another lovely parallel between tape recorders translating

tape into music and ribosomes translating mRNA into proteins. Imagine a

collection of many tape recorders, arranged in a row, evenly spaced. We

might call this array a "polyrecorder". Now imagine a single tape passing

serially through the playing heads of all the component recorders. If the

tape contains a single long melody, then the output will be a many-voiced

canon, of course, with the delay determined by the time it takes the tape to

get from one tape recorder to the next. In cells, such "molecular canons"

do indeed exist, where many ribosomes, spaced out in long lines-forming

what is called a polyribosome-all "play" the same strand of mRNA, produc-

ing identical proteins, staggered in time (see Fig. 97).

Not only this, but nature goes one better. Recall that mRNA is made by

transcription off of DNA; the enzymes which are responsible for this

process are called RNA polymerases ("-ase" is a general suffix for enzymes). It

happens often that a series of RNA polymerases will be at work in parallel

on a single strand of DNA, with the result that many separate (but identi-

cal) strands of mRNA are being produced, each delayed with respect to the

other by the time required for the DNA to slide from one RNA polymerase

to the next. At the same time, there can be several different ribosomes

working on each of the parallel emerging mRNA's. Thus one arrives at a

double-decker, or two-tiered, "molecular canon" (Fig. 98). The corre-

sponding image in music is a rather fanciful but amusing scenario: several

FIGURE 98. Here, an even more complex scheme. Not just one but several strands of

mRNA, all emerging by transcription from a single strand of DNA, are acted upon by

polyribosomes. The result is a two-tiered molecular canon. [From Hanawalt and Haynes, The

Chemical Basis of Life, p. 271.]

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