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

A, those which prefer C, etc. Given the sequence of operations which an
enzyme performs, you can figure out which letter it prefers, but for now I'll
just give them without explanation Here's a sample enzyme, consisting of
three operations:

{

(I) Delete the unit to which the enzyme is bound (and then bind

to the next unit to the right).

(2) Move one unit to the right.

(3) Insert a T (to the immediate right of this unit).

This enzyme happens to like to bind to A initially. And here's a sample

strand:

ACA

What happens if our enzyme binds to the left A and begins acting? Step 1

deletes the A, so we are left with CA--and the enzyme is now bound to the

C. Step 2 slides the enzyme rightwards, to the A, and Step 3 appends a T

onto the end to form the strand CAT. And the enzyme has done its

complete duty: it has transformed ACA into CAT.

What if it had bound itself to the right A of ACA? It would have deleted

that A and moved off the end of the strand. Whenever this happens, the

enzyme quits (this is a general principle). So the entire effect would just be

to lop off one symbol.

Let's see some more examples. Here is another enzyme:

{

(I) Search for the nearest p}rimidine to the right of this unit.

(2) Go into Copy mode.

(3) Search for the nearest purine to the right of of this unit.

(4) Cut the strand here (viz., to the right of the present unit).

Now this contains the terms "pyrimidine" and "purine". They are easy

terms. A and G are called purines, and C and T are called pyrimidines. So

searching for a pyrimidine merely means searching for the nearest C or T.

Copy Mode and Double Strands

The other new term is Copy mode. Any strand can be "copied" onto another

strand, but in a funny way. Instead of copying A onto A, you copy it onto T,

and vice versa. And instead of copying C onto C, you copy it onto G, and

vice versa. Note that a purine copies onto a pyrimidine, and vice versa. This

is called complementary base pairing. The complements are shown below:

complement

{

A .¢:~ T

}

purines pyrimidines

G .¢:~ C

(^506) Self-Ref and Self-Rep