80

7

The Transmission of Information

Table 7.1 The genetic code

First (5 prime5^,)

Second position

Third (3 prime3^,)

U

U

C

A

G

phe

ser

tyr

cys

U

phe

ser

tyr

cys

C

leu

ser

stop

stop

A

leu

ser

stop

trp

G

C

leu

pro

his

arg

U

leu

pro

his

arg

C

leu

pro

gln

arg

A

leu

pro

gln

arg

G

A

ile

thr

asn

ser

U

ile

thr

asn

ser

C

ile

thr

lys

arg

A

met

thr

lys

arg

G

G

val

ala

asp

gly

U

val

ala

asp

gly

C

val

ala

glu

gly

A

val

ala

glu

gly

G

Note The table is given for RNA; for DNA, T must be used in place of U. See Table 15.6 for the

key to the amino acid abbreviations. “stop” is an instruction to stop sequence translation. AUG

encodes the corresponding instruction to “start” (in eukaryotes; sometimes other triplets are used

in prokaryotes)

in any base above 4. As is well known, DNA is encoded by RNA using the transfor-

mation ⁶

↓ ^A

C

T

G

U

G

A

C

by virtue of complementary base-pairing, and RNA triplets are, in turn, encoded by

amino acids (Table 7.1).

Codes used in telecommunications are single-valued and one-to-one transforma-

tions (i.e., bijective functions), which allows unambiguous decoding. The type of

coding found in biology is more akin to that described for the broadcast meteo-

rological bulletin described at the beginning of this chapter, in which the physical

carrier of the information changes and the bare technical content accrues meaning.

In that example, supposing that the satellite was defined as the information source,

6 Since DNA is composed of two complementary strands, one could equally well write the coding

transformation as

↓ ^A

C

T

G

A

C

U

G

.