66

6

The Nature of Information

In much human communication, it is the context-dependent difference between

explicit and implicit meaning that is decisive in determining the ultimate outcome

of the reception of information. In the latter example of the previous paragraph, the

context—here provided by the physical environment—endows the statement with

a large complement of implicit information, which mostly depends on the mental

baggage possessed by the recipient of the information; for example, the meaning

of a Chinese poem may only be understandable to someone who has assimilated

Chinese history and literature since childhood, and will not as a rule be intelligible

to a foreigner armed with a dictionary.

A very similar state of affairs is present in the living cell. A given sequence of

DNA will have a well-defined explicit meaning in terms of the sequence of amino

acids it encodes, and into which it can be translated. In the eukaryotic cell, however,

that amino acid sequence may then be glycosylated and further transformed, but in

a bacterium, it may not be; indeed it may even misfold and aggregate—a concrete

example of implicit meaning dependent on context.

The importance of context in determining implicit meaning is even more graph-

ically illustrated in the case of the developing multicellular organism, in which the

cells are initially all identical; according to chemical signals received from their

environment, they will develop into different kinds of cells. The meaning of the

genotype is the phenotype, and it is implicit rather than explicit meaning, which is,

of course, why the DNA sequence of any earthly organism sent to an alien civiliza-

tion will not allow them to reconstruct the organism. Ultimately, most of the cells

in the developing embryo become irreversibly different from each other (differen-

tiation), but while they are still pluripotent, they may be transplanted into regions

of different chemical composition and change their fate; for example, a cell from

the non-neurogenic region of one embryo transplanted into the neurogenic region

of another may become a neuroblast (Sect. 14.9.2). The mechanism of such trans-

formations will be discussed in a little more detail in Chap. 14, but here this type of

phenomenon serves to illustrate how the implicit meaning of the genome dominates

the explicit meaning. This implicit meaning is called epigenetics, ²⁶ and it seems

clear that we will not truly understand life before we have developed a powerful way

of treating epigenetic phenomena. Shannon’s approach has proved very powerful for

treating the problem of the accurate transmission of signals, but at present we do

not have a comparable foundation for treating the problem of the precise transfer of

meaning. ²⁷

Even at the molecular level, at which phenotype is more circumscribed and could

be considered to be the function (of an enzyme), or simply the structure of a protein,

there is presently little understanding of the relation between sequence and function,

as illustrated by the thousands of known different sequences encoding the same type

of structure and function, or different sequences encoding different structures but the

same type of function, or similar structures with different functions.

26 Cf. Sects. 14.9.2 and 14.9.3.

27 Given that translation (from nucleic acid to protein) is involved, the proverb “traduttori traditori”

is quite apt.