14.8 Gene Expression

201

control over which portion of the protein repertoire will be expressed, determines

the range of environments in which the individual can survive and, hence, is equiv-

alent to adaptedness to future conditions (cf. directive correlation and its degree,

Sect. 3.4). The eukaryotic genome, with its resources of duplicate genes, pseudo-

genes, transposable elements, exon shuffling, polyploidy, and so forth, possesses the

potential of phenotypic variability while retaining genetic stability. Prokaryotes lack

these features, but they can readily acquire new genetic material from their peers or

from viruses.

14.8

Gene Expression

Gene expression refers to the processes (Fig. 14.1,d comma ed, e, andf f ) whereby proteins are

produced (“expressed”) from a DNA template. It thus constitutes the bridge between

genotype and phenotype. Whenever cells are not preparing for division (and many

highly differentiated cells never divide), they are simply living, which means, in

formal terms, that they are engaged in maintaining their essential variables within the

domain corresponding to “alive” (Sects. 3.4 and 3.6). In certain environments, such as

ocean floor sediments several kilometres thick, metabolic activity (of the bacteria that

are presumed to be ubiquitous there) may be barely detectable (the degree of activity

may be many orders of magnitude less than that of familiar laboratory bacteria, or

that of those living parasitically inside a warm-blooded creature). Such environments

are, moreover, unchanging or barely changing; hence, the vital processes can be

maintained with very little need to change any of the parameters controlling them.

Most natural habitats show far more variety of conditions, however. Commonly

encountered environmental disturbances include the fluctuating presence of toxic

molecules and changes of temperature. Hence, cells need the ability to adapt (i.e., to

modify their phenotypes to maintain their essential variables within the vital range).

The formal framework for understanding this process was introduced in Chap. 3.

Here, we examine the molecular mechanisms of regulation that enable adaptation—

the control of expression of different proteins as the cell proceeds around its cycle

(Fig. 14.2) and as an organism develops (Sect. 14.9); development is a consequence

of differential gene expression. The mechanism is essentially the same in all these

cases. The entire process of gene expression is facilitated by many enzymes.

Despite the existence of elaborate machinery for regulating transcription

(Sect. 14.8.2 ff.) stochastic influences on expression and, hence, phenotype are

discernible. ⁴⁸

48 Blake et al. (2003), Raser and O’Shea (2005).