Molecular Biology of the Cell - Volume 2 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1993

Control of Gene Expression

The DNA of any Organism encodes all the Cell/22.html">RNA and Protein molecules that make up its Cells. However, a complete description of a genomic DNA sequence alone—whether it consists of a few million NUCLEOTIDES, as in a bacterium, or three billion, as in a human—reveals very little about how that organism is organized. The Genome can be compared to a dictionary containing all the "words" used by a given individual. Yet, understanding the logic of an organism's design based solely on such a dictionary is no easier than writing a Shakespearean play using an English dictionary. In all cases, one must know how to use these vocabulary units, and the number of their possible combinations is so vast that compiling the dictionary itself must be recognized as the simplest part of the work and merely an initial step in studying the problem.

We are, of course, still a long way from being able to "map out" The Structure of an organism based solely on its genome sequences. Doing so requires a deeper understanding of cell biology and of what thousands of large and small molecules do after they are synthesized. This chapter discusses only one aspect of this issue. We will examine the rules by which specific sets of genes are selectively activated in each cell. It will be shown that mechanisms controlling Gene Expression operate at various levels, each of which is addressed in a separate section of this chapter. Finally, we will consider the evolution of complex gene regulatory systems. However, we must begin with An Overview of some fundamental principles of Genetic control in higher organisms.