Genes VII

Development begins with a single fertilized egg, but gives rise to cells that have different developmental fates. The problem of early development is to understand how this asymmetry is introduced: how does a single initial cell give rise within a few cell divisions to progeny cells that have different properties from one another? The means by which asymmetry is generated varies with the type of organism. The egg itself may be homogeneous, with the acquisition of asymmetry depending on the process of the initial division cycles, as in the case of mammals. Or the egg may have an initial asymmetry in the distribution of its cytoplasmic components, which in turn gives rise to further differences as development proceeds, as in the case of Drosophila.

Early development is defined by the formation of axes. By whatever means are used to develop asymmetry, the early embryo develops differences along the anterior-posterior axis (head-tail) and along the dorso-ventral axis (top-bottom.) At the stage of interpreting the axial information, a relatively restricted set of signaling pathways is employed, and essentially the same pathways are found in flies and mammals.

The paradigm for considering the molecular basis for development is to suppose that each cell type may be characterized by its pattern of gene expression, that is, by the particular gene products that it produces. The principal level for controlling gene expression is at transcription, and components of pathways regulating transcription provide an important class of developmental regulators. We may include a variety of activities within the rubric of transcriptional regulators, which could act to change the structure of a promoter region, to initiate transcription at a promoter, to regulate the activity of an enhancer, or indeed sometimes to repress the action of transcription factors. However, the regulators of transcription most often prove to be DNA-binding proteins that activate transcription at particular promoters or enhancers.