Developmental Processes

Differentiation is the change of unspecialized cells in a simple body pattern to specialized cells in a more complex body pattern. It is highly coordinated with growth and includes morphogenesis, the development of the complex overall body pattern.

Below, we emphasize molecular changes in organisms which lead to development. However, this does not imply that external factors have no role in development. In fact, external factors such as changes in light, temperature, or nutrient availability often elicit chemical changes in developing organisms which profoundly influence development.

The so-called "Central Dogma of Biology" says that spatial and temporal differences in gene expression cause cellular and morphological differentiation. Since DNA makes RNA, and RNA makes protein, there are basically three levels where a cell can modulate gene expression: 1) by altering the transcription of DNA into RNA; 2) by altering the translation of RNA into protein; and 3) by altering the activity of the protein, which is usually an enzyme. Since DNA and RNA are themselves synthesized by proteins, the gene expression patterns of all cells are regulated by highly complex biochemical networks.

A few simple calculations provide a better appreciation of the complexity of the regulatory networks of gene expression which control differentiation. Starting with the simplifying assumption that a given protein (gene product) can be either absent or present in a cell, there are at least ten centillion (a one followed by 6,000 zeros) different patterns of gene expression in a single typical-cell at any time. Given that a multicellular organism contains one quadrillion or more cells, and that gene expression patterns change over time, the number of possible gene expression patterns is enormous.

Perhaps the central question of developmental biology is how an organism can select the proper gene expression pattern among all these possibilities. This question has not yet been satisfactorily answered. However, in a 1952 paper, Alan Turing showed that simple chemical systems, in which the component chemicals diffuse and react with one another over time, can create complex spatial patterns which change over time. Thus, it seems possible that organisms may regulate differentiation by using a Turing-like reaction-diffusion mechanism, in which proteins and other molecules diffuse and interact with one another to modulate gene expression. Turing's original model, while relatively simple, has been a major impetus for research about pattern development in biology.

Lastly, aging must also be considered a phase of development. Many evolutionary biologists believe that all organisms have genes which have multiple effects, called pleiotropic genes, that increase reproductive success when expressed early in development, but cause the onset of old age when expressed later in development. In this view, natural selection has favored genes which cause aging and death because the early effects of these genes outweigh the later effects.