The primary function of the majority of genes in eukaryotic organisms is to coordinate the development of embryos. This module takes a look at some of the basic principles underlying the role of genes in embryonic development.
Development
In most of the other modules of this course, the effects of genes are examined with regard to their effect on the phenotype of the juvenile or adult organism. In reality, however, the vast majority of these phenotypes are established during embryonic development, because most genes function to generate the pattern (the 'shape') of the developing embryo. Loss of function of a particular gene results in an abnormality in development, which manifests itself as a phenotype in the adult. In this sense, virtually all of eukaryotic genetics is developmental genetics, even though we haven't considered it as such.
The field of developmental biology is concerned with the function of genes in embryogenesis. The central question of developmental biology is the following:
How does a single cell, the fertilized egg (or zygote), manage to produce an extremely complex adult organism, which is composed not only of trillions of cells, but of thousands of different types of cells (such as nerve cells, muscle cells, etc.)?
There is a simple answer to this question, and that is that cells differentiate. In other words, different cells become specialized to carry out different functions by following different developmental pathways. But how does this occur? How do cells know what pathway to follow?
The Mosaic Theory of Development
One early theory (in the late 1890's) that attempted to explain the process of differentiation was the mosaic theory, as outlined by Wilhelm Roux and August Weismann. This theory proposed that there were determinants that specified the various differentiation pathways. The theory stated that these determinants would all be present in the zygote, but as cell division occurred after fertilization, the determinants would be unequally inherited by the offspring cells. This is known as qualitative cell division. The set of determinants would thus be divided up until each cell contained only one type of determinant, and that determinant would determine the fate of that cell.
When Mendel's work was rediscovered in the early 1900's and the concept of the 'gene' was developed, it was believed that the genes were the determinants that were divided up during cell division. This idea is an appealing one because of its simplicity, but the theory is impeded by one small problem: IT IS WRONG.
Evidence was accumulated over sixty years that disproved the notion that genes are divided up in development. The final nail in the coffin of this theory came from nuclear transplantation studies done using amphibians in the early 1960's. In these studies, nuclei were isolated from tadpole intestinal cells (differentiated cells) and injected into eggs that had their own haploid nuclei removed. A small percentage of the injected eggs developed into completely normal adult frogs! These frogs had developed using only the genetic information found in a tadpole intestinal nucleus. Therefore, a tadpole intestinal nucleus must contain all of the genetic information necessary to allow the differentiation of every cell type in an adult frog. If the mosaic theory were correct, this would not be the case; a tadpole intestinal nucleus would have only the genetic information necessary to cause the differentiation of an intestinal cell.
Note: The frogs produced by this procedure would be genetically identical to the frog from which the intestinal cell nucleus was obtained. In other words, they would be clones. In fact, these experiments comprised the earliest successful cloning of vertebrate organisms. The recent cloning of Dolly the sheep was done for the same reasons as outlined above: demonstrating that differentiated mammalian nuclei (from mammary gland in this case) have all of the genetic information necessary to drive the development of a normal adult organism.
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