The Molecular Basis of Embryonic Development

¹ Department of Biology, Yale University New Haven, Conn.; Department of Biochemistry, State University of New York at Buffalo, Buffalo, N. Y.

The information for the development of a complex adult organism from a relatively simple fertilized egg is contained within the deoxyribonucleic acid (genes) of the egg. Fundamentally, the genes are responsible for protein biosynthesis. This is accomplished through the mediation of ribonucleic acids (RNA). In the broadest terms, protein biosynthesis involves deoxyribonucleic acid (DNA) transcription into RNA and RNA translation into the linear amino acid sequence or primary structure of the polypeptide chain. Most organisms possess a large variety of proteins. These can be divided into two classes: (1) structural proteins and (2) enzymes. Enzymes control the nature and rates of the metabolic processes of cells. Not all of the metabolic processes characteristic of a given organism are found in every cell type of that organism. In fact, the enzyme complement of different cell types differs both qualitatively and quantitatively. Thus, the individuality of cells (i.e., their phenotype) is a reflection of the activities of their constituent enzymes. Since all cells of an organism possess the same genetic potential, this biochemical individuality is established at some time during embryonic development via the interaction of genome and environment. In essence, then, the basic principle of development in "higher" organisms is differential gene action. At present, little is known about the mechanism of regulation of gene action at the level of the gene. However, through studies of the end products of gene action, RNA, and protein, advances in our knowledge of the interrelationships of development and differential gene action have been made. For example, studies of the enzyme lactate dehydrogenase have been especially useful in providing insight into differential gene action during embryonic and phylogenetic development.