Developmental Processes

Nearly every multicellular organism passes through a life cycle stage where it exists as a single undifferentiated cell or as a small number of undifferentiated cells. This developmental stage contains molecular information which specifies the entire course of development encoded in its many thousands of genes. At the molecular level, genes are used to make proteins, many of which act as enzymes, biological catalysts which drive the thousands of different biochemical reactions inside cells.

Adult multicellular organisms can consist of one quadrillion (a one followed by 15 zeros) or more cells, each of which has the same genetic information. (There are a few notable exceptions, such as the red blood cells of mammals, which do not have DNA, and certain cells in the unfertilized eggs of amphibians, which undergo gene amplification and have multiple copes of some genes.) F. C. Steward first demonstrated the constancy of DNA in all the cells of a multicellular organism in the 1950s. In a classical series of experiments, Steward separated a mature carrot plant into individual cells and showed that each cell, whether it came from the root, stem, or leaf, could be induced to develop into a mature carrot plant which was genetically identical to its parent. Although such experiments cannot typically be done with multicellular animals, animals also have the same genetic information in all their cells.

Many developmental biologists emphasize that there are additional aspects of information transfer during development which do not involve DNA directly. In addition to DNA, a fertilized egg cell contains many proteins and other cellular constituents which are typically derived from the female. These cellular constituents are often asymmetrically distributed during cell division, so that the two daughter cells derived from the fertilized egg have significant biochemical and cytological differences. In many species, these differences act as biological signals which affect the course of development. There are additional spatial and temporal interactions within and among the cells of a developing organism which act as biological signals and provide a form of information to the developing organism.