| Differentiation and Genetic Cascades |
Development of a complex multicellular organism is more than just growth - we certainly do not look like gigantic fertilized eggs. Starting from a single cell, numerous specialized cell types emerge that differ in many ways: size, shape, longevity, biochemistry, and so on. What can account for this great diversity among cell types? What processes underlie this differentiation of a single cell into all the cell types of an adult individual?
Is differentiation due to the loss of certain genes in some cell types? While there are some exceptional cases (for example, mature red blood cells lack nuclei), development as a rule is not due to particular cell types having different genes. With only a few exceptions, all the cells in your body contain the same DNA. Discoveries of adult stem cells show that some adult cells retain the potential to produce many, if not all, of the cell types in the organism. These cells can reverse the process of differentiation, reaching a state where their descendants can redifferentiate into all of the cell types.
If cells of an individual are genetically alike, how does differentiation occur? Recall that proteins, not DNA, carry out most cellular functions. (See the Proteins and Proteomics unit.) DNA serves a blueprint from which RNA is transcribed. Proteins come from the amino acid chains that are translated from the RNA. The levels of transcription and translation of a gene determine how much of that gene's protein will be present in the cell. Gene expression, which encompasses transcription and translation, is the general term to describe the processes in which DNA produces RNA and proteins. It can also include other factors, such as the rate at which RNA is degraded before it can be translated. Differential gene expression will result in varying concentrations and kinds of proteins in cells, causing them to look and function differently. This differential transcription and translation of genes ultimately allows for cellular differentiation. Thus, development is a program that regulates gene expression at the appropriate locations and times.
How is it that, for a given cell type, certain subsets of genes are expressed and other genes are not expressed? As we will see later, the protein product that results from the expression of one gene can influence the expression of several other genes. In turn, the altered expression patterns of these genes can then influence the expression of an even larger number of genes. By this process, called a cascade, a change in one or a few genes can alter the expression patterns of numerous genes.