Within a single bacterial cell, genes are reversibly induced and repressed by transcriptional control in order to adjust the cell’s enzymatic machinery to its immediate nutritional and physical environment. Single-celled eukaryotes, such as yeasts, also possess many genes that are controlled in response to environmental variables (e.g., nutritional status, oxygen tension, and temperature). Even in the organs of higher animals — for example, the mammalian liver — some genes can respond reversibly to external stimuli such as noxious chemicals. In general, however, metazoan cells are protected from immediate outside influences; that is, most cells in metazoans experience a fairly constant environment. Perhaps for this reason, genes that respond to environmental changes constitute a much smaller fraction of the total number of genes in multicellular organisms than in single-celled organisms.
The different cell types in a multicellular organism differ dramatically in both structure and function. If we compare a mammalian neuron with a lymphocyte, for example, the differences are so extreme that it is difficult to imagine that the two cells contain the same genome (Figure 7-1). For this reason, and because cell differentiation is often irreversible, biologists originally suspected that genes might be selectively lost when a cell differentiates. We now know, however, that cell differentiation generally depends on changes in gene expression rather than on any changes in the nucleotide sequence of the cell's genome.Figure 7-1A mammalian neuron and a lymphocyteThe long branches of this neuron from the retina enable it to receive electrical signals from many cells and carry those signals to many neighboring cells. The lymphocyte is a white blood cell involved in the immune response to infection and moves freely through the body. Both of these cells contain the same genome, but they express different RNAs and proteins. (From B.B. Boycott, Essays on the Nervous System [R. Bellairs and E.G. Gray, eds.]. Oxford, UK: Clarendon Press, 1974.)
D. Andrews, J. Holton, and C. Leovy. International Geophysics Academic Press, (1987)https://www.elsevier.com/books/middle-atmosphere-dynamics/andrews/978-0-12-058575-5.