ABSTRACT
We began Part III of Genomes by looking inside the nucleus and asking how the packaging of chromosomal DNA into nucleosomes and chromatin fibers influences the accessibility of individual genes and how that accessibility sets the overall pattern of genome expression. We then remained in the nucleus in Chapters 11 and 12, examining how the interaction between DNA-binding proteins and the genome results in synthesis of the transcriptome, before transferring to the cytoplasm in Chapter 13 to explore the synthesis and maintenance of the proteome. Throughout those four chapters, our attention was firmly on the genome and the RNAs and proteins whose synthesis the genome specifies. However, a genome does not exist purely for its own benefit. The genome resides in a cell that might itself be an organism or which forms part of a larger multicellular organism. Through the transcriptome and proteome, the genome specifies the biochemical activities occurring in the cell in which it resides, enabling the cell to generate energy and to grow and divide. The genome must therefore be responsive to the extracellular environment, so that the biochemical activities of the cell are continually in tune with the available nutrient supply and the prevailing physical and chemical conditions. Cells in multicellular organisms must also respond to hormones and growth factors, which may signal changes in the environment to which the organism must adapt but which also, importantly, coordinate the biochemical activities of different cells so that the multicellular organism functions as a unified entity. The response of the genome to these extracellular signals is usually transient: the expression pattern reverts to its original form when the signals cease or takes on a new pattern if one set of signals is replaced by a second set.
