Gene Expression
CHAPTER EIGHT
8
AN OVERVIEW OF GENE EXPRESSION HOW TRANSCRIPTIONAL SWITCHES WORK THE MOLECULAR MECHANISMS THAT CREATE SPECIALIZED CELL TYPES POST-TRANSCRIPTIONAL CONTROLS
Control of Gene Expression
An organism’s DNA encodes all of the RNA and protein molecules that are needed to make its cells. Yet a complete description of the DNA sequence of an organism—be it the few million nucleotides of a bacterium or the few billion nucleotides in each human cell—does not enable us to reconstruct the organism any more than a list of all the English words in a dictionary enables us to reconstruct a play by Shakespeare. We need to know how the elements in the DNA sequence or the words on a list work together to make the masterpiece. In cell biology, the question comes down to gene expression. Even the simplest single-celled bacterium can use its genes selectively—for example, switching genes on and off to make the enzymes needed to digest whatever food sources are available. And, in multicellular plants and animals, gene expression is under even more elaborate control. Over the course of embryonic development, a fertilized egg cell gives rise to many cell types that differ dramatically in both structure and function. The differences between a mammalian neuron and a lymphocyte, for example, are so extreme that it is difficult to imagine that the two cells contain the same DNA (Figure 8–1). For this reason, and because cells in an adult organism rarely lose their distinctive characteristics, biologists originally suspected that genes might be selectively lost when a cell becomes specialized. We now know, however, that nearly all the cells of a multicellular organism contain the same genome. Cell differentiation is instead achieved by changes in gene expression. Hundreds of different cell types carry out a range of specialized functions that depend upon genes that are only switched on in that cell type: for
270
Chapter 8
Control of Gene Expression example, the
8
AN OVERVIEW OF GENE EXPRESSION HOW TRANSCRIPTIONAL SWITCHES WORK THE MOLECULAR MECHANISMS THAT CREATE SPECIALIZED CELL TYPES POST-TRANSCRIPTIONAL CONTROLS
Control of Gene Expression
An organism’s DNA encodes all of the RNA and protein molecules that are needed to make its cells. Yet a complete description of the DNA sequence of an organism—be it the few million nucleotides of a bacterium or the few billion nucleotides in each human cell—does not enable us to reconstruct the organism any more than a list of all the English words in a dictionary enables us to reconstruct a play by Shakespeare. We need to know how the elements in the DNA sequence or the words on a list work together to make the masterpiece. In cell biology, the question comes down to gene expression. Even the simplest single-celled bacterium can use its genes selectively—for example, switching genes on and off to make the enzymes needed to digest whatever food sources are available. And, in multicellular plants and animals, gene expression is under even more elaborate control. Over the course of embryonic development, a fertilized egg cell gives rise to many cell types that differ dramatically in both structure and function. The differences between a mammalian neuron and a lymphocyte, for example, are so extreme that it is difficult to imagine that the two cells contain the same DNA (Figure 8–1). For this reason, and because cells in an adult organism rarely lose their distinctive characteristics, biologists originally suspected that genes might be selectively lost when a cell becomes specialized. We now know, however, that nearly all the cells of a multicellular organism contain the same genome. Cell differentiation is instead achieved by changes in gene expression. Hundreds of different cell types carry out a range of specialized functions that depend upon genes that are only switched on in that cell type: for
270
Chapter 8
Control of Gene Expression example, the