Yeast RSC chromatin remodeling complex structure determined by electron microscopy.The complex (red) is shown bound to a nucleosome (yellow). [Courtesy of Francisco J. Asturias, 2002, PNAS99:13477.]
In previous chapters we've seen that the actions and properties of each cell type are determined by the proteins it contains. In this and the next chapter, we consider how the kinds and amounts of the various proteins produced by a particular cell type in a multicellular organism are regulated. The basic steps in gene expression, the entire process whereby the information encoded in a particular gene is decoded into a particular protein, are reviewed in Chapter 4. Synthesis of mRNA requires that an RNA polymerase initiate transcription, polymerize ribonucleoside triphosphates complementary to the DNA coding strand, and then terminate transcription (see Figure 4-10). In prokaryotes, ribosomes and translationinitiation factors have immediate access to newly formed RNA transcripts, which function as mRNA without further modification. In eukaryotes, however, the initial RNA transcript is subjected to processing that yields a functional mRNA (see Figure 4-14). The mRNA then is transported from its site of synthesis in the nucleus to the cytoplasm, where it is translated into protein with the aid of ribosomes, tRNAs, and translation factors (see Figure 4-26).
Theoretically, regulation at any one of the various steps in gene expression could lead to differential production of proteins in different cell types or developmental stages or in response to external conditions. Although examples of regulation at each step in gene expression have been found, control of transcription initiation—the first step—is the most important mechanism for determining whether most genes are expressed and how much of the encoded mRNAs and, consequently, proteins are produced. The molecular mecha nisms that regulate transcription initiation are critical to numerous biological phenomena, including the development of a multicellular organism from a single fertilized egg cell, the immune responses that protect us from pathogenic microorganisms, and neurological processes such as learning and memory. When these regulatory mechanisms function improperly, pathological processes such as cancer and aging may occur.
In this chapter, we focus on the molecular events that determine when transcription of eukaryotic genes is initiated
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