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12: Regulation of Transcription and Epigenetic Inheritance
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12.1: Introduction
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Cells regulate their metabolism in several ways. We have already that allosterically regulated enzymes monitor the cellular levels of metabolites. Recall that glycolytic intermediates rise and fall in cells based on cellular energy needs, binding to or dissociating from allosteric sites
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12.2: Gene Regulation in Prokaryotes
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Many prokaryotic genes are organized in operons, linked genes transcribed into a single mRNA encoding two or more proteins. Operons usually encode proteins with related functions. Regulating the activity of an operon (rather than multiple single genes encoding single proteins) allows better coordination of the synthesis of several proteins at once. In E. coli, the regulated lac operon encodes three enzymes involved in the metabolism of lactose (an alternative nutrient to glucose).
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12.3: The Problem with Unregulated (Housekeeping) Genes in All Cells
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Before we turn our attention to the regulation of gene expression in eukaryotes, consider for a moment the expression of constitutive, or housekeeping, genes that are always active. The requirement that some genes are always “on” raises questions about cellular priorities of gene expression. Constitutive gene products are sets of many polypeptides that form large macromolecular complexes in cells, or enzyme sets that participate in vital biochemical pathways.
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12.4: Gene Regulation in Eukaryotes
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Results of this experiment provided the evidence that even very different cells of an organism contain the same genes. In fact, in any multicellular eukaryotic organism, every cell contains the same DNA (genes). Therefore, the different cell types in an organism differ not in which genes they contain, but which sets of genes they express! Looked at another way, cells differentiate when they turn on new genes and turn off old ones.
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12.5: Epigenetics
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Aristotle thought that an embryo emerged from an amorphous mass, a “less fully concocted seed with a nutritive soul and all bodily parts”. The much later development of the microscope led to more detailed (if inaccurate) descriptions of embryonic development. In 1677, no less a luminary than Anton von Leeuwenhoek, looking at a human sperm with his microscope, thought he saw a miniature human inside! The tiny human, or homunculus, became the epitome of preformation theory.
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12.6: Key Words and Terms
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