Lec34n - Lecture 35: Control of Eukaryotic Gene Expression...

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Lecture 35: Control of Eukaryotic Gene Expression Campbell Chap. 19. 6 th Ed. pp. 354-356 & 362-367 (top) 7 th Ed. Chap 17 pp. 359-367 8 th Ed. 320-322, 356-362 Introduction . In any organism, only a subset of genes is expressed and protein products made at any time. All cells can adjust the complement of proteins they make to reflect their specific situation - for instance, their nutritional status. We saw an example of this in the Trp operon in bacteria, which is only expressed when Trp levels are low and the organism needs to make more. Eukaryotic cells can also regulate gene expression to reflect particular needs at a given time. However, multicellular eukaryotes also have an additional type of regulation. Some cells in these organisms make certain specialized proteins that other cells never need to make. That is, some genes are never expressed in most cells in the body, but are only expressed in certain tissues. The amount of a particular protein found in a cell at any time, and the ability of the protein to function, can be regulated at any of several levels (See Fig 19.3, 7 th ed.; 18.6, 8 th Ed.). These include rate of transcription of the gene, RNA processing & transport to the cytoplasm, rate of translation of the mRNA, rate of mRNA degradation, modification of the protein, rate of protein degradation, and transport to the appropriate site in the cell. We already saw an example of how the levels of cyclin proteins are regulated through the cell cycle by protein degradation. We’ll see an example later in the course of how transport to the right place regulates protein function. Recent findings on micro RNA suggest that mRNA degradation plays a more important role in regulating protein levels than we’d realized. However, in most cases, the major factor determining the amount of a protein in the cell is its level of transcription. Generally, if an mRNA is made, it will be translated into functional protein. That is, the amount of a functional protein present in cells is usually proportional to the amount of its mRNA. For this reason, we’ll focus on learning how transcription rate is regulated. Part I: Role of Transcription Factors in Regulating Gene Expression Introduction: Compare with bacteria . Remember that in the Trp operon in bacteria, the Trp repressor (when it’s bound to Trp) binds the operator to block transcription by RNA polymerase. Here are some differences between this situation and how gene regulation works in eukaryotes. We (eukaryotes) have no operons; each of our genes has its own promoter. In addition, we don’t have repressor proteins that act by binding to operators and physically blocking RNA polymerase. (So we don’t have operator sequences next to our genes). In addition, in contrast to bacteria, in eukaryotes it’s relatively rare to have small molecules bind regulatory proteins, and for this binding to determine whether or not the regulatory proteins bind DNA (as was the case for binding of Trp to the Trp repressor in E. coli).
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Lec34n - Lecture 35: Control of Eukaryotic Gene Expression...

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