8: Molecular Genetics II - Regulation of Gene Expression
- Page ID
- 146899
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Why does a human skin cell differ so dramatically from a liver cell, or a muscle cell, or a neuron? After all, every cell in the body (except gametes) has the same genome -- the same complement of DNA. (We'll ignore somatic mutations for the moment.) So why are these cells' "phenotypes" so different, if their genotype is the same?
The answer, of course, is that in skin cells, some genes are expressed and others aren't. In a liver cell, a different set of genes is expressed. Other genes are expressed (or repressed) only in response to an environmental influence or a signal from another cell. The molecular mechanisms that allow cells to regulate gene expression -- to turning genes on and off -- is the main topic of this chapter.
Before we dig into these molecular mechanisms, though, two important points tie these firmly to the realm of genetics and heritability. First, as we will see, the regulation of a gene is dependent on DNA sequences -- promoters, proximal regulatory elements, distal enhancers, splice sites, etc -- which can themselves be polymorphic. That is, the amount that a gene is expressed is itself a heritable trait, subject to selection just like any other trait.
And secondly, have you ever considered that when a liver cell divides, the two daughter cells look like liver cells -- and not skin cells or neurons? (Similarly, when a skin cell in the same organism divides, its two daugher cells also look like skin cells.) That's because in both cases, the daugher cells are expressing the same set of genes as the parent cell. Patterns of gene expression are heritable. We call the inheritance of these patterns of gene expression epigenetics. We'll examine both genetic and epigenetic control of gene expression as the chapter progresses.
If you have mastered the material in this chapter, you should be able to:
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Define the following terms used in prokaryotic gene regulation: promoter, transcription factor, inducer, operon, regulon
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Describe how transcription factors regulate the transcription of operons
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Predict how changes in DNA sequence will affect gene expression
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Define the following terms used in eukaryotic gene regulation: chromatin, histone, methylation, alternative splicing, RNA interference (RNAi)
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Describe how the following affect gene expression:
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Acetylation and deacetylation of chromatin
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Methylation of DNA
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Alternative splicing
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RNAi
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Predict how changes in chromatin, DNA methylation, alternative splicing, and RNAi will affect gene expression