CHAPTER 19 Control of Gene Expression in Eukaryotes 3 22 2014 differential gene expression expression of different sets of genes in cells with the same genome responsible for creating different cell types arranging them into tissues coordinating activity individual result of production or activation of specific regulatory sequences leads to certain proteins expressed only in certain types of cells different cell types express different genes because of different histone modifications and different regulatory proteins present gene definition the coding sequences regulatory sequences that direct production of 1 related proteins or RNAs eukaryotes can control gene expression at levels of transcription translation post translation express 3 more levels of control chromatin remodeling RNA processing regulation of mRNA life span stability o o o o o chromatin remodeling o o o o occurs when DNA near promoter is released from tight interactions with proteins decondensation happens before transcription begins DNA wrapped around proteins to create chromatin a protein DNA complex RNA polymerase cannot access DNA when supercoiled within nucleus chromatin must be relaxed or decondensed for RNA polymerase to bind to promoter promoters sites where RNA polymerase binds to initiate transcription elaborate structure allows DNA to be packaged in nucleus plays key role in regulating gene expression structure chromatin contains nucleosomes repeating beadlike structures nucleosomes made of negatively charged DNA wrapped twice around eight positively charged histone linker stretch of DNA present btwn each pair of nucleosomes histone proteins one of several positively charged proteins associated w DNA in chromatin H1 histone protein maintains structure of each nucleosome may interact with each other and other nucleosome histones to form 30 nanometer fiber tightly packed structure o fibers may form higher order structures chromatin structure may be modified by the use of proteins ATP dependent chromatin remodeling complexes reshape chromatin other enzymes catalyze o acetylation addition of acetyl groups acetylation of histones associated w gene activation histone acetyl transferases HATs type of acetylation enzyme adds negatively charged acetyl groups to positively charged lysine residues in histones reduces positive charge on histones to decondense chromatin and allow gene expression remove acetyl groups from histones reverse effets of acetylation allow chromatin condensation o histone deactylases HDAcs o methylation addition of methyl groups of histones can correlate with activation or inactivation chromatin modifications can be inherited epigenetic inheritance patterns of inheritance not due to differences in gene sequences daughter cells inherit patterns of histone modification epigenetic inheritance daughter cells inherit patterns of gene expression genetic inheritance o o histone code hypothesis patterns of chemical modifications of histones contain information that influences whether a particular gene is expressed 3 22 2014 o o o o o pattern of chemical modification on histones varies from one cell type to another RNA processing transcription results in primary RNA transcript primary RNA transcript has to go RNA processing introns snipped exons spliced to make a mature RNA regulation of mRNA life span stability life span of mature mRNA transcript may be a control point for gene expression life span determines how many protein products it lives to produce default state of eukaryotic genes is off negative control consists of chromatin in condensed form promoter region unavailable eukaryotic genes turned off by regulatory proteins occurs when regulatory proteins bind to silencers or when chromatin remains condensed positive control must be implemented to open up DNA at promoter genes for gene expression to occur eukaryotic genes turned on by regulatory proteins occurs when chromatin is decondensed to expose promoter when specific regulatory proteins bind to enhancers and promoter proximal elements 3 conserved sequences of eukaryotic promoters each eukaryotic promoter has 2 3 conserved sequences o o most common sequence is the TATA box bound by the TATA binding protein TBP o regulatory sequences sections of DNA involved in controlling activity of genes o o gene activity changes when regulatory sequences bind these sequences co regulated genes not clustered together share regulatory DNA sequence bind same regulatory protein promoter proximal elements attach to transcriptional activators to promote initiation of transcription located just upstream of promoter transcription start site have sequences unique to specific genes o o o o o o o o o o o 3 22 2014 provide mechanism for eukaryotic ells to exert precise control over transcription enhancer regulatory sequence in eukaryotic cells binding of specific proteins to an enhancer enhances the transcription of certain genes use positive control when regulatory proteins bind to enhancers transcription begins present in all eukaryotes may be more than 100 000 bases away from promoter may be located in introns untranscribed 5 or 3 sequences flanking the gene they regulate many types many genes may have 1 enhancer function even if normal orientation is flipped or they are moved to new location on same chromosome silencer regulatory sequence in eukaryotic cell similar to enhancer use negative control repress rather than activate gene expression transcription shut down when regulatory proteins bind to silencers discovery of enhancers and silencers resulted in redefining gene as DNA that codes for functional protein or RNA molecule regulatory sequences required for expression 3 conserved sequences of eukaryotic promoters each eukaryotic promoter has 2 3 conserved sequences transcription factors recognize a specific sequence of bases in target DNA via minor and major groove combinations two classes of proteins bind to regulatory sequences at the start of transcription regulatory transcription factors bind to enhancers silences and promoter proximal elements are responsible for expression of particular genes in particular cell types particular stages of development basal transcription factors interact w promoter are not restricted to particular cell types required for transcription do not regulate gene expression mediator complex regulatory proteins in eukaryotes that form a physical link btwn regulatory transcription factors that are bound to DNA the basal
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