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TAMU BIOL 111 - Ch 18 Blueprint
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Rebecca DavenportBIOL 1406-BWI11/6/16Ch 18 Blueprint1. Regulation of Gene Expressiona. Differential Expression of Genesi. Prokaryotes and eukaryotes precisely regulate gene expression in response to environmental conditionsii. In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell typesiii. RNA molecules play many roles in regulating gene expression in eukaryotesb. Bacteria often respond to environmental change by regulating transcriptioni. Natural selection has favored bacteria that produce only the products needed bythat cellii. A cell can regulate the production of enzymes by feedback inhibition or by gene regulationiii. One mechanism for control of gene expression in bacteria is the operon modelc. Operons: the basic concepti. A cluster of functionally related genes can be coordinately controlled by a single “on-off switch”.ii. The “switch” is a segment of DNA called an operator usually positioned within the promoter.iii. An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control.iv. The operon can be switched off by a protein repressorv. The repressor prevents gene transcription by binding to the operator and blocking RNA polymerasevi. The repressor is the product of a separate regulatory genevii. The repressor can be in an active or inactive form, depending on the presence ofother moleculesviii. A corepressor is a molecule that cooperates with a repressor protein to switch an operon offix. For example, E. coli can synthesize the amino acid tryptophan when it has insufficient tryptophanx.d. Repressible and inducible operons: Two types of negative gene regulationi. A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription.ii. The trp operon is a repressible operoniii. An inducible operon is one that is usually off;a molecule called an inducer inactivates the repressor and turns on transcriptioniv. The lac operon is an inducible operonv. By default the trp operon is on and the genes for tryptophan synthesis are transcribedvi. When tryptophan is present, it binds to the trp repressor protein, which turns the operon off vii. The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are highviii. The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactoseix. By itself, the lac repressor is active and switches the lac operon offx. A molecule called an inducer inactivates the repressor to turn the lac operon onxi. Inducible enzymes usually function in catabolic pathways; their synthesis is induced by achemical signalxii. Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end productxiii. Regulation of the trp and lac operons involves negative control of genes becauseoperons are switched off by the active form of the repressor.xiv.e. Eukaryotic gene expression is regulated at many stagesi. All organisms must regulate which genes are expressed at any given timeii. In multicellular organisms regulation of gene expression is essential for cell specializationf. Histone modifications and DNA Methylationi. In histone acetylation, acetyl groups are attached to positively charged lysines inhistone tailsii. This loosens chromatin structure, thereby promoting the initiation of transcriptioniii. The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatiniv. DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some speciesv. DNA methylation can cause long-term inactivation of genes in cellular differentiationvi. In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of developmentg. Epigenetic inheritancei. Although the chromatin modifications just discussed do not alter DNA sequence,theymay be passed to future generations of cellsii. The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritanceh. Regulation of Transcription initiationi. Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machineryi. Organization of a Typical Eukaryotic Genei. Associated with most eukaryotic genes are multiple control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcriptionii. Control elements and the transcription factors they bind are critical to the precise regulation of gene expression in different cell typesj. The roles of transcription Factorsi. To initiate transcription, eukaryotic RNA polymerase requires the assistance of transcription factorsii. General transcription factors are essential for the transcription of all protein-coding genesiii. In eukaryotes, high levels of transcription of particular genes depend on control elements interacting with specific transcription factorsk. Mechanisms of post-Transcriptional Regulationi. Transcription alone does not account for gene expressionii. Regulatory mechanisms can operate at various stages after transcriptioniii. Such mechanisms allow a cell to fine-tune gene expression rapidly in response toenvironmental changesl. RNA Processingi. In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as


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TAMU BIOL 111 - Ch 18 Blueprint

Type: Chapter Summary
Pages: 4
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