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Regulation of Gene Expression
Chapter 18 Regulation of Gene Expression
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Overview: Conducting the Genetic Orchestra
Prokaryotes and eukaryotes alter gene expression in response to their changing environment In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles in regulating gene expression in eukaryotes
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Fig. 18-1 Figure 18.1 What regulates the precise pattern of expression of different genes? Figure 18.1 What regulates the precise pattern of expression of different genes?
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Gene expression in bacteria is controlled by the operon model
Concept 18.1: Bacteria often respond to environmental change by regulating transcription Natural selection has favored bacteria that produce only the products needed by that cell A cell can regulate the production of enzymes by feedback inhibition or by gene regulation Gene expression in bacteria is controlled by the operon model
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Figure 18.2 Regulation of a metabolic pathway
Precursor Figure 18.2 Regulation of a metabolic pathway Feedback inhibition trpE gene Enzyme 1 trpD gene Regulation of gene expression Enzyme 2 trpC gene trpB gene Figure 18.2 Regulation of a metabolic pathway Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production
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Operons: The Basic Concept
An operon is a cluster of genes whose protein product can be turned on or off depending upon the needs of the organism. 3 parts of the Operon: Operator: controls access of RNA Pol to genes (near promoter) Promoter: where RNA Pol attaches Genes of the Operon: stretch of DNA required for enzymes made by the operon
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Regulatory Genes produce repressor proteins
Repressors act like an ‘off’ switch by blocking RNA Pol and therefore transcription of genes Repressors are usually on, but can be inhibited A corepressor is a molecule that cooperates with a repressor protein to switch an operon off For example, E. coli can synthesize the amino acid tryptophan
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By default the trp operon is on and the genes for tryptophan synthesis are transcribed
When tryptophan is present, it binds to the trp repressor protein, which turns the operon off The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription The trp operon is a repressible operon An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose By itself, the lac repressor is active (gene off) and switches the lac operon off
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Fig. 18-4 Regulatory gene Promoter Operator DNA lacI lacZ Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacI lacZ lacY lacA RNA polymerase Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes For the Cell Biology Video Cartoon Rendering of the lac Repressor from E. coli, go to Animation and Video Files. 3 mRNA mRNA 5 5 Protein -Galactosidase Permease Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on
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Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by the end product (higher levels) Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor
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Positive Gene Regulation
Some operons are controlled through a stimulatory protein such as catabolite activator protein (CAP), an activator of transcription Ex. Low glucose in E. coli=CAP activation (through binding of cyclic AMP) Activated CAP attaches to the promoter of the lac operon and increases attraction of RNA polymerase= faster transcription High glucose levels=CAP detaches from the lac operon, and transcription slows
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Fig. 18-5 Promoter Operator DNA lacI lacZ CAP-binding site RNA polymerase binds and transcribes Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP) Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter Operator DNA lacI lacZ Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP) CAP-binding site RNA polymerase less likely to bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
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Concept 18.2: Eukaryotic gene expression can be regulated at any stage
All organisms must regulate which genes are expressed at any given time Differences between cell types result from differential gene expression: the expression of different genes by cells with the same genome Errors in gene expression can lead to diseases including cancer Gene expression is regulated at many stages
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Fig. 18-6 Signal NUCLEUS Chromatin Chromatin modification DNA Gene available for transcription Gene Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells Transcription RNA Exon Primary transcript Intron RNA processing Tail Cap mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells Polypeptide Protein processing Active protein Degradation of protein Transport to cellular destination Cellular function
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Regulation of Chromatin Structure
Genes within highly packed heterochromatin are usually not expressed Less accessible for transcription Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression Histone acetylation Addition of methyl groups (methylation) Addition of phosphate groups (phosphorylation)
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Histone tails Histone acetylation , acetyl groups are attached to positively charged lysines in histone tail loosens chromatin structure, promoting transcription Amino acids available for chemical modification DNA double helix (a) Histone tails protrude outward from a nucleosome Figure 18.7 A simple model of histone tails and the effect of histone acetylation Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription
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DNA Methylation (on DNA)
Addition of methyl groups (methylation) condenses chromatin; inhibits transcription In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development Addition of phosphate groups (phosphorylation) loosens chromatin, promoting transcription
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Regulation of Transcription Initiation
Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery
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(distal control elements)
Fig Poly-A signal sequence Enhancer (distal control elements) Proximal control elements Termination region Exon Intron Exon Intron Exon DNA Upstream Downstream Promoter Transcription Primary RNA transcript Exon Intron Exon Intron Exon Cleaved 3 end of primary transcript 5 RNA processing Intron RNA Poly-A signal Figure 18.8 A eukaryotic gene and its transcript Coding segment mRNA 3 Start codon Stop codon 5 Cap 5 UTR 3 UTR Poly-A tail Associated with most eukaryotic genes are control elements, segments of noncoding DNA that help regulate transcription by binding certain proteins
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Enhancers and Specific Transcription Factors
To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors Proximal control elements are located close to the promoter Distal control elements, groups of which are called enhancers, may be far away from a gene or even located in an intron
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Transcription initiation complex
Fig Promoter Activators Gene DNA Distal control element Enhancer TATA box Transcription initiation complex An activator is a protein that binds to an enhancer and stimulates transcription of a gene Bound activators cause mediator proteins to interact with proteins at the promoter General transcription factors DNA-bending protein Group of mediator proteins RNA polymerase II Figure 18.9 A model for the action of enhancers and transcription activators RNA polymerase II Transcription initiation complex RNA synthesis
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Mechanisms of Post-Transcriptional Regulation
Transcription alone does not account for gene expression Regulatory mechanisms can operate at various stages after transcription Such mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental changes
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Exons DNA Troponin T gene Primary RNA transcript RNA splicing mRNA or
Fig alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, (intron and exon segments are changed) Exons DNA Troponin T gene Primary RNA transcript Figure Alternative RNA splicing of the troponin T gene RNA splicing mRNA or
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Initiation of Translation
The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA Alternatively, translation of all mRNAs in a cell may be regulated simultaneously
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Fig After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control Proteasomes are giant protein complexes that bind protein molecules and degrade them Proteasome and ubiquitin to be recycled Ubiquitin Proteasome Protein to be degraded Ubiquitinated protein Protein fragments (peptides) Protein entering a proteasome Figure Degradation of a protein by a proteasome
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Concept 18.3: Noncoding RNAs play multiple roles in controlling gene expression
Only a small fraction of DNA codes for proteins, rRNA, and tRNA A significant amount of the genome may be transcribed into noncoding RNAs Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration
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These can degrade mRNA or block its translation
Fig Hairpin miRNA Hydrogen bond Dicer miRNA miRNA- protein complex 5 3 (a) Primary miRNA transcript MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA These can degrade mRNA or block its translation Figure Regulation of gene expression by miRNAs mRNA degraded Translation blocked (b) Generation and function of miRNAs
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Small RNAs may also block transcription of specific genes
The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi) RNAi is caused by small interfering RNAs (siRNAs) siRNAs play a role in heterochromatin formation and can block large regions of the chromosome Small RNAs may also block transcription of specific genes
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Gene expression orchestrates the developmental programs of animals
Concept 18.4: A program of differential gene expression leads to the different cell types in a multicellular organism During embryonic development, a fertilized egg gives rise to many different cell types Cell types are organized successively into tissues, organs, organ systems, and the whole organism Gene expression orchestrates the developmental programs of animals
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A Genetic Program for Embryonic Development
The transformation from zygote to adult results from: Cell division: series of mitotic divisions (increase # of cells) Cell differentiation: process by which cells become specialized in structure and function Morphogenesis: physical processes that give an organism its shape (a) Fertilized eggs of a frog (b) Newly hatched tadpole
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What controls differentiation and morphogenesis?
Cytoplasmic determinants Cell-cell signals Determination Pattern formation
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Fig Cytoplasmic determinants are maternal substances in the egg that influence early development. These are distributed unevenly in the early cells of the embryo and results in different effects Unfertilized egg cell Sperm Nucleus Fertilization Two different cytoplasmic determinants NUCLEUS Early embryo (32 cells) Zygote Signal transduction pathway Mitotic cell division Signal receptor Figure Sources of developmental information for the early embryo Two-celled embryo Signal molecule (inducer) (a) Cytoplasmic determinants in the egg (b) Induction by nearby cells
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Animation: Cell Signaling
2. Cell-cell signals In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells and induce differentiation of specialized cell types Animation: Cell Signaling
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3. Determination commits a cell to its final fate
After cell-cell signaling irreversible Cell differentiation is marked by the production of tissue-specific proteins
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Myoblasts produce muscle-specific proteins and form skeletal muscle cells
MyoD is one of several “master regulatory genes” that produce proteins that commit the cell to becoming skeletal muscle The MyoD protein is a transcription factor that binds to enhancers of various target genes
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4. Pattern formation is the development of a spatial organization of tissues and organs
In animals, pattern formation begins with the establishment of the major axes Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells gradients of substances called morphogens establish an embryo’s axes and other features
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Fig Head Thorax Abdomen 0.5 mm Dorsal Right In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization After fertilization, the embryo develops into a segmented larva with three larval stages BODY AXES Anterior Posterior Left Ventral (a) Adult Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell 2 Unfertilized egg Egg shell Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Figure Key developmental events in the life cycle of Drosophila Embryonic development 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva
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Fig EXPERIMENT Tail One maternal effect gene, the bicoid gene, affects the front half of the body An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends Head T1 T2 A8 T3 A7 A1 A2 A3 A4 A5 A6 Wild-type larva Tail Tail A8 A8 A7 A6 A7 Mutant larva (bicoid) RESULTS Figure Is Bicoid a morphogen that determines the anterior end of a fruit fly? Fertilization, translation of bicoid mRNA 100 µm Anterior end Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo
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