Ch. 18 Regulation of Gene Expression Objectives: LO 3.18 The student is able to describe the connection between the regulation of gene expression and observed differences between different kinds of organisms. LO 3.19 The student is able to describe the connection between the regulation of gene expression and observed differences between individuals in a population. LO 3.20 The student is able to explain how the regulation of gene expression is essential for the processes and structures that support efficient cell function. LO 3.21 The student can use representations to describe how gene regulation influences cell products and function. LO 3.22 The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production.. LO 3.23 The student can use representations to describe mechanisms of the regulation of gene expression. 18.1 Bacteria Often Respond to Environmental Change by Regulating Transcription Conserve resources 1 of 2 ways: • Feedback inhibition (discussed in Ch. 8) • Regulation of gene expression (discussed here) Precursor Feedback inhibition trpE gene Enzyme 1 trpD gene Enzyme 2 Regulation of gene expression trpC gene trpB gene Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production Operons: The Basic Concept and Negative Gene Regulation Operons: • Operator (“on/off switch”), promoter, and genes. – Repressible (anabolic) operons: Always “on” until repressor is bound. (inhibited) • Corepressor is like feedback inhibition (product works with repressor) • Ex: tryptophan producing genes trp operon Promoter DNA Regulatory gene mRNA Promoter Genes of operon trpD trpC trpE trpR 3 RNA polymerase Operator Start codon mRNA 5 5 Inactive repressor (a) Tryptophan absent, repressor inactive, operon on D No RNA made mRNA Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off C B A Polypeptide subunits that make up enzymes for tryptophan synthesis DNA Protein trpA Stop codon E Protein trpB Active repressor • Inducible (catabolic) operons are usually off but can be induced. – Inducer inactivates the repressor – Ex: lac (lactose) operon Regulatory gene DNA Promoter Operator lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon lacI DNA lacZ lacY lacA RNA polymerase 3 mRNA 5 mRNA 5 -Galactosidase Protein Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on Permease Transacetylase Positive Gene Regulation • Gene is always on but activator stimulates transcription. – Ex: cAMP Promoter Promoter DNA lacI CAP-binding site lacZ Operator RNA polymerase less likely to bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized DNA lacI lacZ CAP-binding site Active CAP cAMP Operator RNA polymerase binds and transcribes Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized 18.2 Eukaryotic Gene Expression is Regulated at Many Stages Signal NUCLEUS Chromatin • Each cell of multicellular organisms contain all genetic info; only some is expressed (differential gene expression). – Each process has the potential for regulation. DNA Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation Gene available for transcription Gene Transcription RNA Exon Primary transcript Intron RNA processing Cap Tail mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Polypeptide Protein processing, such as cleavage and chemical modification Degradation of protein Active protein Transport to cellular destination Cellular function (such as enzymatic activity, structural support) Regulation of Chromatin Structure Histone tails DNA double helix Amino acids available for chemical modification Nucleosome (end view) (a) Histone tails protrude outward from a nucleosome Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription • Histone Modifications: acetylation loosens chromatin easier protein access. • DNA Methylation: addition of methyl group to gene turns it off. • Epigenetic Inheritance: gene regulation passed on to offspring. Regulation of Transcription Initiation • Control elements/enhancers upstream from a gene can activate or repress transcription factors to regulate gene expression. • Combination of control elements and their activators. – Like genes use similar control elements and activators. Mechanisms of Post-Transcriptional Regulation • mRNA degradation • Alternative RNA splicing: different intron/exons spliced together. Exons DNA 1 3 2 4 5 4 5 Troponin T gene Primary RNA transcript 3 2 1 RNA splicing mRNA 1 2 3 5 or 1 2 4 5 Animation: Blocking Translation Right-click slide / select “Play” © 2011 Pearson Education, Inc. Animation: Protein Processing Right-click slide / select “Play” © 2011 Pearson Education, Inc. 18.3 Noncoding RNAs Play Multiple Roles in Controlling Gene Expression • Parts of DNA that make very small RNA (ncRNA) but not proteins; regulate gene expression. – Bind to a complementary sequence of mRNA, blocking translation. – Bind to DNA changing chromatin structure 1. microRNAs (miRNA): begins as hairpin 2. Small interfering RNAs (siRNA): begins as double strand Hairpin Hydrogen bond miRNA Dicer 5 3 (a) Primary miRNA transcript miRNA miRNAprotein complex mRNA degraded Translation blocked (b) Generation and function of miRNAs 18.4 A Program of Differential Gene Expression Leads to the Different Cell Types in a Multicellular Organism • Embryonic development: division differentiation morphogenesis Cytoplasmic Determinants • RNA and proteins from mom’s cell unevenly distributed giving rise to different cells during 1st divisions. (a) Cytoplasmic determinants in the egg Unfertilized egg Sperm Fertilization Zygote (fertilized egg) Mitotic cell division Two-celled embryo Nucleus Molecules of two different cytoplasmic determinants Induction is how embryonic cells effect one another due to cell-surface molecules or growth factors. (b) Induction by nearby cells Early embryo (32 cells) Determination due to the expression of genes for tissue-specific proteins. NUCLEUS Pattern Formation puts determined cells in their “proper places” for the resulting organism. Signal transduction pathway Morphogens (proteins) establish an embryo’s axes Signaling molecule (inducer) Signal receptor Animation: Development of Head-Tail Axis in Fruit Flies Right-click slide / select “Play” © 2011 Pearson Education, Inc.