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 Enzyme 1 Enzyme 2 Enzyme 3 Tryptophan (a) (b) Regulation of enzyme activity Regulation of enzyme production Regulation of gene expression  trpE gene trpD gene trpC gene trpB gene trpA gene

Operons: The Basic Concept and Negative Gene Regulation 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 Promoter DNA Regulatory gene mRNA trpR 5 3 Protein Inactive repressor RNA polymerase trp operon Genes of operon Operator mRNA 5 Start codon Stop codon trpE trpD trpC trpB trpA E D C B A Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on (b) Tryptophan present, repressor active, operon off Tryptophan (corepressor) Active repressor No RNA made Trp operon – always on unless repressed by the repressor + tryptophan. Repressor is always available but in inactive form until tryptophan binds to it.

Inducible (catabolic) operons are usually off but can be induced. Inducer inactivates the repressor Ex: lac (lactose) operon (a) Lactose absent, repressor active, operon off (b) Lactose present, repressor inactive, operon on Regulatory gene Promoter Operator DNA lacZ lacI mRNA 5 3 No RNA made RNA polymerase Active repressor Protein lac operon lacY lacA mRNA 5 Inactive repressor Allolactose (inducer) -Galactosidase Permease Transacetylase Lac Operon is always off unless induced by inducer. Lac repressor is active once made.

Positive Gene Regulation Gene is always on but activator stimulates transcription. Ex: cAMP Promoter DNA CAP-binding site lacZ lacI RNA polymerase binds and transcribes Operator cAMP Active CAP Inactive CAP Allolactose Inactive lac repressor (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA CAP-binding site lacZ lacI Operator RNA polymerase less likely to bind Inactive lac repressor Inactive CAP (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

18.2 Eukaryotic Gene Expression is Regulated at Many Stages Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation DNA Gene Gene available for transcription RNA Exon Primary transcript Transcription Intron RNA processing Cap Tail mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Translation Degradation of mRNA Polypeptide Protein processing, such as cleavage and chemical modification Active protein Degradation of protein Transport to cellular destination Cellular function (such as enzymatic activity, structural support) Each cell of multicellular organisms contain all genetic info; only some is expressed (differential gene expression). Each process has the potential for regulation.

Regulation of Chromatin Structure Amino acids available for chemical modification Histone tails DNA double helix 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 Troponin T gene Primary RNA transcript RNA splicing or mRNA 1 2 3 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 microRNAs (miRNA): begins as hairpin Small interfering RNAs (siRNA): begins as double strand (a) Primary miRNA transcript Hairpin miRNA Hydrogen bond Dicer miRNA- protein complex mRNA degraded Translation blocked (b) Generation and function of miRNAs 5 3

Embryonic development: division  differentiation  morphogenesis 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

Morphogens (proteins) establish an embryo’s axes Induction is how embryonic cells effect one another due to cell-surface molecules or growth factors. Determination due to the expression of genes for tissue-specific proteins. Pattern Formation puts determined cells in their “proper places” for the resulting organism. Morphogens (proteins) establish an embryo’s axes (b) Induction by nearby cells Early embryo (32 cells) NUCLEUS Signal transduction pathway Signal receptor Signaling molecule (inducer)

Animation: Development of Head-Tail Axis in Fruit Flies Right-click slide / select “Play” © 2011 Pearson Education, Inc.