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How is Gene Expression Controlled?

How is Gene Expression Controlled?. Transcriptional Control (whether gene is transcribed or not) Operon : series of genes that code for specific products, including regulators that control whether these genes are transcribed

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How is Gene Expression Controlled?

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  1. How is Gene Expression Controlled? • Transcriptional Control (whether gene is transcribed or not) • Operon: series of genes that code for specific products, including regulators that control whether these genes are transcribed • Example: lac operon (bacteria) – genes for lactose metabolism only activated if lactose is present (when lactose not present, a repressor blocks transcription; if present, lactose blocks repressor, and transcription occurs) • Regulator genes control the expression of suites of genes; many control development and/or body patterns (Hox genes in animals) • Post-transcriptional Control: editing of exons • Translational Control • Involves whether or not m-RNA is used or stored in cytoplasm • Ex., egg cells often with large amounts of m-RNA “ready for use” • Post-translational Control • Polypeptides may be inactive; may need to join another polypeptide or may become activated by a co-factor

  2. Signal NUCLEUS Chromatin Chromatin modification DNA Gene available for transcription Gene Transcription Fig. 18.6 RNA Exon Primary transcript Intron RNA processing Tail mRNA in nucleus Cap Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Translation Degradation of mRNA Polypeptide Protein processing Active protein Degradation of protein Transport to cellular destination Cellular function

  3. Precursor Feedback inhibition trpE gene Enzyme 1 trpD gene Fig. 18.2 Regulation of gene expression trpC gene Enzyme 2 trpB gene Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production

  4. Figure 16.21a

  5. trp operon Promoter Promoter Genes of operon DNA trpD trpB trpA trpE trpC trpR Operator Regulatory gene Stop codon Start codon 3 mRNA 5 RNA polymerase mRNA 5 Fig. 18.3 D E C B A Protein Inactive repressor Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

  6. Regulatory gene Promoter Operator lacZ lacI DNA No RNA made 3 mRNA RNA polymerase 5 Fig. 18.4 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon lacZ DNA lacI lacY lacA RNA polymerase 3 mRNA mRNA 5 5 Permease -Galactosidase Transacetylase Protein Inactive repressor Allolactose (inducer) (b) Lactose present, repressor inactive, operon on

  7. Eye Fig. 18.18 Leg Antenna Wild type Mutant

  8. Thorax Head Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Left Ventral (a) Adult Fig. 18.17 Follicle cell Egg cell developing within ovarian follicle 1 Nucleus Egg cell Nurse cell Egg shell Unfertilized egg 2 Depleted nurse cells Fertilization Laying of egg Fertilized egg 3 Embryonic development Segmented embryo 4 0.1 mm Body segments Hatching Larval stage 5 (b) Development from egg to larva

  9. Adult fruit fly Fruit fly embryo (10 hours) Figure 21.17 Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse

  10. Exons (regions of genes coding for protein or giving rise to rRNA or tRNA) (1.5%) Repetitive DNA that includes transposable elements and related sequences (44%) Introns and regulatory sequences (24%) Fig. 21.7 Unique noncoding DNA (15%) L1 sequences (17%) Repetitive DNA unrelated to transposable elements (15%) Alu elements (10%) Simple sequence DNA (3%) Large-segment duplications (5–6%)

  11. How do Cells Become Specialized? • Cell Differentiation: a process where a generalized cell changes in form and function to a specialized cell (ex. neurons, RBCs) • Often triggered chemically by neighbor cells (induction) • Cell Fate: specialized function that cell acquires • Cell Potency: range of cell types that cell could acquire if exposed to different inductive environ- ments; potency always includes fate • Totipotent cells: unlimited potency • Pluripotent cells: high, but not unlimited potency • Cell Determination: when potency becomes restricted to fate; timing can vary • Heterotopic transplantation: method for testing potency and timing of cell determination

  12. Fig. 18.14 (a) Fertilized eggs of a frog (b) Newly hatched tadpole

  13. Unfertilized egg cell Sperm Nucleus Fig. 18.15 Fertilization Two different cytoplasmic determinants NUCLEUS Early embryo (32 cells) Zygote Signal transduction pathway Mitotic cell division Signal receptor Signal molecule (inducer) Two-celled embryo (b) Induction by nearby cells (a) Cytoplasmic determinants in the egg

  14. Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Fig. 18-16-3 OFF mRNA MyoD protein (transcription factor) Myoblast (determined) mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins MyoD Another transcription factor Part of a muscle fiber (fully differentiated cell)

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