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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 Fig. 18.6 Signal NUCLEUS Chromatin Chromatin modification DNA Gene available for transcription Gene Transcription RNA Exon Primary transcript Intron RNA processing Tail Cap mRNA in nucleus Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Polypeptide Protein processing Active protein Degradation of protein Transport to cellular destination Cellular function Precursor Fig. 18.2 Feedback inhibition trpE gene Enzyme 1 trpD gene Regulation of gene expression Enzyme 2 trpC gene trpB gene Enzyme 3 trpA gene Tryptophan (a) Regulation of enzyme activity (b) Regulation of enzyme production Figure 16.21a trp operon Promoter Promoter Genes of operon DNA trpR Regulatory gene mRNA 5 Protein trpE 3 Operator Start codon mRNA 5 RNA polymerase Inactive repressor E trpD trpC trpB trpA B A Stop codon D C Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on DNA No RNA made mRNA Active repressor Protein Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off Fig. 18.3 Regulatory gene Promoter Operator Fig. 18.4 lacZ lacI DNA No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacZ lacY -Galactosidase Permease lacI 3 mRNA 5 RNA polymerase mRNA 5 Protein Allolactose (inducer) lacA Inactive repressor (b) Lactose present, repressor inactive, operon on Transacetylase Fig. 18.18 Eye Leg Antenna Wild type Mutant Head Thorax Abdomen Fig. 18.17 0.5 mm Dorsal BODY Anterior AXES Left Right Posterior Ventral (a) Adult Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell Egg shell 2 Unfertilized egg Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Embryonic development 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva Figure 21.17 Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome Mouse chromosomes Mouse embryo (12 days) Adult mouse Exons (regions of genes coding for protein or giving rise to rRNA or tRNA) (1.5%) Fig. 21.7 Repetitive DNA that includes transposable elements and related sequences (44%) L1 sequences (17%) Introns and regulatory sequences (24%) Unique noncoding DNA (15%) Repetitive DNA unrelated to transposable elements (15%) Alu elements (10%) Simple sequence Large-segment DNA (3%) duplications (5–6%) 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 environments; 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 Fig. 18.14 (a) Fertilized eggs of a frog (b) Newly hatched tadpole Fig. 18.15 Unfertilized egg cell Sperm Fertilization Nucleus Two different cytoplasmic determinants Zygote Mitotic cell division Two-celled embryo (a) Cytoplasmic determinants in the egg Early embryo (32 cells) Signal transduction pathway Signal receptor Signal molecule (inducer) (b) Induction by nearby cells NUCLEUS Fig. 18-16-3 Nucleus Master regulatory gene myoD Embryonic precursor cell Other muscle-specific genes DNA Myoblast (determined) OFF OFF mRNA OFF MyoD protein (transcription factor) mRNA MyoD Part of a muscle fiber (fully differentiated cell) mRNA Another transcription factor mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins