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

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

Figure 16.21a

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

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

Fig Antenna Mutant Wild type Eye Leg

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

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

Fig 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%) Unique noncoding DNA (15%) Repetitive DNA unrelated to transposable elements (15%) L1 sequences (17%) Alu elements (10%) Simple sequence DNA (3%) Large-segment 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 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

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

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

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