Figure 18.3 trp operon Promoter Promoter Genes of operon DNA trpR trpE trpD trpC trpB trpA Operator Regulatory gene RNA polymerase Start codon Stop codon 3 mRNA 5 mRNA 5 E D 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 Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

Tryptophan (corepressor) Figure 18.3b-2 DNA No RNA made mRNA Protein Active repressor Figure 18.3 The trp operon in E. coli: regulated synthesis of repressible enzymes. Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off

Figure 18.4 Regulatory gene Promoter Operator DNA DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacI lacZ lacY lacA RNA polymerase Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. 3 mRNA mRNA 5 5 Protein -Galactosidase Permease Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off Figure 18.4a Regulatory gene Promoter Operator DNA DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Figure 18.4 The lac operon in E. coli: regulated synthesis of inducible enzymes. Active repressor Protein (a) Lactose absent, repressor active, operon off

RNA polymerase binds and transcribes Operator Figure 18.5 Promoter DNA lacI lacZ CAP-binding site RNA polymerase binds and transcribes Operator Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter DNA lacI lacZ Figure 18.5 Positive control of the lac operon by catabolite activator protein (CAP). CAP-binding site Operator RNA polymerase less likely to bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

Figure 18.6 Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethylation 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 Figure 18.6 Stages in gene expression that can be regulated in eukaryotic cells. 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)

Amino acids available for chemical modification Figure 18.7 Histone tails DNA double helix Amino acids available for chemical modification Nucleosome (end view) (a) Histone tails protrude outward from a nucleosome Figure 18.7 A simple model of histone tails and the effect of histone acetylation. Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription

Enhancer (distal control elements) Proximal control elements Figure 18.8-3 Enhancer (distal control elements) Proximal control elements Poly-A signal sequence Transcription start site Transcription termination region DNA Exon Intron Exon Intron Exon Upstream Downstream Promoter Transcription Poly-A signal Primary RNA transcript (pre-mRNA) Exon Intron Exon Intron Exon Cleaved 3 end of primary transcript 5 RNA processing Intron RNA Figure 18.8 A eukaryotic gene and its transcript. Coding segment mRNA G P P P AAA  AAA 3 Start codon Stop codon 5 Cap 5 UTR 3 UTR Poly-A tail

Distal control element Enhancer TATA box General transcription factors Figure 18.10-3 Promoter Activators Gene DNA Distal control element Enhancer TATA box General transcription factors DNA- bending protein Group of mediator proteins RNA polymerase II Figure 18.10 A model for the action of enhancers and transcription activators. RNA polymerase II Transcription initiation complex RNA synthesis

Albumin gene not expressed Figure 18.11 Enhancer Promoter Control elements Albumin gene Crystallin gene LIVER CELL NUCLEUS LENS CELL NUCLEUS Available activators Available activators Albumin gene not expressed Figure 18.11 Cell type–specific transcription. Albumin gene expressed Crystallin gene not expressed Crystallin gene expressed (a) Liver cell (b) Lens cell

Primary RNA transcript Figure 18.13 Exons DNA 1 2 3 4 5 Troponin T gene Primary RNA transcript 1 2 3 4 5 Figure 18.13 Alternative RNA splicing of the troponin T gene. RNA splicing mRNA 3 or 1 2 5 1 2 4 5

Proteasome and ubiquitin to be recycled Ubiquitin Figure 18.14 Proteasome and ubiquitin to be recycled Ubiquitin Proteasome Protein to be degraded Ubiquitinated protein Protein fragments (peptides) Protein entering a proteasome Figure 18.14 Degradation of a protein by a proteasome.

(a) Primary miRNA transcript miRNA miRNA- protein complex Figure 18.15 Hairpin Hydrogen bond miRNA Dicer 5 3 (a) Primary miRNA transcript miRNA miRNA- protein complex Figure 18.15 Regulation of gene expression by miRNAs. mRNA degraded Translation blocked (b) Generation and function of miRNAs

within a control element Figure 18.23 Proto-oncogene DNA Translocation or transposition: gene moved to new locus, under new controls Gene amplification: multiple copies of the gene Point mutation: within a control element within the gene New promoter Oncogene Oncogene Figure 18.23 Genetic changes that can turn proto-oncogenes into oncogenes. Normal growth- stimulating protein in excess Normal growth-stimulating protein in excess Normal growth- stimulating protein in excess Hyperactive or degradation- resistant protein

Protein that inhibits the cell cycle Figure 18.24b 2 Protein kinases MUTATION Defective or missing transcription factor, such as p53, cannot activate transcription. 3 UV light Active form of p53 1 DNA damage in genome DNA Figure 18.24 Signaling pathways that regulate cell division. Protein that inhibits the cell cycle (b) Cell cycle–inhibiting pathway