Regulation of Gene Expression

Slide 1 ______Chapter 18

Regulation of Gene ______Expression ______

PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece ______

______

Slide 2 Changes in the environment can regulate cell activity ______

– Natural selection has favored single-celled organisms that produce only the products needed by that cell – The fastest way is to directly control enzyme cascades ______– In bacteria it is nearly as fast to change gene expression. This is controlled by the operon model – one promoter, many genes ______

– Eukaryotes also have coordinately controlled gene expression – but, like everything else about them it’s more complicated (more on that later....) ______

______

Fig. 18-2 Slide 3 Precursor ______

trpE gene

Enzyme 1 Feedback trpD gene Operon: Loop: ______Gene Enzyme expression Enzyme 2 trpC gene activity in a cluster of directly functionally controlled by related genes trpB gene ______presence or can be absence of Enzyme 3 coordinated substrate by a single trpA gene on-off Tryptophan “switch” ______

______

______Slide 4 Operons: the entire stretch of DNA that includes ... ______

• The promoter which binds RNA polymerase. ______• The operator: a regulatory “switch” in the promoter that reacts to environmental change • All of the related genes that they control ______

______

Slide 5 Negative vs. Positive Operons ______

• Two Types of Negative, or “Repressor”, Gene Regulation – Operons that involve negative control of genes are switched off by ______the active form of a repressor – A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription – An inducible operon is one that is usually off; a molecule called ______an inducer inactivates the repressor and turns on transcription

• One Type of Positive, or “Activator”, Gene Regulation – An activator of transcription is a stimulatory protein that acts to ______bind RNA polymerase much like an eukaryotic transcription factor

______

Slide 6 Example 1 of Negative, or “Repressor”, Gene Regulation ______

• Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product – The trp operon is a repressible operon: By default it is on and the ______genes for tryptophan synthesis are transcribed – Tryptophan is a corepressor and when it is present, it binds to the trp repressor protein, which turns the operon off ______

______

______Slide 7 Fig. 18-3a ______

trp operon

Promoter Promoter Genes of operon ______DNA trpR trpE trpD trpC trpB trpA Regulatory Operator gene Start codon Stop codon 3′ mRNA RNA mRNA 5′ 5′ polymerase E D C B A ______Protein Inactive Polypeptide subunits that make up repressor enzymes for tryptophan synthesis ______

Tryptophan absent, repressor inactive, operon on

______

Slide 8 Fig. 18-3b-1 ______

DNA No RNA made ______

mRNA

Protein Active repressor ______Tryptophan (corepressor)

______Tryptophan present, repressor active, operon off

______

Slide 9 Fig. 18-3b-2 ______

DNA No RNA made ______

mRNA

Protein Active repressor ______Tryptophan (corepressor)

______Tryptophan present, repressor active, operon off

______

______Slide 10 ______Example 2 of Negative, or “Repressor”, Gene Regulation

• Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal ______–The lac operon is an inducible operon and by itself, the lac repressor is active and switches the lac operon off

– Lactose is an inducer and inactivates the repressor to turn the lac operon on ______

______

Slide 11 Fig. 18-4a ______Regulatory Promoter gene Operator DNA lacI lacZ ______No RNA made 3′ mRNA RNA 5′ polymerase ______

Active Protein repressor ______

Lactose absent, repressor active, operon off ______

______

Slide 12 Fig. 18-4b ______

lac operon

DNA lacI lacZlacY lacA ______

RNA polymerase 3′ mRNA mRNA 5′ 5′

β-Galactosidase Permease Transacetylase ______Protein

Allolactose Inactive (inducer) repressor ______

Lactose present, repressor inactive, operon on

______

______Slide 13 Positive Gene Regulation ______

• Positive secondary control of the speed of an operon • Sometimes lactose is present but glucose is scarce ______• Catabolite Activator Protein (CAP) is activated by binding with cyclic AMP • Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus ______accelerating transcription • When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate ______• CAP helps regulate other operons that encode enzymes used in catabolic pathways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings ______

______

Fig. 18-5 Promoter Slide 14 Operator ______

DNA lacI lacZ CAP-binding site Lactose present, RNA polymerase binds Active and transcribes glucose scarce cAMP CAP (cAMP level high): ______abundant lac mRNA Inactive Inactive lac synthesized CAP repressor

Promoter Operator ______DNA lacI lacZ Lactose present, CAP-binding site glucose present RNA polymerase less (cAMP level low): likely to bind Inactive less lac mRNA CAP Inactive lac synthesized repressor ______

______

Slide 15 Differential gene expression in multicellular ______development and cell specialization

– Regulation of Chromatin Structure – Regulation of Transcription Initiation ______– Mechanisms of Post-Transcriptional Regulation – Mechanisms of Post-Translational Regulation ______

______

______Fig. 18-6a Slide 16 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

______

Slide 17 Fig. 18-6b ______CYTOPLASM mRNA in cytoplasm

Translation Degradation of mRNA ______

Polypeptide

Protein processing ______Active protein Degradation of protein Transport to cellular destination

Cellular function ______

______

Slide 18 Regulation of Chromatin Structure ______

• Genes within highly packed heterochromatin are usually not expressed ______• Chemical modifications to histones and DNA influence chromatin structure and gene expression by making a region of DNA either more or less able to bind the transcription machinery ______• The histone code hypothesis proposes that specific combinations of modifications help determine chromatin configuration and influence transcription ______

______

______Fig. 18-7 Slide 19 In histone ______acetylation, acetyl groups Histone tails are attached to positively Amino acids The addition of ______available charged DNA for chemical methyl groups modification lysines in double helix (methylation) can histone tails condense chromatin (a) Histone tails protrude outward from a This process nucleosome ______loosens The addition of phosphate chromatin groups structure, (phosphorylation) thereby next to a Unacetylated histones Acetylated histones ______initiating methylated transcription amino acid can loosen chromatin ______

______

Slide 20 DNA Methylation ______

• DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with ______reduced transcription in some species • DNA methylation can cause long-term inactivation of genes in cellular differentiation ______• In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development ______

______

Slide 21 Epigenetic Inheritance ______

• Although the chromatin modifications just discussed do not alter DNA sequence, they may be passed to future generations of cells ______• The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic ______inheritance ______

______

______Fig. 18-8-1 Slide 22 Organization of a Typical Eukaryotic Gene ______

Poly-A signal sequence Enhancer Proximal Termination (distal control elements) control elements region ExonIntronExon Intron Exon ______DNA Upstream Downstream Promoter

______

Fig. 18-8-2 Slide 23 Organization of a Typical Eukaryotic Gene ______

Poly-A signal sequence Enhancer Proximal Termination (distal control elements) control elements region ExonIntronExon Intron Exon ______DNA Upstream Downstream Promoter Transcription

ExonIntron ExonIntron Exon Primary RNA Cleaved 3′ end transcript 5′ of primary transcript Poly-A ______signal

______

Fig. 18-8-3 Slide 24 Organization of a Typical Eukaryotic Gene ______

Poly-A signal sequence Enhancer Proximal Termination (distal control elements) control elements region ExonIntronExon Intron Exon ______DNA Upstream Downstream Promoter Transcription

ExonIntron ExonIntron Exon Primary RNA Cleaved 3′ end transcript 5′ of primary RNA processing transcript

Intron RNA Poly-A ______signal Coding segment mRNA 3′ Start Stop 5′ Cap 5′ UTR codon codon 3′ UTR Poly-A tail ______What do the GTP Cap and Poly-A Tail do again?

______

______Fig. 18-9-1 Activators Promoter Slide 25 Gene ______DNA Enhancer Distal control TATA element box ______

______

Fig. 18-9-2 Activators Promoter Slide 26 Gene ______DNA Enhancer Distal control TATA element box General transcription factors ______DNA-bending protein Tissue-specific transcription factors ______

______

Fig. 18-9-3 Activators Promoter Slide 27 Gene ______DNA Enhancer Distal control TATA element box General transcription factors ______DNA-bending protein Tissue-specific transcription factors

RNA ______polymerase II

RNA polymerase II ______

Transcription initiation complex RNA synthesis

______

______Fig. 18-10 Slide 28 Enhancer Promoter ______A particular Control Albumin gene combination elements of control elements can Crystallin gene activate transcription LIVER CELL LENS CELL ______only when the NUCLEUS NUCLEUS appropriate Av ailable Unlike activator proteins Available proteins prokaryotes, proteins and coordinately transcription controlled factors are eukaryotic present ______Albumin gene genes can be not expressed Albumin gene scattered over expressed different chromosomes, but each has Crystallin gene the same ______not expressed Crystallin gene combination of expressed control (a) Liver cell (b) Lens cell elements

______

Fig. 18-11 Slide 29 Exons ______

DNA ______Troponin T gene

Primary ______RNA transcript RNA splicing ______mRNA or

______

Slide 30 Fig. 18-12 ______Proteasomes are giant protein complexes that bind protein molecules and degrade them

Proteasome Ubiquitin and ubiquitin ______Proteasome to be recycled

Protein to Ubiquitinated Protein ______be degraded protein fragments Protein entering a (peptides) proteasome

After translation, various types of protein processing, ______including cleavage and the addition of chemical groups, are subject to control

______

______Noncoding RNAs can control gene expression Slide 31 through RNA Interference (RNAi). ______

• Only a small fraction of DNA codes for proteins, rRNA, and tRNA. A significant amount of the genome may be transcribed into noncoding RNAs ______• MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA and degrade it or block its translation • siRNAs and miRNAs are similar but form from different ______RNA precursors • siRNAs play a role in heterochromatin formation and can block large regions of the chromosome ______• Small RNAs may also block transcription of specific genes

______

Fig. 18-UN5 Slide 32 Chromatin modification ______• Small RNAs can promote the formation of Chromatin modification heterochromatin in certain regions, blocking transcription.

Transcription Translation ______RNA processing • miRNA or siRNA can block the translation of specific mRNAs.

mRNA Translation degradation

Protein processing and degradation ______

mRNA degradation

• miRNA or siRNA can target specific mRNAs ______for destruction.

______

Fig. 18-13 Hairpin Slide 33 miRNA Hydrogen ______bond

Dicer ______

miRNA miRNA- 5′ 3′ protein (a) Primary miRNA transcript complex ______

______mRNA degraded Translation blocked (b) Generation and function of miRNAs

______

______Slide 34 Fig. 18‐14 ______The differentiation of cell types in a multicellular organism results from orchestrated changes in gene expression ______

______

______Fertilized eggs are single cells This tadpole has neurons, muscle, gills, fins, etc.

______

Fig. 18‐15a Slide 35 Unfertilized egg cell ______An egg’s cytoplasm Sperm Nucleus contains RNA, proteins, and Fertilization other Two different ______cytoplasmic substances determinants that are distributed unevenly in Zygote the ______unfertilized Mitotic egg cell division

Two‐celled embryos ______have two different cell types

______

Fig. 18‐15b In the process called induction, Slide 36 signal molecules from nearby cells ______cause transcriptional changes in embryonic target cells

Early embryo NUCLEUS ______(32 cells)

Signal transduction pathway ______Signal receptor

Signal molecule (inducer) ______

______

______Fig. 18‐17b Follicle cell 1 Egg cell Nucleus Slide 37 developing within ______ovarian follicle Egg cell Nurse cell

2 Unfertilized egg Egg Ovary cells shell Depleted ______can tell the nurse cells Fertilization unfertilized Laying of egg

egg which 3 Fertilized egg end is which Embryonic ______development

4 Segmented embryo 0.1 mm Body Hatching segments ______5 Larval stage

______

Slide 38 Fig. 18‐19c ______Bicoid: Nurse cells help localize the mRNA for a protein that is required to make head structures ______

Nurse cells Egg

bicoid mRNA ______Developing egg Bicoid mRNA in mature Bicoid protein unfertilized egg in early embryo ______

______

Fig. 18‐19a Slide 39 EXPERIMENT ______Tail

Head ______T1 T2 A8 T3 A7 A6 A1 A2 A3 A4 A5 Wild‐type larva Tail Tail ______

A8 ______A8 A7 A6 A7 Mutant larva (bicoid)

______

______Fig. 18‐16‐1 Nucleus Slide 40 Master regulatory gene myoD Other muscle‐specific genes ______DNA Embryonic precursor cell OFF OFF ______

______

Fig. 18‐16‐2 Nucleus Slide 41 Master regulatory gene myoD Other muscle‐specific genes ______DNA Embryonic precursor cell OFF OFF

mRNA OFF ______

MyoD protein Myoblast (transcription (determined) factor) ______

______

Fig. 18‐16‐3 Nucleus Slide 42 Master regulatory gene myoD Other muscle‐specific genes ______DNA Embryonic precursor cell OFF OFF

mRNA OFF ______

MyoD protein Myoblast (transcription (determined) factor) ______

mRNA mRNA mRNA mRNA

Myosin, other ______muscle proteins, MyoD Another and cell cycle– Part of a muscle fiber transcription blocking proteins (fully differentiated cell) factor

______