regulation of gene expression - csus.edu · lectures by chris romero, updated by erin barley with...

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

PowerPoint® Lecture Presentations for

BiologyEighth Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Chapter 18

Regulation of Gene Expression

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Slide 2

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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....)

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Slide 3 Fig. 18-2

trpE gene

trpD gene

trpC gene

trpB gene

trpA gene

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

Precursor

FeedbackLoop:

Enzyme activitydirectly controlled by presence or absence of substrate

Operon:

Gene expression in a cluster of functionally related genes can be coordinated by a single on-off “switch”

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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

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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

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Slide 6


• 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

Example 1 of Negative, or “Repressor”, Gene Regulation

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Fig. 18-3a

Polypeptide subunits that make upenzymes for tryptophan synthesis

Tryptophan absent, repressor inactive, operon on

DNA

mRNA 5′

Protein Inactiverepressor

RNApolymerase

Regulatorygene

Promoter Promoter

trp operon

Genes of operon

OperatorStop codonStart codon

mRNA

trpA

5′

3′

trpR trpE trpD trpC trpB

ABCDE

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Slide 8 Fig. 18-3b-1

Tryptophan present, repressor active, operon off

Tryptophan(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

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Slide 9 Fig. 18-3b-2

Tryptophan present, repressor active, operon off

Tryptophan(corepressor)

No RNA made

Activerepressor

mRNA

Protein

DNA

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• 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

Example 2 of Negative, or “Repressor”, Gene Regulation

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Slide 11 Fig. 18-4a

Lactose absent, repressor active, operon off

DNA

Protein Activerepressor

RNApolymerase

Regulatorygene

Promoter

Operator

mRNA5′

3′

NoRNAmade

lacI lacZ

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Slide 12 Fig. 18-4b

Lactose present, repressor inactive, operon on

mRNA

Protein

DNA

mRNA 5′

Inactiverepressor

Allolactose(inducer)

5′

3′RNApolymerase

Permease Transacetylase

lac operon

β-Galactosidase

lacYlacZ lacAlacI

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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 lacoperon, and transcription returns to a normal rate

• CAP helps regulate other operons that encode enzymes used in catabolic pathways

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Slide 14 Fig. 18-5

Lactose present, glucose present (cAMP level low):

less lac mRNA synthesized

cAMP

DNA

Inactive lacrepressorInactive

CAP

lacICAP-binding site

Promoter

ActiveCAP

Operator

lacZ

RNA polymerase bindsand transcribes

Inactive lacrepressor

lacZ

OperatorPromoter

DNA

CAP-binding site

lacI

RNA polymerase lesslikely to bind

InactiveCAP

Lactose present, glucose scarce (cAMP level high):

abundant lac mRNA synthesized

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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

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Fig. 18-6a

DNA

Signal

Gene

NUCLEUS

Chromatin modification

Chromatin

Gene availablefor transcription

Exon

Intron

Tail

RNA

Cap

RNA processing

Primary transcript

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

Transcription

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Slide 17 Fig. 18-6b

mRNA in cytoplasm

Translation

CYTOPLASM

Degradationof mRNA

Protein processing

Polypeptide

Active protein

Cellular function

Transport to cellulardestination

Degradationof protein

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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

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Fig. 18-7

Histonetails

DNAdouble helix

(a) Histone tails protrude outward from anucleosome

Acetylated histones

Aminoacidsavailablefor chemicalmodification

Unacetylated histones

In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails

This process loosens chromatin structure, thereby initiating transcription

The addition of methyl groups (methylation) can condense chromatin

The addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin

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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

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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

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Fig. 18-8-1

Enhancer(distal control elements)

Proximalcontrol elements

Poly-A signalsequence

Terminationregion

DownstreamPromoterUpstreamDNA

ExonExon ExonIntron Intron

Organization of a Typical Eukaryotic Gene

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Slide 23 Fig. 18-8-2




Terminationregion


Exon Exon ExonIntronIntron Cleaved 3′ endof primarytranscript

Primary RNAtranscript

Poly-Asignal

Transcription

5′



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Terminationregion



Exon Exon ExonIntronIntron Cleaved 3′ endof primarytranscript

Primary RNAtranscript

Poly-Asignal

Transcription

5′RNA processing

Intron RNA

Coding segmentmRNA

5′ Cap 5′ UTRStart

codonStop

codon 3′ UTR Poly-Atail

3′


What do the GTP Cap and Poly-A Tail do again?

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Fig. 18-9-1

Enhancer TATAbox

PromoterActivatorsDNA

Gene

Distal controlelement

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Enhancer TATAbox


Gene


Tissue-specifictranscription factors

DNA-bendingprotein

Generaltranscriptionfactors

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Enhancer TATAbox


Gene


Tissue-specifictranscription factors

DNA-bendingprotein

Generaltranscriptionfactors

RNApolymerase II

RNApolymerase II

Transcriptioninitiation complex RNA synthesis

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Fig. 18-10

Controlelements

Enhancer

Availableproteins

Albumin gene

(b) Lens cell

Crystallin geneexpressed

Availableproteins

LENS CELLNUCLEUS

LIVER CELLNUCLEUS

Crystallin gene

Promoter

(a) Liver cell

Crystallin genenot expressed

Albumin geneexpressed

Albumin genenot expressed

A particular combination of control elements can activate transcription only when the appropriate activator proteins and transcription factors are present

Unlike prokaryotes, coordinately controlled eukaryotic genes can be scattered over different chromosomes, but each has the same combination of control elements

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Slide 29 Fig. 18-11

or

RNA splicing

mRNA

PrimaryRNAtranscript

Troponin T gene

Exons

DNA

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Slide 30 Fig. 18-12

Proteasomeand ubiquitinto be recycledProteasome

Proteinfragments(peptides)Protein entering a

proteasome

Ubiquitinatedprotein

Protein tobe degraded

Ubiquitin

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

Proteasomes are giant protein complexes that bind protein molecules and degrade them

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Noncoding RNAs can control gene expression 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

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Slide 32 Fig. 18-UN5


RNA processing

TranslationmRNAdegradation

Protein processingand degradation

mRNA degradation

• miRNA or siRNA can target specific mRNAsfor destruction.

• miRNA or siRNA can block the translationof specific mRNAs.

Transcription

• Small RNAs can promote the formation ofheterochromatin in certain regions, blocking transcription.


Translation

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Slide 33 Fig. 18-13

miRNA-proteincomplex(a) Primary miRNA transcript

Translation blocked

Hydrogenbond

(b) Generation and function of miRNAs

Hairpin miRNA

miRNA

Dicer

3′

mRNA degraded

5′

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Fig. 18‐14

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

The differentiation of cell types in a multicellular organism results from orchestrated changes in gene expression

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Slide 35 Fig. 18‐15a

Two differentcytoplasmicdeterminants

Unfertilized egg cell

Sperm

Fertilization

Zygote

Mitoticcell division

Two‐celledembryoshave twodifferentcell types

Nucleus

An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg

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Slide 36 Fig. 18‐15b

Signalmolecule(inducer)

Signaltransductionpathway

Early embryo(32 cells)

NUCLEUS

Signalreceptor

In the process called induction, signal molecules from nearby cells cause transcriptional changes in embryonic target cells

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Fig. 18‐17b

Follicle cellNucleus

Eggcell

Nurse cell

Egg celldeveloping withinovarian follicle

Unfertilized egg

Fertilized egg

Depletednurse cells

Eggshell

FertilizationLaying of egg

Bodysegments

Embryonicdevelopment

Hatching

0.1 mm

Segmentedembryo

Larval stage

1

2

3

4

5

Ovary cells can tell the unfertilized egg which end is which

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Slide 38 Fig. 18‐19c

bicoid mRNA

Nurse cells

Egg

Developing egg Bicoid mRNA in matureunfertilized egg

Bicoid proteinin early embryo

Bicoid: Nurse cells help localize the mRNA for a protein that is required to make head structures

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Slide 39 Fig. 18‐19a

T1 T2T3

A1 A2 A3 A4 A5 A6A7

A8

A8A7 A6 A7

Tail

TailTail

Head

Wild‐type larva

Mutant larva (bicoid)

EXPERIMENT

A8

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Fig. 18‐16‐1

Embryonicprecursor cell

Nucleus

OFF

DNA

Master regulatory gene myoD Other muscle‐specific genes

OFF

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Slide 41 Fig. 18‐16‐2


Nucleus

OFF

DNA


OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Myoblast(determined)

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Slide 42 Fig. 18‐16‐3


Nucleus

OFF

DNA


OFF

OFFmRNA

MyoD protein(transcriptionfactor)

Myoblast(determined)

mRNA mRNA mRNA mRNA

Myosin, othermuscle proteins,and cell cycle–blocking proteinsPart of a muscle fiber

(fully differentiated cell)

MyoD Anothertranscriptionfactor

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regulation of gene expression - csus.edu · lectures by chris romero, updated by erin barley with...

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