regulation of gene expression -...

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LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION

Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

© 2011 Pearson Education, Inc.

Lectures by Erin Barley

Kathleen Fitzpatrick

Regulation of Gene Expression

Chapter 18 Overview: Conducting the Genetic Orchestra

•  Prokaryotes and eukaryotes alter gene expression in response to their changing environment

•  In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types

•  RNA molecules play many roles in regulating gene expression in eukaryotes


Figure 18.1

Concept 18.1: Bacteria often respond to environmental change by regulating transcription

•  Natural selection has favored bacteria that produce only the products needed by that cell

•  A cell can regulate the production of enzymes by feedback inhibition or by gene regulation

•  Gene expression in bacteria is controlled by the operon model


Precursor Feedback inhibition

Enzyme 1

Enzyme 2

Enzyme 3

Tryptophan

(a) (b) Regulation of enzyme activity

Regulation of enzyme production

Regulation of gene expression

-

-

trpE gene

trpD gene

trpC gene

trpB gene

trpA gene

Figure 18.2

Operons: The Basic Concept

•  A cluster of functionally related genes can be under coordinated control by a single “on-off switch”

•  The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter

•  An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control


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•  The operon can be switched off by a protein repressor

•  The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase

•  The repressor is the product of a separate regulatory gene


•  The repressor can be in an active or inactive form, depending on the presence of other molecules

•  A corepressor is a molecule that cooperates with a repressor protein to switch an operon off

•  For example, E. coli can synthesize the amino acid tryptophan


•  By default the trp operon is on and the genes for tryptophan synthesis are transcribed

•  When tryptophan is present, it binds to the trp repressor protein, which turns the operon off

•  The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high


Promoter

DNA

Regulatory gene

mRNA

trpR

5ʹ′

3ʹ′

Protein Inactive repressor

RNA polymerase

Promoter

trp operon

Genes of operon

Operator

mRNA 5ʹ′

Start codon Stop codon

trpE trpD trpC trpB trpA

E D C B A

Polypeptide subunits that make up enzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

(b) Tryptophan present, repressor active, operon off

DNA

mRNA

Protein

Tryptophan (corepressor)

Active repressor

No RNA made

Figure 18.3

Figure 18.3a

Promoter

DNA

Regulatory gene mRNA

trpR

5ʹ′

3ʹ′

Protein Inactive repressor

RNA polymerase

Promoter

trp operon

Genes of operon

Operator

mRNA 5ʹ′

Start codon Stop codon

trpE trpD trpC trpB trpA

E D C B A

Polypeptide subunits that make up enzymes for tryptophan synthesis

(a) Tryptophan absent, repressor inactive, operon on

Figure 18.3b-1


DNA

mRNA

Protein


Active repressor

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Figure 18.3b-2


DNA

mRNA

Protein


Active repressor

No RNA made

Repressible and Inducible Operons: Two Types of Negative Gene Regulation

•  A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription

•  The trp operon is a repressible operon •  An inducible operon is one that is usually off; a

molecule called an inducer inactivates the repressor and turns on transcription


•  The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose

•  By itself, the lac repressor is active and switches the lac operon off

•  A molecule called an inducer inactivates the repressor to turn the lac operon on


(a) Lactose absent, repressor active, operon off

(b) Lactose present, repressor inactive, operon on

Regulatory gene

Promoter Operator

DNA lacZ lacI

lacI

DNA

mRNA 5ʹ′

3ʹ′

No RNA made

RNA polymerase

Active repressor Protein

lac operon

lacZ lacY lacA DNA

mRNA 5ʹ′

3ʹ′

Protein

mRNA 5ʹ′

Inactive repressor

RNA polymerase

Allolactose (inducer)

β-Galactosidase Permease Transacetylase

Figure 18.4

Figure 18.4a

(a) Lactose absent, repressor active, operon off

Regulatory gene

Promoter Operator

DNA lacZ lacI DNA

mRNA 5ʹ′

3ʹ′

No RNA made

RNA polymerase

Active repressor Protein

Figure 18.4b

(b) Lactose present, repressor inactive, operon on

lacI

lac operon

lacZ lacY lacA DNA

mRNA 5ʹ′

3ʹ′

Protein

mRNA 5ʹ′

Inactive repressor

RNA polymerase

Allolactose (inducer)

β-Galactosidase Permease Transacetylase

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•  Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal

•  Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product

•  Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor


Positive Gene Regulation

•  Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription

•  When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP (cAMP)

•  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


Figure 18.5 Promoter

DNA

CAP-binding site

lacZ lacI

RNA polymerase binds and transcribes

Operator

cAMP Active CAP

Inactive CAP

Allolactose

Inactive lac repressor

(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

Promoter

DNA

CAP-binding site

lacZ lacI

Operator RNA polymerase less likely to bind


Inactive CAP

(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

Figure 18.5a

Promoter

DNA

CAP-binding site

lacZ lacI

RNA polymerase binds and transcribes

Operator

cAMP Active CAP

Inactive CAP

Allolactose


(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

Figure 18.5b

Promoter

DNA

CAP-binding site

lacZ lacI

Operator RNA polymerase less likely to bind


Inactive CAP

(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

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Concept 18.2: Eukaryotic gene expression is regulated at many stages

•  All organisms must regulate which genes are expressed at any given time

•  In multicellular organisms regulation of gene expression is essential for cell specialization


Differential Gene Expression

•  Almost all the cells in an organism are genetically identical

•  Differences between cell types result from differential gene expression, the expression of different genes by cells with the same genome

•  Abnormalities in gene expression can lead to diseases including cancer

•  Gene expression is regulated at many stages


Figure 18.6 Signal

NUCLEUS Chromatin

Chromatin modification: DNA unpacking involving histone acetylation and

DNA demethylation DNA

Gene

Gene available for transcription

RNA Exon Primary transcript

Transcription

Intron RNA processing

Cap Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM mRNA in cytoplasm

Translation Degradation of mRNA

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)

Figure 18.6a Signal

NUCLEUS Chromatin

Chromatin modification: DNA unpacking involving histone acetylation and

DNA demethylation DNA

Gene

Gene available for transcription

RNA Exon Primary transcript

Transcription

Intron RNA processing

Cap Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

Figure 18.6b CYTOPLASM

mRNA in cytoplasm

Translation Degradation of mRNA

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)

Regulation of Chromatin Structure

•  Genes within highly packed heterochromatin are usually not expressed

•  Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression


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Histone Modifications

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

•  This loosens chromatin structure, thereby promoting the initiation of 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


Animation: DNA Packing

Figure 18.7

Amino acids available for chemical modification

Histone tails

DNA double helix

Nucleosome (end view)

(a) Histone tails protrude outward from a nucleosome

Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin

structure that permits transcription

•  The histone code hypothesis proposes that specific combinations of modifications, as well as the order in which they occur, help determine chromatin configuration and influence transcription


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


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


Regulation of Transcription Initiation

•  Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery


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Organization of a Typical Eukaryotic Gene

•  Associated with most eukaryotic genes are multiple control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription

•  Control elements and the transcription factors they bind are critical to the precise regulation of gene expression in different cell types


Figure 18.8-1

Enhancer (distal control

elements) DNA

Upstream Promoter

Proximal control

elements Transcription

start site Exon Intron Exon Exon Intron

Poly-A signal

sequence Transcription termination

region

Downstream

Figure 18.8-2


elements) DNA

Upstream Promoter

Proximal control



Poly-A signal


region

Downstream Poly-A signal

Exon Intron Exon Exon Intron

Transcription

Cleaved 3ʹ′ end of primary transcript

5ʹ′ Primary RNA transcript (pre-mRNA)

Figure 18.8-3


elements) DNA

Upstream Promoter

Proximal control



Poly-A signal


region

Downstream Poly-A signal

Exon Intron Exon Exon Intron

Transcription

Cleaved 3ʹ′ end of primary transcript

5ʹ′ Primary RNA transcript (pre-mRNA)

Intron RNA

RNA processing

mRNA

Coding segment

5ʹ′ Cap 5ʹ′ UTR Start

codon Stop

codon 3ʹ′ UTR

3ʹ′

Poly-A tail

P P P G AAA ⋅⋅⋅ AAA

The Roles of Transcription Factors

•  To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors

•  General transcription factors are essential for the transcription of all protein-coding genes

•  In eukaryotes, high levels of transcription of particular genes depend on control elements interacting with specific transcription factors


•  Proximal control elements are located close to the promoter

•  Distal control elements, groupings of which are called enhancers, may be far away from a gene or even located in an intron

Enhancers and Specific Transcription Factors


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•  An activator is a protein that binds to an enhancer and stimulates transcription of a gene

•  Activators have two domains, one that binds DNA and a second that activates transcription

•  Bound activators facilitate a sequence of protein-protein interactions that result in transcription of a given gene


Animation: Initiation of Transcription

Figure 18.9

DNA

Activation domain

DNA-binding domain

•  Some transcription factors function as repressors, inhibiting expression of a particular gene by a variety of methods

•  Some activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription


Activators DNA

Enhancer Distal control element

Promoter Gene

TATA box

Figure 18.10-1

Activators DNA


Promoter Gene

TATA box General transcription factors

DNA- bending protein

Group of mediator proteins

Figure 18.10-2

Activators DNA


Promoter Gene

TATA box General transcription factors

DNA- bending protein

Group of mediator proteins

RNA polymerase II

RNA polymerase II

RNA synthesis Transcription initiation complex

Figure 18.10-3

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

Combinatorial Control of Gene Activation


Figure 18.11

Control elements

Enhancer Promoter

Albumin gene

Crystallin gene

LIVER CELL NUCLEUS

Available activators

Albumin gene expressed

Crystallin gene not expressed

(a) Liver cell

LENS CELL NUCLEUS


Albumin gene not expressed

Crystallin gene expressed

(b) Lens cell

Control elements

Enhancer Promoter

Albumin gene

Crystallin gene

LIVER CELL NUCLEUS


Albumin gene expressed

Crystallin gene not expressed

(a) Liver cell

Figure 18.11a

Control elements

Enhancer Promoter

Albumin gene

Crystallin gene

LENS CELL NUCLEUS


Albumin gene not expressed

Crystallin gene expressed

(b) Lens cell

Figure 18.11b

Coordinately Controlled Genes in Eukaryotes

•  Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter and control elements

•  These genes can be scattered over different chromosomes, but each has the same combination of control elements

•  Copies of the activators recognize specific control elements and promote simultaneous transcription of the genes


Nuclear Architecture and Gene Expression

•  Loops of chromatin extend from individual chromosomes into specific sites in the nucleus

•  Loops from different chromosomes may congregate at particular sites, some of which are rich in transcription factors and RNA polymerases

•  These may be areas specialized for a common function


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Figure 18.12

Chromosome territory

Chromosomes in the interphase nucleus

Chromatin loop

Transcription factory

10 µm

Figure 18.12a

Chromosomes in the interphase nucleus

10 µm

Mechanisms of Post-Transcriptional Regulation

•  Transcription alone does not account for gene expression

•  Regulatory mechanisms can operate at various stages after transcription

•  Such mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental changes


RNA Processing

•  In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns


Animation: RNA Processing

Exons

DNA

Troponin T gene

Primary RNA transcript

RNA splicing

or mRNA

1

1

1 1

2

2

2 2

3

3

3

4

4

4

5

5

5 5

Figure 18.13

mRNA Degradation

•  The life span of mRNA molecules in the cytoplasm is a key to determining protein synthesis

•  Eukaryotic mRNA is more long lived than prokaryotic mRNA

•  Nucleotide sequences that influence the lifespan of mRNA in eukaryotes reside in the untranslated region (UTR) at the 3ʹ′ end of the molecule


Animation: mRNA Degradation

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Initiation of Translation

•  The initiation of translation of selected mRNAs can be blocked by regulatory proteins that bind to sequences or structures of the mRNA

•  Alternatively, translation of all mRNAs in a cell may be regulated simultaneously

•  For example, translation initiation factors are simultaneously activated in an egg following fertilization


Animation: Blocking Translation

Protein Processing and Degradation

•  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


Animation: Protein Processing

Animation: Protein Degradation

Figure 18.14

Protein to be degraded

Ubiquitin

Ubiquitinated protein

Proteasome

Protein entering a proteasome

Proteasome and ubiquitin to be recycled

Protein fragments (peptides)

Concept 18.3: Noncoding RNAs play multiple roles in controlling gene expression

•  Only a small fraction of DNA codes for proteins, and a very small fraction of the non-protein-coding DNA consists of genes for RNA such as rRNA and tRNA

•  A significant amount of the genome may be transcribed into noncoding RNAs (ncRNAs)

•  Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration


Effects on mRNAs by MicroRNAs and Small Interfering RNAs

•  MicroRNAs (miRNAs) are small single-stranded RNA molecules that can bind to mRNA

•  These can degrade mRNA or block its translation


(a) Primary miRNA transcript

Hairpin miRNA

miRNA

Hydrogen bond

Dicer

miRNA- protein complex

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

5ʹ′ 3ʹ′

Figure 18.15

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•  The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi)

•  RNAi is caused by small interfering RNAs (siRNAs)

•  siRNAs and miRNAs are similar but form from different RNA precursors


Chromatin Remodeling and Effects on Transcription by ncRNAs

•  In some yeasts siRNAs play a role in heterochromatin formation and can block large regions of the chromosome

•  Small ncRNAs called piwi-associated RNAs (piRNAs) induce heterochromatin, blocking the expression of parasitic DNA elements in the genome, known as transposons

•  RNA-based mechanisms may also block transcription of single genes


The Evolutionary Significance of Small ncRNAs

•  Small ncRNAs can regulate gene expression at multiple steps

•  An increase in the number of miRNAs in a species may have allowed morphological complexity to increase over evolutionary time

•  siRNAs may have evolved first, followed by miRNAs and later piRNAs


Concept 18.4: A program of differential gene expression leads to the different cell types in a multicellular organism

•  During embryonic development, a fertilized egg gives rise to many different cell types

•  Cell types are organized successively into tissues, organs, organ systems, and the whole organism

•  Gene expression orchestrates the developmental programs of animals


A Genetic Program for Embryonic Development

•  The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis


Figure 18.16

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

1 mm 2 mm

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Figure 18.16a

(a) Fertilized eggs of a frog

1 mm

Figure 18.16b

(b) Newly hatched tadpole

2 mm

•  Cell differentiation is the process by which cells become specialized in structure and function

•  The physical processes that give an organism its shape constitute morphogenesis

•  Differential gene expression results from genes being regulated differently in each cell type

•  Materials in the egg can set up gene regulation that is carried out as cells divide


Cytoplasmic Determinants and Inductive Signals

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

•  Cytoplasmic determinants are maternal substances in the egg that influence early development

•  As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression


Figure 18.17

(a) Cytoplasmic determinants in the egg (b) Induction by nearby cells

Unfertilized egg

Sperm

Fertilization

Zygote (fertilized egg)

Mitotic cell division

Two-celled embryo

Nucleus

Molecules of two different cytoplasmic determinants

Early embryo (32 cells)

NUCLEUS

Signal transduction pathway

Signal receptor

Signaling molecule (inducer)

Figure 18.17a (a) Cytoplasmic determinants in the egg

Unfertilized egg

Sperm

Fertilization

Zygote (fertilized egg)

Mitotic cell division

Two-celled embryo

Nucleus

Molecules of two different cytoplasmic determinants

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•  The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells

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

•  Thus, interactions between cells induce differentiation of specialized cell types


Animation: Cell Signaling

Figure 18.17b (b) Induction by nearby cells

Early embryo (32 cells)

NUCLEUS

Signal transduction pathway

Signal receptor

Signaling molecule (inducer)

Sequential Regulation of Gene Expression During Cellular Differentiation

•  Determination commits a cell to its final fate •  Determination precedes differentiation •  Cell differentiation is marked by the production of

tissue-specific proteins


•  Myoblasts produce muscle-specific proteins and form skeletal muscle cells

•  MyoD is one of several “master regulatory genes” that produce proteins that commit the cell to becoming skeletal muscle

•  The MyoD protein is a transcription factor that binds to enhancers of various target genes


Nucleus

Embryonic precursor cell

DNA

Master regulatory gene myoD

OFF OFF

Other muscle-specific genes

Figure 18.18-1 Nucleus


Myoblast (determined)

DNA


OFF OFF

OFF mRNA


MyoD protein (transcription factor)

Figure 18.18-2

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Nucleus


Myoblast (determined)

Part of a muscle fiber (fully differentiated cell)

DNA


OFF OFF

OFF mRNA


MyoD protein (transcription factor)

mRNA mRNA mRNA mRNA

MyoD Another transcription factor

Myosin, other muscle proteins, and cell cycle– blocking proteins

Figure 18.18-3

Pattern Formation: Setting Up the Body Plan

•  Pattern formation is the development of a spatial organization of tissues and organs

•  In animals, pattern formation begins with the establishment of the major axes

•  Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells


•  Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster

•  Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans


The Life Cycle of Drosophila

•  In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization

•  After fertilization, the embryo develops into a segmented larva with three larval stages


Head Thorax Abdomen

0.5 mm

BODY AXES

Anterior Left

Ventral

Dorsal Right

Posterior

(a) Adult

Egg developing within ovarian follicle

Follicle cell Nucleus

Nurse cell

Egg

Unfertilized egg Depleted nurse cells

Egg shell

Fertilization

Laying of egg

Fertilized egg

Embryonic development

Segmented embryo

Body segments

Hatching

0.1 mm

Larval stage

(b) Development from egg to larva

2

1

3

4

5

Figure 18.19 Figure 18.19a

Head Thorax Abdomen

0.5 mm

BODY AXES

Anterior Left

Ventral

Dorsal Right

Posterior

(a) Adult

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Figure 18.19b

Egg developing within ovarian follicle

Follicle cell Nucleus

Nurse cell

Egg

Unfertilized egg Depleted nurse cells

Egg shell

Fertilization Laying of egg

Fertilized egg

Embryonic development

Segmented embryo

Body segments Hatching 0.1 mm

Larval stage

(b) Development from egg to larva

5

4

3

2

1

Genetic Analysis of Early Development: Scientific Inquiry

•  Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus won a Nobel Prize in 1995 for decoding pattern formation in Drosophila

•  Lewis discovered the homeotic genes, which control pattern formation in late embryo, larva, and adult stages


Figure 18.20

Wild type Mutant

Eye

Antenna Leg

Figure 18.20a

Wild type

Eye

Antenna

Figure 18.20b

Mutant

Leg

•  Nüsslein-Volhard and Wieschaus studied segment formation

•  They created mutants, conducted breeding experiments, and looked for corresponding genes

•  Many of the identified mutations were embryonic lethals, causing death during embryogenesis

•  They found 120 genes essential for normal segmentation


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Axis Establishment

•  Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila

•  These maternal effect genes are also called egg-polarity genes because they control orientation of the egg and consequently the fly


Animation: Development of Head-Tail Axis in Fruit Flies

•  One maternal effect gene, the bicoid gene, affects the front half of the body

•  An embryo whose mother has no functional bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends

Bicoid: A Morphogen Determining Head Structures


Figure 18.21

Head Tail

Tail Tail

Wild-type larva

Mutant larva (bicoid)

250 µm

T1 T2 T3 A1 A2 A3 A4 A5 A6

A7 A8

A8 A7 A6 A7

A8

Figure 18.21a

Head Tail

Wild-type larva 250 µm

T1 T2 T3 A1 A2 A3 A4 A5 A6

A7 A8

Figure 18.21b

Tail Tail

Mutant larva (bicoid)

A8 A7 A6 A7

A8

•  This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end

•  This hypothesis is an example of the morphogen gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features


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Figure 18.22

Bicoid mRNA in mature unfertilized egg


Fertilization, translation of bicoid mRNA

Anterior end 100 µm

Bicoid protein in early embryo


RESULTS

Figure 18.22a


Figure 18.22b


Anterior end 100 µm

•  The bicoid research is important for three reasons – It identified a specific protein required for some

early steps in pattern formation – It increased understanding of the mother’s role in

embryo development – It demonstrated a key developmental principle that

a gradient of molecules can determine polarity and position in the embryo


Concept 18.5: Cancer results from genetic changes that affect cell cycle control

•  The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development


Types of Genes Associated with Cancer

•  Cancer can be caused by mutations to genes that regulate cell growth and division

•  Tumor viruses can cause cancer in animals including humans


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•  Oncogenes are cancer-causing genes •  Proto-oncogenes are the corresponding normal

cellular genes that are responsible for normal cell growth and division

•  Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle


Figure 18.23

Proto-oncogene

DNA

Translocation or transposition: gene moved to new locus, under new controls

Gene amplification: multiple copies of the gene

New promoter

Normal growth- stimulating protein in excess

Normal growth-stimulating protein in excess

Point mutation: within a control

element within

the gene

Oncogene Oncogene

Normal growth- stimulating protein in excess

Hyperactive or degradation- resistant protein

•  Proto-oncogenes can be converted to oncogenes by

– Movement of DNA within the genome: if it ends up near an active promoter, transcription may increase

– Amplification of a proto-oncogene: increases the number of copies of the gene

– Point mutations in the proto-oncogene or its control elements: cause an increase in gene expression


Tumor-Suppressor Genes

•  Tumor-suppressor genes help prevent uncontrolled cell growth

•  Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset

•  Tumor-suppressor proteins – Repair damaged DNA

– Control cell adhesion

–  Inhibit the cell cycle in the cell-signaling pathway


Interference with Normal Cell-Signaling Pathways

•  Mutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancers

•  Mutations in the ras gene can lead to production of a hyperactive Ras protein and increased cell division


Figure 18.24

Growth factor

1

2

3

4

5

1

2

Receptor

G protein

Protein kinases (phosphorylation cascade)

NUCLEUS Transcription factor (activator)

DNA

Gene expression

Protein that stimulates the cell cycle

Hyperactive Ras protein (product of oncogene) issues signals on its own.

(a) Cell cycle–stimulating pathway

MUTATION

Ras

Ras

GTP

GTP

P P P P

P P

(b) Cell cycle–inhibiting pathway

Protein kinases

UV light

DNA damage in genome

Active form of p53

DNA

Protein that inhibits the cell cycle

Defective or missing transcription factor,

such as p53, cannot

activate transcription.

MUTATION

EFFECTS OF MUTATIONS

(c) Effects of mutations

Protein overexpressed

Cell cycle overstimulated

Increased cell division

Protein absent

Cell cycle not inhibited

3

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Growth factor

1

Receptor

G protein

Protein kinases (phosphorylation cascade)

NUCLEUS Transcription factor (activator)

DNA

Gene expression

Protein that stimulates the cell cycle

Hyperactive Ras protein (product of oncogene) issues signals on its own.

(a) Cell cycle–stimulating pathway

MUTATION

Ras

GTP

P P P

P P

P

2

3

4

5

Ras

GTP

Figure 18.24a Figure 18.24b

(b) Cell cycle–inhibiting pathway

Protein kinases

UV light

DNA damage in genome

Active form of p53

DNA

Protein that inhibits the cell cycle

Defective or missing transcription factor,

such as p53, cannot

activate transcription.

MUTATION 2

1

3

•  Suppression of the cell cycle can be important in the case of damage to a cell’s DNA; p53 prevents a cell from passing on mutations due to DNA damage

•  Mutations in the p53 gene prevent suppression of the cell cycle


Figure 18.24c

EFFECTS OF MUTATIONS

(c) Effects of mutations

Protein overexpressed

Cell cycle overstimulated

Increased cell division

Protein absent

Cell cycle not inhibited

The Multistep Model of Cancer Development

•  Multiple mutations are generally needed for full-fledged cancer; thus the incidence increases with age

•  At the DNA level, a cancerous cell is usually characterized by at least one active oncogene and the mutation of several tumor-suppressor genes


Figure 18.25

Colon

Normal colon epithelial cells

Loss of tumor- suppressor gene APC (or other)

1

2

3

4

5 Colon wall

Small benign growth (polyp)

Activation of ras oncogene

Loss of tumor- suppressor gene DCC

Loss of tumor- suppressor gene p53

Additional mutations

Malignant tumor (carcinoma)

Larger benign growth (adenoma)

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Figure 18.25a

Colon

Normal colon epithelial cells

Colon wall

Figure 18.25b

Small benign growth (polyp)

Loss of tumor- suppressor gene APC (or other)

1

Figure 18.25c

Activation of ras oncogene

Loss of tumor-suppressor gene DCC Larger benign

growth (adenoma)

2

3

Figure 18.25d

Loss of tumor-suppressor gene p53

Additional mutations

Malignant tumor (carcinoma)

4

5

Inherited Predisposition and Other Factors Contributing to Cancer

•  Individuals can inherit oncogenes or mutant alleles of tumor-suppressor genes

•  Inherited mutations in the tumor-suppressor gene adenomatous polyposis coli are common in individuals with colorectal cancer

•  Mutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers, and tests using DNA sequencing can detect these mutations


Figure 18.26

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Figure 18.UN01

Operon

Promoter Genes

RNA polymerase

Operator

Polypeptides A B C

A B C

Figure 18.UN02

Genes expressed Genes not expressed Promoter

Genes

Operator

Corepressor Inactive repressor: no corepressor present

Active repressor: corepressor bound

Figure 18.UN03

Genes expressed Genes not expressed Promoter

Genes Operator

Active repressor: no inducer present Inactive repressor:

inducer bound

Figure 18.UN04 Chromatin modification

• Genes in highly compacted chromatin are generally not transcribed. • Histone acetylation seems to loosen chromatin structure, enhancing transcription.

• DNA methylation generally reduces transcription.

mRNA degradation

• Each mRNA has a characteristic life span, determined in part by sequences in the 5ʹ′ and 3ʹ′ UTRs.

• Regulation of transcription initiation: DNA control elements in enhancers bind specific transcription factors.

Bending of the DNA enables activators to contact proteins at the promoter, initiating transcription. • Coordinate regulation:

Enhancer for liver-specific genes

Enhancer for lens-specific genes

Transcription

RNA processing

• Alternative RNA splicing:


mRNA or

• Initiation of translation can be controlled via regulation of initiation factors.

• Protein processing and degradation by proteasomes are subject to regulation.

Translation

Protein processing and degradation

Chromatin modification

Transcription

RNA processing

mRNA degradation

Translation



• Genes in highly compacted chromatin are generally not transcribed. • Histone acetylation seems to loosen chromatin structure, enhancing transcription.

• DNA methylation generally reduces transcription.

• Regulation of transcription initiation: DNA control elements in enhancers bind specific transcription factors.

Bending of the DNA enables activators to contact proteins at the promoter, initiating transcription. • Coordinate regulation:

Enhancer for liver-specific genes

Enhancer for lens-specific genes

Transcription

RNA processing

• Alternative RNA splicing:


mRNA or


Transcription

RNA processing

mRNA degradation

Translation


Figure 18.UN04a

mRNA degradation

• Each mRNA has a characteristic life span, determined in part by sequences in the 5ʹ′ and 3ʹ′ UTRs.

• Initiation of translation can be controlled via regulation of initiation factors.

• Protein processing and degradation by proteasomes are subject to regulation.

Translation



Transcription

RNA processing

mRNA degradation

Translation


Figure 18.UN04b

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Figure 18.UN05


Transcription

RNA processing

mRNA degradation

Translation



Translation

mRNA degradation

• miRNA or siRNA can target specific mRNAs for destruction.

• miRNA or siRNA can block the translation of specific mRNAs.

• Small or large noncoding RNAs can promote the formation of heterochromatin in certain regions, blocking transcription.

Figure 18.UN06

Enhancer Promoter

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

Figure 18.UN07

Enhancer Promoter

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

Figure 18.UN08

Enhancer Promoter

Gene 1

Gene 2

Gene 3

Gene 4

Gene 5

regulation of gene expression -...

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