control of gene expression © 2012 pearson education, inc
TRANSCRIPT
CONTROL OF GENE EXPRESSION
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Gene regulation is the turning on and off of genes.
Gene expression is the overall process of information flow from genes to proteins.
Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes
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Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes
A cluster of genes with related functions, along with the control sequences, is called an operon.
Found mainly in prokaryotes.
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Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes
When an E. coli encounters lactose, all enzymes needed for its metabolism are made at once using the lactose operon.
The lactose (lac) operon includes
1. Three adjacent lactose-utilization genes
2. A promoter sequence where RNA polymerase binds and initiates transcription of all three lactose genes
3. An operator sequence where a repressor can bind and block RNA polymerase action.
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Proteins interacting with DNA turn prokaryotic genes on or off in response to environmental changes
Regulation of the lac operon– A regulatory gene, codes for a repressor protein
– outside of the operon
– always being transcribed & translated
– In the absence of lactose, the repressor protein binds the operator and prevents transcription
– When Lactose is present, it inactivates the repressor protein so,
– the operator is unblocked
– RNA polymerase can bind the promoter
– all three genes of the operon are transcribed
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Operon turned off (lactose is absent):OPERON
Regulatorygene
Promoter Operator Lactose-utilization genes
RNA polymerase cannotattach to the promoter
Activerepressor
Protein
mRNA
DNA
Protein
mRNA
DNA
Operon turned on (lactose inactivates the repressor):
RNA polymerase isbound to the promoter
LactoseInactiverepressor
Translation
Enzymes for lactose utilization
Multiple mechanisms regulate gene expression in eukaryotes
Multiple control points exist in Eukaryotic gene expression
Genes can be turned on or off, or sped up, or slowed down.
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Multiple mechanisms regulate gene expression in eukaryotes
These control points include:
1. Chromosome changes and DNA unpacking
2. Control of transcription
3. Control of RNA processing including the
– addition of a cap and tail and
– splicing
4. Flow through the nuclear envelope
5. Breakdown of mRNA
6. Control of translation
7. Control after translation
– cleavage/modification/activation of proteins
– protein degradation
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Chromosome Chromosome
DNA unpackingOther changes to the DNA
Gene DNA
TranscriptionGene
Exon
Intron
Tail
Cap
Addition of a cap and tail
RNA transcript
Splicing
mRNA in nucleus
NUCLEUSFlow through nuclear envelope
CYTOPLASM
Chromosome structure and chemical modifications can affect gene expression
Eukaryotic chromosomes undergo multiple levels of folding and coiling, called DNA packing.
– Nucleosomes are formed when DNA is wrapped around histone proteins.
– Nucleosomes appear as “beads on a string”.
– Each nucleosome bead includes DNA plus 8 histones.
– At the next level of packing, the beaded string is wrapped into a tight helical fiber (30nm).
– This fiber coils further into a thick supercoil (300nm).
– Looping and folding further compacts DNA into a metaphase chromosome
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DNA double helix(2-nm diameter)
“Beads ona string”
Linker
Histones Supercoil(300-nm diameter)
Nucleosome(10-nm diameter)
Tight helicalfiber (30-nmdiameter)
Metaphasechromosome
700 nm
Chromosome structure and chemical modifications can affect gene expression
DNA packing can prevent gene expression by preventing RNA polymerase & other proteins from contacting the DNA.
Cells seem to use higher levels of packing for long-term inactivation of genes.
Highly compacted chromatin is generally not expressed
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Chromosome structure and chemical modifications can affect gene expression
Epigenetic inheritance
– Inheritance of traits transmitted by mechanisms that do not alter the sequence of nucleotides in DNA
– Chemical modification of DNA bases or histone proteins can result in epigenetic inheritance
– Ex. Enzymatic addition of a methyl group (CH3) to DNA
– Genes are not expressed when methylated
– Removal of the extra methyl groups can turn on some of these genes
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Chromosome structure and chemical modifications can affect gene expression
X-chromosome inactivation
– In female mammals either the maternal or paternal chromosome is randomly inactivated.
– occurs early in embryonic development; all cellular descendants have the same inactivated chromosome.
– An inactivated X chromosome = Barr body.
– Tortoiseshell fur coloration is due to inactivation of X chromosomes in heterozygous female cats.
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Cell divisionand random
X chromosomeinactivation
Early Embryo Adult
X chromo-somes
Allele fororange fur
Allele forblack fur
Two cell populations
Active X
Inactive X
Active X
Inactive XBlack fur
Orange fur
Complex assemblies of proteins control eukaryotic transcription
Prokaryotes and eukaryotes employ regulatory proteins (activators and repressors) that
– bind to specific segments of DNA
– either promote or block the binding of RNA polymerase, turning the transcription of genes on and off.
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Complex assemblies of proteins control eukaryotic transcription
Eukaryotic RNA polymerase requires the assistance of proteins = transcription factors.
Examples of Transcription Factors:
– Activator proteins-bind to DNA sequences called enhancers.
– A DNA bending protein bends DNA, bringing bound activators closer to promoter.
– Once bent activators interact with other proteins, allows binding of RNA pol to the promoter, leading to transcription.
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Enhancers
DNA
PromoterGene
Transcriptionfactors
Activatorproteins
Otherproteins
RNA polymerase
Bendingof DNA
Transcription
Figure 11.7_2
mRNA in cytoplasm CYTOPLASM
Breakdown of mRNA
Translation
PolypeptidePolypeptide
Broken-downmRNA
Cleavage, modification,activation
Active protein Activeprotein
Amino acids
Breakdownof protein
Small RNAs play multiple roles in controlling gene expression
A significant amount of the genome codes for microRNAs
microRNAs (miRNAs) can bind to complementary sequences on mRNA molecules. This can lead to
– degradation of the target mRNA
– blocking its translation
RNA interference (RNAi) is the use of miRNA to control gene expression
can inject miRNAs into a cell to turn off a specific gene sequence.
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miRNA
Target mRNA
mRNA degraded
or
Translationblocked
miRNA-proteincomplex
Protein
3
2
1
4
Later Stages of Gene Expression are Subject to Regulation
Breakdown of mRNA
– Enzymes in the cytoplasm destroy mRNA
– mRNA’s of eukaryotes have lifetimes from hours to weeks
Folding of the polypeptideand the formation ofS—S linkages
Initial polypeptide(inactive)
Folded polypeptide(inactive)
Active formof insulin
Cleavage
S S
SS
S
SS
S
SS
SS
SHSH
SH
SH
SH
SH
Protein Activation: The role of polypeptide cleavage
Protein Breakdown
– Final control mechanism
– Cells can adjust the kinds and amounts of its proteins in response to environmental changes
– Damaged proteins are usually broken down right away
Later Stages of Gene Expression are Subject to Regulation
CLONING OF PLANTS AND ANIMALS
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Plant cloning shows that differentiated cells may retain all of their genetic potential
Most differentiated cells retain a full set of genes, even though only a subset may be expressed. Evidence is available from
– plant cloning, in which a root cell can divide to form an adult plant
– salamander limb regeneration, in which the cells in the leg stump dedifferentiate, divide, and then redifferentiate, giving rise to a new leg.
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Root ofcarrot plant
Root cells culturedin growth medium
Cell divisionin culture
Single cell
Plantlet Adult plant
Nuclear transplantation can be used to clone animals
Animal cloning can be achieved using nuclear transplantation: the nucleus of an egg cell or zygote is replaced with a nucleus from an adult somatic cell.
Using nuclear transplantation to produce new organisms is called reproductive cloning (first used in mammals in 1997 to produce Dolly)
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Nuclear transplantation can be used to clone animals
Another way to clone uses embryonic stem (ES) cells harvested from a blastocyst. This procedure can be used to produce
– cell cultures for research
– stem cells for therapeutic treatments.
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Figure 11.13
The nucleus isremoved froman egg cell.
Donor cell
A somatic cellfrom an adult donoris added.
Nucleus fromthe donor cell
Reproductivecloning
Blastocyst
The blastocyst isimplanted in asurrogate mother.
A clone of thedonor is born.
The cell grows inculture to producean early embryo(blastocyst).
Therapeuticcloning
Embryonic stem cellsare removed from theblastocyst and grownin culture.
The stem cells areinduced to formspecialized cells.
Figure 11.13_1
The nucleus isremoved froman egg cell.
Donor cell
A somatic cellfrom an adult donoris added.
Nucleus fromthe donor cell
Blastocyst
The cell grows inculture to producean early embryo(blastocyst).
Figure 11.13_2
Reproductivecloning
BlastocystThe blastocyst isimplanted in asurrogate mother.
A clone of thedonor is born.
Therapeuticcloning
Embryonic stem cellsare removed from theblastocyst and grownin culture.
The stem cells areinduced to formspecialized cells.
Reproductive cloning has valuable applications, but human reproductive cloning raises ethical issues
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Since Dolly’s landmark birth in 1997, researchers have cloned many other mammals, including mice, cats, horses, cows, mules, pigs, rabbits, ferrets, and dogs.
Cloned animals can show differences in anatomy and behavior due to
– environmental influences and
– random phenomena
Reproductive cloning has valuable applications, but human reproductive cloning raises ethical issues
Reproductive cloning is used to produce animals with desirable traits to
– produce better agricultural products
– produce therapeutic agents
– restock populations of endangered animals
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Therapeutic cloning can produce stem cells with great medical potential
When grown in laboratory culture, stem cells can
– divide indefinitely
– give rise to many types of differentiated cells
Adult stem cells can give rise to many, but not all, types of cells
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Therapeutic cloning can produce stem cells with great medical potential
Embryonic stem cells are considered more promising than adult stem cells for medical applications.
The ultimate aim of therapeutic cloning is to supply cells for the repair of damaged or diseased organs.
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Figure 11.15
Blood cells
Nerve cells
Heart muscle cells
Different types ofdifferentiated cells
Different cultureconditions
Culturedembryonicstem cells
Adult stemcells in bone
marrow
THE GENETIC BASIS OF CANCER
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Cancer results from mutations in genes that control cell division
Mutations in two types of genes can cause cancer.
1. Oncogenes
– Proto-oncogenes are normal genes that promote cell division.
– Mutations to proto-oncogenes create cancer-causing oncogenes that often stimulate cell division.
2. Tumor-suppressor genes
– Tumor-suppressor genes normally inhibit cell division or function in the repair of DNA damage.
– Mutations inactivate the genes and allow uncontrolled division to occur.
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Oncogene
Hyperactivegrowth-stimulatingprotein in anormal amount
Normal growth-stimulatingprotein in excess
Normal growth-stimulatingprotein in excess
New promoter
The gene is moved toa new DNA locus,
under new controls
Multiple copiesof the gene
A mutation withinthe gene
Proto-oncogene(for a protein that stimulates cell division)
DNA
Tumor-suppressor gene Mutated tumor-suppressor gene
Normalgrowth-inhibitingprotein
Cell divisionunder control
Defective,nonfunctioningprotein
Cell divisionnot under control
Colon wall
DNAchanges:
Cellularchanges:
Growth of a polyp
An oncogeneis activated
Increasedcell division
A tumor-suppressorgene is inactivated
A second tumor-suppressor gene
is inactivated
Growth of amalignant tumor
1 2 3
Figure 11.17B
Chromosomes1
mutation2
mutations3
mutations4
mutations
Normalcell
Malignantcell
Lifestyle choices can reduce the risk of cancer
After heart disease, cancer is the second-leading cause of death in most industrialized nations.
Cancer can run in families if an individual inherits an oncogene or a mutant allele of a tumor-suppressor gene that makes cancer one step closer.
But most cancers cannot be associated with an inherited mutation.
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Lifestyle choices can reduce the risk of cancer
Carcinogens are cancer-causing agents that alter DNA.
Most mutagens (substances that promote mutations) are carcinogens.
The one substance known to cause more cases and types of cancer than any other single agent is tobacco smoke.
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