control of gene expression © 2012 pearson education, inc

CONTROL OF GENE EXPRESSION

© 2012 Pearson Education, Inc.

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



A cluster of genes with related functions, along with the control sequences, is called an operon.

Found mainly in prokaryotes.



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.



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


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.


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


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


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


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



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



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.


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.


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.


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.


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


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.


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)


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.


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


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


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


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.


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


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.


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.


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.


control of gene expression © 2012 pearson education, inc

Documents

lactose genes

lactose operon

control of gene expression

lactose utilization

prokaryotic genes

lactose lac operon

chromosome dna

pearson education