Download Tumor-suppressor genes

CONTROL OF GENE
EXPRESSION
© 2012 Pearson Education, Inc.
Proteins interacting with DNA turn prokaryotic
genes on or off in response to environmental
changes
 Gene regulation is the
turning on and off of
genes.
 Gene expression is the
overall process of
information flow from
genes to proteins.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Proteins interacting with DNA turn prokaryotic
genes on or off in response to environmental
changes
 Regulation of the lac operon
– A regulatory gene, located outside the operon, codes
for a repressor protein, that is always being transcribed &
translated.
– In the absence of lactose, the repressor binds to the
operator and prevents RNA polymerase action.
– Lactose inactivates the repressor, so
– The operator is unblocked
– RNA polymerase can bind to the promoter, and
– all three genes of the operon are transcribed.
© 2012 Pearson Education, Inc.
Operon turned off (lactose is absent):
OPERON
Regulatory
gene
Promoter Operator
Lactose-utilization genes
DNA
RNA polymerase cannot
attach to the promoter
mRNA
Protein
Active
repressor
Operon turned on (lactose inactivates the repressor):
DNA
RNA polymerase is
bound to the promoter
mRNA
Translation
Protein
Lactose
Inactive
repressor
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.
© 2012 Pearson Education, Inc.
Chromosome
Chromosome
DNA unpacking
Other changes to the DNA
DNA
Gene
Gene
Transcription
Exon
RNA transcript
Intron
Addition of a cap and tail
Splicing
Tail
Cap
mRNA in nucleus
Flow through
NUCLEUS nuclear envelope
CYTOPLASM
Figure 11.7_2
mRNA in cytoplasm
CYTOPLASM
Breakdown of mRNA
Brokendown
mRNA
Translation
Polypeptide
Polypeptide
Cleavage, modification,
activation
Active
protein
Active protein
Breakdown
of protein
Amino
acids
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
© 2012 Pearson Education, Inc.
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 eight 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
© 2012 Pearson Education, Inc.
DNA double helix
(2-nm diameter)
Metaphase
chromosome
Nucleosome
(10-nm diameter)
Linker
“Beads on
a string”
Histones Supercoil
(300-nm diameter)
Tight helical
fiber (30-nm
diameter)
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Early Embryo
Adult
Two cell populations
X chromosomes
Allele for
orange fur
Cell division
and random
X chromosome Active X
inactivation Inactive X
Allele for
black fur
Inactive X
Active X
Orange
fur
Black 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.
© 2012 Pearson Education, Inc.
Complex assemblies of proteins control
eukaryotic transcription
 Eukaryotic RNA polymerase requires the
assistance of proteins = transcription factors.
 Transcription Factors Include:
– 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,
allowing RNA pol to bind the promoter, leading to
transcription.
© 2012 Pearson Education, Inc.
Enhancers
Promoter
Gene
DNA
Activator
proteins
Transcription
factors
Other
proteins
RNA polymerase
Bending
of DNA
Transcription
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 either
– degrading the target mRNA or
– blocking its translation
 RNA interference (RNAi) is the use of miRNA to
artificially control gene expression by injecting
miRNAs into a cell to turn off a specific gene
sequence.
© 2012 Pearson Education, Inc.
Protein
miRNA
1
miRNAprotein
complex
2
Target mRNA
3
or
4
Translation
blocked
mRNA degraded
Protein Activation: The role of polypeptide
cleavage
Folding of the polypeptide
and the formation of
S—S linkages
Cleavage
S S
Initial polypeptide
(inactive)
Folded polypeptide
(inactive)
S S
Active form
of insulin
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
 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
CLONING OF PLANTS
AND ANIMALS
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Root of
carrot plant
Single cell
Root cells cultured
in growth medium
Cell division
in culture
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)
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Figure 11.13_1
Donor
cell
Nucleus from
the donor cell
Blastocyst
The nucleus is
removed from
an egg cell.
A somatic cell
from an adult donor
is added.
The cell grows in
culture to produce
an early embryo
(blastocyst).
Figure 11.13_2
Reproductive
cloning
Blastocyst
The blastocyst is
implanted in a
surrogate mother.
A clone of the
donor is born.
Therapeutic
cloning
Embryonic stem cells
are removed from the
blastocyst and grown
in culture.
The stem cells are
induced to form
specialized 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.
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Figure 11.15
Blood cells
Adult stem
cells in bone
marrow
Nerve cells
Cultured
embryonic
stem cells
Heart muscle cells
Different culture
conditions
Different types of
differentiated cells
THE GENETIC BASIS
OF CANCER
© 2012 Pearson Education, Inc.
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.
© 2012 Pearson Education, Inc.
Proto-oncogene
(for a protein that stimulates cell division)
DNA
A mutation within
the gene
Multiple copies
of the gene
Oncogene
Hyperactive
growthstimulating
protein in a
normal amount
The gene is moved to
a new DNA locus,
under new controls
New promoter
Normal growthstimulating
protein
in excess
Normal growthstimulating
protein
in excess
Tumor-suppressor gene
Normal
growthinhibiting
protein
Cell division
under control
Mutated tumor-suppressor gene
Defective,
nonfunctioning
protein
Cell division
not under control
An oncogene A tumor-suppressor
DNA
changes: is activated gene is inactivated
A second tumorsuppressor gene
is inactivated
Cellular
Increased
changes: cell division
1
Growth of a
malignant tumor
3
Colon wall
Growth of a polyp
2
Figure 11.17B
1
Chromosomes mutation
Normal
cell
2
mutations
3
4
mutations mutations
Malignant
cell
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.
© 2012 Pearson Education, Inc.
Lifestyle choices can reduce the risk of cancer
 Carcinogens are cancer-causing agents that alter
DNA.
 Most mutagens (substances that promote
mutations) are carcinogens.
 Two of the most potent carcinogens (mutagens)
are
– X-rays
– ultraviolet radiation in sunlight.
© 2012 Pearson Education, Inc.
Lifestyle choices can reduce the risk of cancer
 The one substance known to cause more cases and
types of cancer than any other single agent is
tobacco smoke.
© 2012 Pearson Education, Inc.