Cloning has been attempted to save endangered
species
– A clone is produced by asexual reproduction and is
genetically identical to its parent
– Dolly the sheep was the first cloned mammal
– Endangered animals that were cloned include cows,
oxen, sheep, wildcats, and wolves
Disadvantages of cloning
– Does not increase genetic diversity
– Cloned animals may have health problems related to
abnormal gene regulation
Gene expression
is the overall process of
information flow from genes to proteins
– Mainly controlled at the level of transcription
– A gene that is “turned on” is being transcribed to
produce mRNA that is translated to make its corresponding protein
– Organisms respond to environmental changes by
controlling gene expression
Copyright © 2009 Pearson Education, Inc.
11.1 Proteins interacting with DNA turn
11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
An
operon
is a group of genes under coordinated
control in bacteria
The lactose (
lac
) operon includes
– Three adjacent genes for lactose-utilization enzymes
– Promoter sequence where RNA polymerase binds
– Operator sequence is where a repressor can bind
11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
Regulation of the
lac
operon
– Regulatory gene codes for a repressor protein
– 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
11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
Types of operon control
– Inducible operon (lac operon)
– Active repressor binds to the operator
– Inducer (lactose) binds to and inactivates the repressor
– Repressible operon (trp operon)
– Repressor is initially inactive
– Corepressor (tryptophan) binds to the repressor and makes it active
– For many operons, activators enhance RNA
DNA
DNA
RNA polymerase cannot attach to promoter Lactose-utilization genes Promoter Operator Regulatory gene OPERON Protein mRNA Inactive repressor
Lactose Enzymes for lactose utilization
RNA polymerase bound to promoter mRNA
Active repressor
DNA
RNA polymerase cannot attach to promoter
Lactose-utilization genes Promoter Operator
Regulatory gene
OPERON
mRNA
Active repressor
DNA
Protein
Inactive repressor
Lactose Enzymes for lactose utilization
RNA polymerase bound to promoter
Operon turned on (lactose inactivates repressor)
DNA
Inactive repressor
Active repressor
Inactive repressor
Active repressor
Lactose Promoter
Tryptophan Operator Gene
11.2 Differentiation results from the expression
of different combinations of genes
Differentiation
involves cell specialization, in
both structure and function
11.3 DNA packing in eukaryotic chromosomes
helps regulate gene expression
Eukaryotic chromosomes undergo multiple levels
of folding and coiling, called DNA packing
– Nucleosomes are formed when DNA is wrapped
around histone proteins
– “Beads on a string” appearance
– Each bead includes DNA plus 8 histone molecules
– String is the linker DNA that connects nucleosomes
– Tight helical fiber is a coiling of the nucleosome
string
– Supercoil is a coiling of the tight helical fiber
– Metaphase chromosome represents the highest level
of packing
DNA packing can prevent transcription
11.3 DNA packing in eukaryotic chromosomes
helps regulate gene expression
DNA double helix (2-nm diameter)
“Beads on a string”
Linker
Histones
Metaphase chromosome Tight helical fiber
(30-nm diameter)
Nucleosome (10-nm
diameter)
Supercoil
(300-nm diameter)
11.4 In female mammals, one X chromosome is
inactive in each somatic cell
X-chromosome inactivation
– In female mammals, one of the two X chromosomes
is highly compacted and transcriptionally inactive
– Random inactivation of either the maternal or
paternal chromosome
– Occurs early in embryonic development and all
cellular descendants have the same inactivated chromosome
– Inactivated X chromosome is called a Barr body
– Tortoiseshell fur coloration is due to inactivation of X
Two cell populations in adult X chromosomes Early embryo Allele for black fur Inactive X Black fur Allele for orange fur Orange fur Cell division and random X chromosome
inactivation Active X
11.5 Complex assemblies of proteins control
eukaryotic transcription
Eukaryotic genes
– Each gene has its own promoter and terminator
– Are usually switched off and require activators to be
turned on
– Are controlled by interactions between numerous
11.5 Complex assemblies of proteins control
eukaryotic transcription
Regulatory proteins that bind to control sequences
– Transcription factors promote RNA polymerase binding to the promoter
– Activator proteins bind to DNA enhancers and
interact with other transcription factors
– Silencers are repressors that inhibit transcription
Control sequences
– Promoter
– Enhancer
– Related genes located on different chromosomes can be controlled by similar enhancer sequences
11.5 Complex assemblies of proteins control
eukaryotic transcription
Enhancers
Other proteins DNA
Transcription factors
Activator proteins
RNA polymerase
Promoter
Gene
Bending of DNA
11.6 Eukaryotic RNA may be spliced in more
than one way
Alternative RNA splicing
– Production of different mRNAs from the same
transcript
– Results in production of more than one polypeptide
from the same gene
– Can involve removal of an exon with the introns on
either side
1
or Exons
DNA
RNA splicing RNA
transcript
mRNA
2 3 4 5
1 2 3 4 5
1 2 4 5
11.7 Small RNAs play multiple roles in
controlling gene expression
RNA interference
(
RNAi
)
– Prevents expression of a gene by interfering with
translation of its RNA product
– Involves binding of small, complementary RNAs to
mRNA molecules
– Leads to degradation of mRNA or inhibition of
translation
MicroRNA
– Single-stranded chain about 20 nucleotides long
– Binds to protein complex
– MicroRNA + protein complex binds to complementary
11.7 Small RNAs play multiple roles in
controlling gene expression
Copyright © 2009 Pearson Education, Inc.
Animation: Blocking Translation
miRNA 1
Target mRNA
Protein
miRNA-protein complex
2
11.8 Translation and later stages of gene
expression are also subject to regulation
Control of gene expression also occurs with
– Breakdown of mRNA
– Initiation of translation
– Protein activation
– Protein breakdown
Folding of
polypeptide and formation of
S—S linkages
Initial polypeptide
(inactive) Folded polypeptide(inactive)
11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
Many possible control points exist; a given gene
may be subject to only a few of these
– Chromosome changes (1)
– DNA unpacking
– Control of transcription (2)
– Regulatory proteins and control sequences
– Control of RNA processing
– Addition of 5’ cap and 3’ poly-A tail (3)
– Splicing (4)
– Flow through nuclear envelope (5)
11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
Many possible control points exist; a given gene
may be subject to only a few of these
– Breakdown of mRNA (6)
– Control of translation (7)
– Control after translation
– Cleavage/modification/activation of proteins (8)
Applying Your Knowledge
For each of the following, determine whether an
increase or decrease in the amount of gene
product is expected
– The mRNA fails to receive a poly-A tail during
processing in the nucleus
– The mRNA becomes more stable and lasts twice as
long in the cell cytoplasm
– The region of the chromatin containing the gene
becomes tightly compacted
– An enzyme required to cleave and activate the
protein product is missing
Copyright © 2009 Pearson Education, Inc.
11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
NUCLEUS
DNA unpacking Other changes to DNA
Addition of cap and tail Chromosome Gene RNA transcript Gene Transcription Intron Exon Splicing Cap mRNA in nucleus
Tail Flow through nuclear envelope Broken-down mRNA CYTOPLASM
Breakdown of mRNA Translation mRNA in cytoplasm
Broken-down protein Cleavage / modification /
activation
Breakdown of protein Polypeptide
NUCLEUS
DNA unpacking
Other changes to DNA
Addition of cap and tail Chromosome
Gene
RNA transcript
Gene Transcription
Intron Exon
Splicing
Cap mRNA in nucleus
Tail
Broken-down mRNA CYTOPLASM
Breakdown of mRNA
Translation mRNA in cytoplasm
Broken-down protein Cleavage / modification /
activation
Breakdown of protein Polypeptide
11.10 Cascades of gene expression direct the
development of an animal
Role of gene expression in fruit fly development
– Orientation from head to tail
– Maternal mRNAs present in the egg are translated and influence formation of head to tail axis
– Segmentation of the body
– Protein products from one set of genes activate other sets of genes to divide the body into segments
– Production of adult features
– Homeotic genes are master control genes that
Animation: Development of Head-Tail Axis in Fruit Flies
11.10 Cascades of gene expression direct the
development of an animal
Copyright © 2009 Pearson Education, Inc.
Antenna Eye
Head of a normal fruit fly
11.11 CONNECTION: DNA microarrays test for
the transcription of many genes at once
DNA microarray
– Contains DNA sequences arranged on a grid
– Used to test for transcription
– mRNA from a specific cell type is isolated
– Fluorescent cDNA is produced from the mRNA
– cDNA is applied to the microarray
– Unbound cDNA is washed off
cDNA Nonfluorescent spot Fluorescent spot Actual size (6,400 genes) Each well contains DNA
from a particular gene
DNA microarray
mRNA isolated
DNA of an
expressed gene DNA of anunexpressed gene Reverse transcriptase
11.12 Signal transduction pathways convert
messages received at the cell surface to
responses within the cell
Signal transduction pathway
is a series of
molecular changes that converts a signal at the
cell’s surface to a response within the cell
– Signal molecule is released by a signaling cell
– Signal molecule binds to a receptor on the surface
11.12 Signal transduction pathways convert
messages received at the cell surface to
responses within the cell
– Relay proteins are activated in a series of reactions
– A transcription factor is activated and enters the
nucleus
– Specific genes are transcribed to initiate a cellular
response
Copyright © 2009 Pearson Education, Inc.
11.13 EVOLUTION CONNECTION:
Cell-signaling systems appeared early in the
evolution of life
Yeast mating is controlled by a signal
transduction pathway
– Yeast have two mating types: a and
– Each produces a chemical factor that binds to
receptors on cells of the opposite mating type
– Binding to receptors triggers growth toward the
other cell and fusion
Cell signaling processes in multicellular
organisms are adaptations of those in unicellular
organisms such as bacteria and yeast
Yeast cell, mating type a factor
factor Receptor
a
Yeast cell,
CLONING OF PLANTS AND
ANIMALS
11.14 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
– A root cell can divide to form an adult plant
– Animal limb regeneration
– Remaining cells divide to form replacement structures
Root of
carrot plant
Root cells cultured
in nutrient medium Cell divisionin culture Single
cell
11.15 Nuclear transplantation can be used to
clone animals
Nuclear transplantation
– Replacing the nucleus of an egg cell or zygote with a
nucleus from an adult somatic cell
– Early embryo (blastocyst) can be used in
– Reproductive cloning
– Implant embryo in surrogate mother for development
– New animal is genetically identical to nuclear donor
– Therapeutic cloning
– Remove embryonic stem cells and grow in culture for medical treatments
Remove nucleus from egg cell
Implant blastocyst in surrogate mother
Add somatic cell from adult donor
Donor cell
Remove embryonic stem cells from blastocyst and grow in culture Reproductive
cloning Nucleus from
donor cell
Grow in culture to produce an early embryo (blastocyst)
Therapeutic cloning
Clone of
donor is born
Remove nucleus from egg
Add somatic cell from adult donor
Donor
cell Nucleus from donor cell
Implant blastocyst in surrogate mother
Remove embryonic stem cells from
blastocyst and grow in culture Reproductive
cloning
Therapeutic cloning
Clone of
donor is born
Induce stem cells to form
11.16 CONNECTION: Reproductive cloning has
valuable applications, but human
reproductive cloning raises ethical issues
Cloned animals can show differences from their
parent due to a variety of influences during
development
Reproductive cloning is used to produce animals
with desirable traits
– Agricultural products
– Therapeutic agents
– Restoring endangered animals
Human reproductive cloning raises ethical
11.17 CONNECTION: Therapeutic cloning can
produce stem cells with great medical
potential
Stem cells can be induced to give rise to
differentiated cells
– Embryonic stem cells can differentiate into a variety
of types
– Adult stem cells can give rise to many but not all types of cells
Therapeutic cloning can supply cells to treat
human diseases
Research continues into ways to use and produce
Blood cells
Different types of differentiated cells
Nerve cells Adult stem
cells in bone marrow
Different culture conditions
Cultured embryonic
stem cells
11.18 Cancer results from mutations in genes
that control cell division
Mutations in two types of genes can cause cancer
– Oncogenes
– Proto-oncogenes normally promote cell division
– Mutations to oncogenes enhance activity
– Tumor-suppressor genes
– Normally inhibit cell division
– Mutations inactivate the genes and allow uncontrolled division to occur
11.18 Cancer results from mutations in genes
that control cell division
Oncogenes
– Promote cancer when present in a single copy
– Can be viral genes inserted into host chromosomes
– Can be mutated versions of proto-oncogenes, normal genes that promote cell division and differentiation
– Converting a proto-oncogene to an oncogene can occur by
– Mutation causing increased protein activity
– Increased number of gene copies causing more protein to be produced
11.18 Cancer results from mutations in genes
that control cell division
Tumor-suppressor genes
– Promote cancer when both copies are mutated
Mutation within the gene Hyperactive growth-stimulating protein in normal Proto-oncogene DNA Multiple copies
of the gene new DNA locus,Gene moved to under new controls
Oncogene New promoter
Mutated tumor-suppressor gene Tumor-suppressor gene
Defective,
nonfunctioning protein
Normal growth-inhibiting protein
Cell division
11.19 Multiple genetic changes underlie the
development of cancer
Four or more somatic mutations are usually
required to produce a cancer cell
One possible scenario for colorectal cancer
includes
– Activation of an oncogene increases cell division
– Inactivation of tumor suppressor gene causes
formation of a benign tumor
1
Colon wall
Cellular changes:
DNA
changes: Oncogeneactivated Increased cell division
Tumor-suppressor gene inactivated Growth of polyp
Second tumor-suppressor gene inactivated
Growth of malignant tumor (carcinoma)
Chromosomes 1
mutation
Normal cell
4
mutations 3
mutations 2
mutations
11.20 Faulty proteins can interfere with normal
signal transduction pathways
Path producing a product that stimulates cell
division
Product of ras proto-oncogene relays a signal when
growth hormone binds to receptor
Product of ras oncogene relays the signal in the
absence of hormone binding, leading to uncontrolled growth
11.20 Faulty proteins can interfere with normal
signal transduction pathways
Path producing a product that inhibits cell division
– Product of p53 tumor-suppressor gene is a
transcription factor
– p53 transcription factor normally activates genes for
factors that stop cell division
– In the absence of functional p53, cell division
Growth factor Protein that Stimulates cell division Translation Nucleus DNA Target cell Normal product of ras gene
Receptor Relay proteins Transcription factor (activated) Hyperactive relay protein (product of
ras oncogene) issues signals on its own
Growth-inhibiting factor Protein that inhibits Translation Normal product of p53 gene Receptor Relay proteins Transcription factor (activated) Nonfunctional transcription factor (product of faulty p53
11.21 CONNECTION: Lifestyle choices can
reduce the risk of cancer
Carcinogens
are cancer-causing agents that
damage DNA and promote cell division
– X-rays and ultraviolet radiation
– Tobacco
Healthy lifestyle choices
– Avoiding carcinogens
– Avoiding fat and including foods with fiber and
antioxidants
– Regular medical checkups
Regulatory
gene Promoter
DNA
Gene 1 Operator
A typical operon
Gene 2
Gene 3
Encodes repressor that in active form attaches to operator
RNA
polymerase binding site
Nucleus from donor cell
Early embryo resulting from nuclear trans-plantation
Surrogate
Nucleus from donor cell
Early embryo resulting from nuclear trans-plantation
Embryonic stem cells in culture
Gene regulation (a) prokaryotic genes often grouped into in eukaryotes
may involve when abnormal may lead to
can produce
is a normal gene that can be mutated to an
oncogene can cause (c) (g) (f) multiple mRNAs (e) (d) (b) operons are switched on/off by in active
form binds to controlled by protein called
