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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 10
Controls Over Genes
(Sections 10.1 - 10.5)
Albia Dugger • Miami Dade College
10.1 Between You and Eternity
• Many gene expression
controls regulate cell
growth and division
• When those controls
fail, cancer results
• Robin Shoulla was
diagnosed with breast
cancer when she was
seventeen
Cancer
• Cancer is a multistep process in which abnormally growing
and dividing cells disrupt body tissues
• Cancer often begins with a mutation in a gene whose product
is part of a system of controls over cell growth and division
• cancer
• Disease that occurs when uncontrolled growth of body
cells physically and metabolically disrupts tissues
A Case of Breast Cancer
• Irregular clusters of cancer cells in human breast tissue
10.2 Eukaryotic Gene Expression
• Differentiation occurs as different cell lineages begin to
express different subsets of their genes
• Which genes a cell uses determines the molecules it will
produce, which in turn determines what kind of cell it will be
• differentiation
• Process by which cells become specialized
Eukaryotic Gene Expression
• Which genes are expressed at any given time depends on
many factors, such as conditions in the cytoplasm and
extracellular fluid, and type of cell
• These factors affect controls governing all steps of gene
expression, starting with transcription and ending with delivery
of an RNA or protein product to its final destination
• Controls may start, enhance, slow, or stop gene expression
Control of Transcription
• Whether and how fast a gene is transcribed depends on
which transcription factors are bound to the DNA
• Activators speed transcription by binding to DNA enhancers
• Repressors slow or stop transcription
Key Terms
• transcription factor
• Regulatory protein that influences transcription; e.g. an activator or
repressor
• activator
• Regulatory protein that increases the rate of transcription when it
binds to a promoter or enhancer
• enhancer
• Binding site in DNA for proteins that enhance transcription rate
• repressor
• Regulatory protein that blocks transcription
Control of Transcription (cont.)
• Interactions between DNA and histone proteins also affect
transcription
• Molecules that methylate DNA prevent transcription
• Number of gene copies affects how fast its product is made
• Polytene chromosomes consists of hundreds or thousands
of copies of the same DNA molecule
Drosophila Polytene Chromosomes
• Giant polytene
chromosomes form in
Drosophila salivary
gland cells by repeated
DNA replication
• Transcription is visible
(white arrows) where
DNA has loosened
mRNA Processing
• Before eukaryotic mRNAs leave the nucleus, they are
modified—spliced, capped, and finished with a poly-A tail
• Controls over these modifications can affect the form of a
protein product and when it will appear in the cell
Translational Control
• Most controls over eukaryotic gene expression affect
translation
• Many controls govern the production or function of various
molecules that carry out translation
• Others affect mRNA stability: Depends on base sequence,
length of poly-A tail, and which proteins are attached to it
Post-Translational Modification
• Many newly-synthesized polypeptide chains must be modified
before they become functional
• Post-translational modifications inhibit, activate, or stabilize
many molecules, including the enzymes that participate in
transcription and translation
Points of
Control Over
Eukaryotic
Gene
Expression
DNA
new RNA
transcript
mRNA
Nucleus
1 Transcription
Binding of transcription factors to
special sequences in DNA slows or
speeds transcription. Chemical
modifications and chromosome
duplications affect RNA polymerase’s
physical access to genes.
2 mRNA Processing
New mRNA cannot leave the nucleus
before being modified, so controls over
mRNA processing affect the timing of
transcription. Controls over alternative
splicing influence the final form of the
protein.
3 mRNA Transport
RNA cannot pass through a nuclear pore
unless bound to certain proteins.
Transport protein binding affects where
the transcript will be delivered in the cell.
Cytoplasm
mRNA
4 Translation
An mRNA’s stability influences how
long it is translated. Proteins that
attach to ribosomes or initiation factors
can inhibit translation. Double-stranded
RNA triggers degradation of
polypeptide complementary mRNA.
chain
5 Protein Processing
A new protein molecule may become
activated or disabled by enzymemediated modifications, such as
phosphorylation or cleavage. Controls
active
over these enzymes influence many
protein
other cell activities.
Stepped Art
Fig 10.2, p. 152
Key Concepts
• Gene Control in Eukaryotes
• A variety of molecules and processes alter gene
expression in response to changing conditions both inside
and outside the cell
• Selective gene expression also results in differentiation, by
which cell lineages become specialized
10.3 There’s a Fly in My Research
• The fruit fly Drosophila melanogaster has been the subject of
of many research experiments on eukaryotic gene expression
• It reproduces quickly and has a short life cycle
Homeotic Genes
• Homeotic genes encode transcription factors with a
homeodomain – a region of about sixty amino acids that can
bind to a promoter or some other DNA sequence in a
chromosome
• homeotic gene
• Type of master gene
• Its expression controls formation of specific body parts
during development
Master Genes
• Master genes affect the expression of many other genes.
• Expression of a master gene causes other genes to be
expressed, allowing completion of intricate tasks such as eye
formation during embryonic development
• master gene
• Gene encoding a product that affects the expression of
many other genes
Knockouts
• Researchers inactivate a homeotic gene by introducing a
mutation or deleting it entirely (knockout)
• Any differences in an organism with a knocked-out gene may
be clues to the function of the missing gene product
• knockout
• An experiment in which a gene is deliberately inactivated
in a living organism
Homeotic Genes: Eyes and eyeless
• A normal fruit fly, a fruit fly with eyeless mutation, and
expression of eyeless on a fly’s head and wing
Homeotic Genes: PAX6
• Humans and other
animals have a gene
similar to eyeless
• A mutation in PAX6
results in missing irises
(aniridia)
Filling in Details of Body Plans
• As an embryo develops, its differentiating cells form tissues,
organs, and body parts
• Some cells alternately migrate and stick to other cells to
develop structures that weave through tissues
• These events fill in the body’s details, and are driven by
cascades of master gene expression
Pattern Formation
• Pattern formation determines the body plan of an embryo:
• Protein products diffuse in gradients along the embryo
• Cells translate different master genes, depending on
where they fall within those gradients
• Some master gene products cause undifferentiated cells
to differentiate into specialized tissues
• pattern formation
• Process by which a complex body forms from local
processes during embryonic development
Pattern Formation in a Fruit Fly
• The master gene even-skipped is expressed (red) where two
gene products (blue and green) overlap
Pattern Formation in a Fruit Fly
• Products of several master genes confine expression of evenskipped (red) to seven stripes, where segments develop
Key Concepts
• Mechanisms of Control
• All cells in an embryo inherit the same genes, but they
start using different subsets of those genes during
development
• The orderly, localized expression of master genes gives
rise to the body plan of complex multicelled organisms
10.4 Outcomes of Gene Controls
• Many traits of humans and other eukaryotic organisms arise
as an outcome of gene expression controls
• Examples:
• X-chromosome inactivation
• Male sex determination
• Flower formation
X Chromosome Inactivation
• A female’s cells each contain two X chromosomes, one
inherited from her mother, the other from her father
• One X chromosome is always tightly condensed (Barr bodies)
• This X chromosome inactivation ensures that only one of
the two X chromosomes in a female’s cells is active
• According to the dosage compensation theory, this
equalizes expression of X chromosome genes between sexes
Key Terms
• X chromosome inactivation
• Shutdown of one of the two X chromosomes in the cells of
female mammals
• Caused by transcription of the XIST gene which keeps the
chromosome from transcribing other genes
• dosage compensation
• Theory that X chromosome inactivation equalizes gene
expression between males and females
X Chromosome Inactivation
• Barr bodies (red spots) in nuclei of XX cells, compared to
nuclei of XY cells
Male Sex Determination in Humans
• The human Y chromosome carries the SRY gene—the master
gene for male sex determination in mammals
• An early human embryo appears neither male nor female
• SRY expression in XY embryos triggers formation of testes
(male gonads) that secrete testosterone, which initiates
development of other male traits
• In an XX embryo, ovaries form in the absence of the Y
chromosome and its SRY gene
Developing
Reproductive
Organs
Structures that will give rise
to external genitalia appear
at seven weeks
SRY expressed
no SRY present
penis
vaginal
opening
birth approaching
Fig 10.8, p. 156
Flower Formation
• Activation of three sets of master genes (A, B, and C) guide
flower formation – the differentiation of leaf cells into floral
parts (sepals, petals, stamens, and carpels)
• These genes are switched on by environmental cues such as
seasonal changes in the length of night
Control of Flower
Formation
petals
carpel
sepals stamens
A The pattern in which the
floral identity genes A, B,
and C are expressed
affects differentiation
of cells growing in whorls
in the plant’s tips. Their
gene products guide
expression of other genes
in cells of each whorl; a
flower results.
Fig 10.9a, p. 157
Mutations in Arabidopsis thaliana
B Mutations in Arabidopsis
ABC genes result in
malformed
flowers.
Top left, right: A gene
mutations lead to petal-less
flowers with no structures in
place of missing petals.
Bottom left: B gene
mutations lead to flowers
with sepals instead of petals.
Bottom right: C gene
mutations lead to flowers
with petals instead of sepals
and carpels. Compare the
normal flower in A.
Fig 10.9b, p. 157
Key Concepts
• Examples in Eukaryotes
• One of the two X chromosomes is inactivated in every cell
of female mammals
• The Y chromosome carries a master gene that causes
male traits to develop in the human fetus
• Flower development is orchestrated by a set of homeotic
genes
10.5 Gene Control in Bacteria
• Bacteria and archaea do not use master genes
• Bacteria control gene expression mainly by adjusting the rate
of transcription
• Genes that are used together often occur together on the
chromosome – all are transcribed together, so their
transcription is controllable in a single step
The Lactose Operon
• E. coli uses three enzymes whose genes are transcribed
together to break down lactose molecules
• Unless lactose is present, these genes are turned off
• There is one promoter for all three genes
• A promoter and one or more operators that control
transcription of multiple genes are called an operon
Key Terms
• operon
• Group of genes together with a promoter–operator DNA
sequence that controls their transcription
• Occur in bacteria, archaeans, and eukaryotes
• operator
• Part of an operon; a DNA binding site for a repressor
which stops transcription
The Lactose Operon
(1) The operon consists of a promoter flanked by two
operators, and three genes for lactose-metabolizing enzymes
(2) In the absence of lactose, a repressor binds the two
operators, preventing RNA polymerase from attaching to the
promoter – transcription does not occur
(3) When lactose is present, some binds to the repressor,
altering the shape of the repressor so it releases the
operators – RNA polymerase attaches to the promoter and
transcribes the genes
The Lactose Operon
1 The lac operon in the E. coli chromosome.
Lactose absent
2 In the absence of lactose, a repressor binds to the two
Repressor protein
operators. Binding prevents RNA polymerase from attaching
to the promoter, so transcription of the operon genes does
not occur.
Lactose present
lactose
3 When lactose is present, some is converted to a form that
binds to the repressor. Binding alters the shape of the
repressor such that it releases the operators. RNA
polymerase can now attach to the promoter
and transcribe the operon genes.
Stepped Art
Fig 10.10, p. 158
Key Concepts
• Gene Control in Bacteria
• Bacterial gene controls govern responses to short-term
changes in nutrient availability and other aspects of the
environment
• The main gene controls bring about fast adjustments in
the rate of transcription
Lactose Intolerance
• In most people, production the enzyme lactase (which digests
lactose in food) starts declining at age five
• The inability to produce lactase is called lactose intolerance
• People who carry a mutation in one of the genes responsible
for programmed lactase shutdown make enough lactase to
continue drinking milk without problems into adulthood