Transcript gene regulation

Chapter 11
The Control of Gene
Expression
PowerPoint Lectures for
Biology: Concepts and Connections, Fifth Edition
– Campbell, Reece, Taylor, and Simon
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
To Clone or Not to Clone?
• A clone is an individual created by asexual
reproduction and thus is genetically identical to
a single parent
– Cloning an animal using a transplanted
nucleus shows that an adult somatic cell
contains a complete genome
• Cloning has potential benefits but evokes
many concerns
– Does not increase genetic diversity
– May produce less healthy animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
GENE REGULATION
11.1 Proteins interacting with DNA turn
prokaryotic genes on or off in response to
environmental changes
• Gene regulation is the "turning on" and "turning
off" of genes
– Helps organisms respond to
environmental changes
• Gene expression is the process by which
information flows from genes to protein
• Early understanding of gene control came from
studies of the bacterium Escherichia coli
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
GENE REGULATION
• An operon is a cluster of genes with related
functions, along with two control sequences
– Promoter: A sequence of genes where the
RNA polymerase attaches and initiates
transcription
– Operator: A sequence of genes between
the operon and the promoter that acts as a
switch for the binding of RNA polymerase
• A repressor binds to the operator, stopping
transcription
• A regulatory gene, located outside the
operon, codes for the repressor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
GENE REGULATION
• The lac operon contains the genes that code
for the enzymes that metabolize lactose
– Repressor is active when alone and inactive
when bound to lactose
• The trp operon allows bacteria to stop
making tryptophan when it is already
present
– Repressor is inactive alone; must bind to the
amino acid tryptophan to be active
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-1b
OPERON
Regulatory
gene
Promoter Operator
Lactose-utilization genes
DNA
mRNA
Protein
RNA polymerase
cannot attach to
promoter
Active
repressor
Operon turned off (lactose absent)
DNA
RNA polymerase
bound to promoter
mRNA
Protein
Lactose
Inactive
repressor
Operon turned on (lactose inactivates repressor)
Enzymes for lactose utilization
LE 11-1c
Promoter
Operator
Genes
DNA
Active
repressor
Active
repressor
Tryptophan
Inactive
repressor
Inactive
repressor
Lactose
lac operon
trp operon
GENE REGULATION
11.2 Differentiation yields a variety of cell
types, each expressing a different combination of
genes
• Gene regulation is much more complex in
eukaryotes than in prokaryotes
– In multicellular eukaryotes, cells become
specialized as a zygote develops into a
mature organism
– The particular genes that are active in
each type of cell are the source of its
particular function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-2
Muscle cell
Pancreas cells
Blood cells
GENE REGULATION
11.3 Differentiated cells may retain all of their
genetic potential
• Though differentiated cells express only a
small percentage of their genes, they retain a
complete set of genes
– Allows for propagation of crop plants
– In animal cells can lead to regeneration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-3
Root of
carrot plant
Single cell
Root cells cultured
in nutrient medium
Cell division
in culture
Plantlet
Adult plant
GENE REGULATION
11.4 DNA packing in eukaryotic chromosomes
helps regulate gene expression
• DNA can fit into a chromosome because of
packing
– DNA winds around clusters of histone
proteins, forming a string of bead-like
nucleosomes
– The beaded fiber coils, supercoils, and
further folds into chromosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-4
DNA double
helix (2-nm
diameter)
Histones
“Beads on
a string”
Linker
Nucleosome
(10-nm diameter)
Tight helical fiber
(30-nm diameter)
Supercoil
(300-nm
diameter)
700
nm
Metaphase chromosome
DNA Splicing
11.7 Eukaryotic RNA may be spliced in more than
one way
• After transcription, splicing removes noncoding
introns
• Alternative splicing may generate two or more
types of mRNA from the same transcript
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-7
Exons
DNA
RNA
transcript
RNA splicing
mRNA
or
DNA Regulation
11.8 Translation and later stages of gene
expression are also subject to regulation
• After eukaryotic mRNA is processed and
transported to the cytoplasm, there are
additional opportunities for regulation
– Breakdown of mRNA
– Initiation of translation
– Protein activation
– Protein breakdown
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Regulation
11.9 Review: Multiple mechanisms regulate gene
expression in eukaryotes
• Cellular differentiation results from selective
turning on or off of genes at multiple control
points
– In nucleus
• DNA unpacking and other changes
• Transcription
• Addition of cap and tail
• Splicing
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA REGULATION
– In cytoplasm
• Breakdown of mRNA
• Translation
• Cleavage/modification/activation
• Breakdown of protein
• Each differentiated cell still retains its full
genetic potential
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
ANIMAL CLONING
11.10 Nuclear transplantation can be used to
clone animals
• Nuclear transplantation
– Nucleus of a somatic cell is transplanted
into a surrogate egg stripped of nucleus
– Cell divides to the blastocyst stage
• Reproductive cloning
– Blastocycst is implanted into uterus
– Live animal is born
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Therapeutic cloning
– Embryonic stem cells are harvested from
blastocyst
– These cells give rise to all the specialized
cells of the body
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-10
Donor
cell
Nucleus from
donor cell
Implant blastocyst in
surrogate mother
Remove nucleus Add somatic cell
from adult donor
from egg cell
Clone of donor is born
(reproductive cloning)
Grow in culture to produce an
early embryo (blastocyst)
Remove embryonic stem
cells from blastocyst and
grow in culture
Induce stem cells to
form specialized cells
(therapeutic cloning)
CONNECTION
11.11 Reproductive cloning has valuable
applications, but human reproductive cloning
raises ethical issues
• Reproductive cloning of nonhuman mammals
is useful in research, agriculture, and medicine
• There are many obstacles, both practical and
ethical, to human cloning
– Research continues in the absence of
consensus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
THE GENETIC CONTROL OF EMBRYONIC DEVELOPMENT
11.13 Cascades of gene expression and cell-tocell signaling direct the development of an animal
• Studies of mutant fruit flies led to early
understanding of gene expression and
embryonic development
• Before fertilization, communication between
the egg and adjacent cells determines body
polarity
• A cascade of gene expression controls
development of an animal from a fertilized egg
• Master control homeotic genes regulate
batteries of genes that shape anatomical parts
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-13a
Eye
Antenna
Leg
Head of a normal fruit fly
Head of a developmental mutant
Transduction
11.14 Signal transduction pathways convert
messages received at the cell surface to
responses within the cell
• Signal transduction pathway
– Signaling cell secretes signal molecules
– Signal molecules bind to receptors on
target cell's plasma membrane
– Cascade of events leads to the activation
of a specific transcription factor
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Transduction
– Transcription factor triggers
transcription of a specific gene
– Translation of the mRNA produces a
protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
THE GENETIC BASIS OF CANCER
11.16 Cancer results from mutations in genes
that control cell division
• An oncogene can cause cancer when present
in a single copy in a cell
• A cell can acquire an oncogene from
– A virus
– A mutation in a proto-oncogene, a normal
gene with the potential to become an
oncogene
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
THE GENETIC BASIS OF CANCER
• Tumor-suppressor genes
– Normally code for proteins that inhibit cell
division
– When inactivated by mutation, can lead to
uncontrolled cell division and tumors
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 11-16b
Tumor-suppressor gene
Mutated tumor-suppressor gene
Normal
growthinhibiting
protein
Defective,
nonfunctioning
protein
Cell division
under control
Cell division not
under control