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Chapter 21
The Genetic Basis
of Development
張學偉 生物醫學暨環境生物學系 助理教授
http://genomed.dlearn.kmu.edu.tw
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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• Overview:
From Single Cell to Multicellular Organism
Use mutations to deduce developmental pathways
Extra small eye on its
antenna
Figure 21.1
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• 如何藉由某種生物來了解”development”
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Model organism
The organism chosen for understand broad
biological principles is called a model organism.
DROSOPHILA MELANOGASTER
(FRUIT FLY)
CAENORHABDITIS ELEGANS
(NEMATODE)
0.25 mm
Figure 21.2
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MUS MUSCULUS
(MOUSE)
ARABIDOPSIS THAMANA
(COMMON WALL CRESS)
DANIO RERIO
(ZEBRAFISH)
No human,
why?
• The fruit fly Drosophila melanogaster was first
chosen as a model organism
• The fruit fly is small and easily grown in the laboratory.
– It has a generation time of only two weeks and produces
many offspring.
– Embryos develop outside the mother’s body.
– Sequencing of Drosophila genome was completed in
2000.
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The nematode Caenorhabditis elegans normally
lives in the soil but is easily grown in petri dishes.
• millimeter long, transparent body with only a few cell
types and grows from zygote to mature adult in only three
and a half days.
• genome sequenced.
• Because individuals are hermaphrodites, it is easy to
detect recessive mutations.
• Self-fertilization of heterozygotes will produce some
homozygous recessive offspring with mutant
phenotypes.
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© 2005©Pearson
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as Benjamin
Cummings
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2002 Pearson
Inc.,
publishing
as Benjamin
Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The mouse Mus musculus has a long history
as a mammalian model of development.
– Genome and genes roughly the same as human.
– Researchers are adepts at manipulating mouse
genes to make transgenic mice and mice in which
particular genes are “knocked out” by mutation.
– Generation time of 9 weeks, but embryo
development inside mother is disadvantages for
studies.
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© 2005©Pearson
Education,Education,
Inc. publishing
as Benjamin
Cummings
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2002 Pearson
Inc.,
publishing
as Benjamin
Cummings
• A second vertebrate model, the zebrafish Danio rerio, has
some unique advantages.
– These small fish (2 - 4 cm long) are easy to breed in the
laboratory in large numbers.
– Transparent embryos develop outside the mother’s body.
– Although generation time is two to four months, the early
stages of development proceed quickly.
• By 24 hours after fertilization, most tissues and early
versions of the organs have formed.
• After two days, the fish hatches out of the egg case.
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• For studying the molecular genetics of plant development,
researchers are focusing on a small weed Arabidopsis
thaliana (a member of the mustard family).
– One plant can grow and produce thousands of progeny
after eight to ten weeks.
– A hermaphrodite, each flower makes ova and sperm.
– Easy for gene manipulation research (genetic
transformation)
– Its relatively small genome, completely sequenced.
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Feedback problems
• What organism can be the model organism?
• What is the common criteria for these model
organism?
• What is the advantages of using @ for model
organism for genetic studies?
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• Concept 21.1: Embryonic development
involves cell division, cell differentiation,
and morphogenesis
• In the embryonic development of most organisms
– A single-celled zygote gives rise to cells of
many different types, each with a different
structure and corresponding function
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• The transformation from a zygote into an
organism has three interrelated processes:
cell division, cell differentiation, and
morphogenesis
Figure 21.3a, b
(a) Fertilized eggs
of a frog
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(b) Tadpole hatching
from egg
• Through a succession of mitotic cell divisions
– The zygote gives rise to a large number of cells
• In cell differentiation
– Cells become specialized in structure and
function
• Morphogenesis (creation of form) encompasses
the processes
– give shape to the organism and its various parts
Morphogenesis = morphology + genesis
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• Embryo development in plant and animal
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Fig. 21.4
Cell division
Morphogenesis
Observable cell differentiation
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• The overall schemes of morphogenesis in animals
and plants are very different.
• In animals, movements of cells and tissues transform the
embryo.
• In plants, morphogenesis and growth in overall size are
not limited to embryonic and juvenile periods.
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• Apical meristems, perpetually embryonic regions in the
tips of shoots and roots, are responsible for the plant’s
continual growth and formation of new organs, such as
leaves and roots.
• In animals, ongoing development in adults is restricted to
the differentiation of cells, such as blood cells, that must be
continually replenished.
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• Why different cell types are found in organisms?
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• Concept 21.2: Different cell types result from
differential gene expression in cells with the
same DNA
Evidence for Genomic Equivalence
• Nearly all the cells of an organism have
genomic equivalence.
• that is, they have the same genes, regardless
of their state of differentiation.
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• The de-differentiated potential for plant and
animal.
 Totipotent vs multipotent
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Totipotency in Plants
• One experimental approach for testing genomic
equivalence
– Is to see whether a differentiated cell can
generate a whole organism
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Transverse
section of
carrot root
EXPERIMENT
Can a differentiated
plant cell develop into a
whole plant?
Figure 21.5
RESULTS
A single somatic (nonreproductive) carrot
cell developed into a mature
carrot plant. The new plant
was a genetic duplicate (clone)
of the parent plant.
2-mg
fragments
Fragments cultured in nutrient
medium; stirring causes
single cells to
shear off into
liquid.
Single cells
free in
suspension
begin to
divide.
Embryonic
plant develops
from a cultured
single cell.
Plantlet is cultured on agar
medium. Later
it is planted
in soil.
Adult plant
CONCLUSION
At least some differentiated (somatic) cells in plants are toipotent, able to
reverse their differentiation and then give rise to all the cell types in a mature
plant.
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• A totipotent cell
– Is one capable of generating a complete new
organism
• Cloning
– Is using one or more somatic cells from a
multicellular organism to make another
genetically identical individual
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Nuclear Transplantation in Animals
• In nuclear transplantation
– The nucleus of an unfertilized egg cell or
zygote is replaced with the nucleus of a
differentiated cell
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• Experiments with frog embryos
 transplanted nucleus support normal development of egg
Frog embryo
EXPERIMENT
Frog egg cell
Frog tadpole
Fully
differentiated
(intestinal) cell
Less
differentiated
cell
Researchers enucleated frog
egg cells by exposing them to
ultraviolet light, which
destroyed the nucleus.
Enucleated
egg cell
Donor
nucleus
transplanted
Nuclei from cells of embryos
up to the tadpole stage were
transplanted into the
enucleated egg cells.
Donor
nucleus
transplanted
Most develop
into tadpoles
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<2% develop
into tadpoles
Figure 21.6
RESULTS
Most of the recipient eggs developed into tadpoles when the
transplanted nuclei came from cells of an early embryo,
which are relatively undifferentiated cells. But with nuclei
from the fully differentiated intestinal cells of a tadpole, fewer
than 2% of the eggs developed into normal tadpoles, and
most of the embryos died at a much earlier developmental
stage.
CONCLUSION
With age increase
The nucleus from a differentiated frog cell can direct
development of a tadpole. However, its ability to do so
decreases as the donor cell becomes more differentiated,
presumably because of changes in the nucleus.
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• Clone & Cloning
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• Reproductive Cloning of Mammals
• In 1997, Scottish researchers
– Cloned a lamb from an adult sheep by nuclear
transplantation
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APPLICATION
This method is used to produce
cloned animals whose nuclear
genes are identical to the donor
Cultured mammary
animal supplying the nucleus.
TECHNIQUE
Shown here is the procedure
used to produce Dolly, the first
reported case of a mammal
cloned using the nucleus of a
differentiated cell.
Egg cell
donor
Mammary
cell donor
1
cells are semistarved,
arresting the cell cycle
and causing
dedifferentiation
2
Egg cell
from ovary
3 Cells fused
Nucleus
Nucleus
removed
removed
Nucleus from
mammary cell
4 Grown in culture
5 Implanted in uterus
of a third sheep
Early embryo
RESULTS
The cloned animal is identical in
appearance and genetic makeup
to the donor animal supplying the
nucleus, but differs from the egg
cell donor and surrogate mother.
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6 Embryonic
development
Lamb (“Dolly”)
genetically identical to
mammary cell donor
Surrogate
mother
Figure 21.7
• “Copy Cat”
– Was the first cat ever cloned
Cute ! vs Kill ?
Figure 21.8
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• Problems Associated with Animal Cloning
• In most nuclear transplantation studies
performed thus far
– Only a small percentage of cloned embryos
develop normally to birth
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• Stem cells
(可以跟植物Totipotent相比較的細胞)
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30 MARCH 2001 VOL 291 SCIENCE Page. 2552
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SCIENCE VOL 293 6 JULY 2001 page.95
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The Stem Cells of Animals
• A stem cell
– Is a relatively unspecialized cell
– Can reproduce itself indefinitely (renewal)
– Can differentiate into specialized cells of one
or more types, given appropriate conditions
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Embryonic stem cells
Adult stem cells
Early human embryo
at blastocyst stage
(mammalian equivalent of blastula)
From bone marrow
in this example
• Embryonic
Stem cells can
be isolated
 From early
embryos at the
blastocyst stage
• Adult stem
cells  Are said
to be pluripotent,
able to give rise to
multiple but not all
cell types
Totipotent
cells
Pluripotent
cells
Cultured
stem cells
Different culture
conditions
Liver cells
Different types of
Differentiated cells
Figure 21.9
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Nerve cells
Blood cells
Transcriptional Regulation of Gene Expression
During Development
• Cell determination
– Precedes differentiation and involves the
expression of genes for tissue-specific proteins
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Figure 21.10 Determination and differentiation of muscle cells (layer 1)
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
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OFF
OFF
Figure 21.10 Determination and differentiation of muscle cells (layer 2)
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
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OFF
OFF
mRNA
MyoD protein
(transcription
factor)
Figure 21.10 Determination and differentiation of muscle cells (layer 3)
Master control
gene myoD
Nucleus
Embryonic
precursor cell
Other musclespecific genes
DNA
OFF
OFF
1 Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
Myoblast
(determined)
OFF
mRNA
MyoD protein
(transcription
factor)
2 Differentiation. MyoD protein stimulates
the myoD gene further, and activates
genes encoding other muscle-specific
transcription factors, which in turn
activate genes for muscle proteins. MyoD
also turns on genes that block the cell
cycle, thus stopping cell division. The
nondividing myoblasts fuse to become
mature multinucleate muscle cells, also
called muscle fibers.
mRNA
MyoD
Muscle cell
(fully differentiated)
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mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Cytoplasmic Determinants and Cell-Cell Signals in
Cell Differentiation
Unfertilized egg cell
•Maternal effect genes
Fertilization
•zygote effect genes
Sperm
Molecules of a
a cytoplasmic
determinant
Molecules of
another cytoplasmic determinant
Sperm
Fertilization
Nucleus
Zygote
(fertilized egg)
Mitotic cell division
Animation:21fly
(a)
Two-celled
embryo
Cytoplasmic determinants in the egg. The unfertilized egg cell has molecules in its
cytoplasm, encoded by the mother’s genes, that influence development. Many of these
cytoplasmic determinants, like the two shown here, are unevenly distributed in the egg. After
fertilization and mitotic division, the cell nuclei of the embryo are exposed to different sets of
cytoplasmic determinants and, as a result, express different genes.
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Figure 21.11a
• In the process called
induction
 Signal molecules from
embryonic cells cause
transcriptional changes in
nearby target cells
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
Animation:21signal
(b) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are
releasing chemicals that signal nearby cells to change their gene expression.
Figure 21.11b
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• Concept 21.3: Pattern formation in animals and
plants results from similar genetic and cellular
mechanisms
• Pattern formation
– Is the development of a spatial organization of
tissues and organs
– Occurs continually in plants
Slide p.17
– Is mostly limited to embryos and juveniles in
animals
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• Positional information
– Consists of molecular cues that control pattern
formation
– Tells a cell its location relative to the body’s
axes and to other cells
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Drosophila Development: A Cascade of Gene Activations
• Pattern formation- extensively studied in Drosophila
melanogaster
The Life Cycle of Drosophila
• The fruit fly Drosophila melanogaster was first
chosen as a model organism
• The fruit fly is small and easily grown in the laboratory.
– It has a generation time of only two weeks and produces
many offspring.
– Embryos develop outside the mother’s body.
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• Key developmental events
in life cycle of Drosophila
Egg cell
developing within
ovarian
Nurse
follicle
cell
Follicle cell
Egg cell
Fertilization
Laying of egg
Fertilized egg
Egg
shell
Nucleus
positional information for the
two developmental axes before
fertilization.
Embryo
Multinucleate
single cell
1
Early blastoderm
Plasma
membrane
formation
2
Late blastoderm
3
After fertilization
Yolk
Cells of
embryo
positional information for
number of correctly oriented
segments.
finally each segment’s
characteristic structures.
Nucleus
Body
segments
4
Segmented
embryo
0.1 mm
5
Hatching
Larval stages (3)
6
Pupa
Metamorphosis
Head
7
Thorax
Abdomen
Adult fly
0.5 mm
Dorsal
Figure 21.12
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BODY
AXES
Anterior
Posterior
Ventral
Genetic Analysis of Early Development: Scientific Inquiry
Axis Establishment
• Maternal effect genes
– Encode for cytoplasmic determinants that
initially establish the axes of the body of
Drosophila
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Maternal effect genes
• Flies with the bicoid mutation
– Do not develop a body axis correctly
Tail
Head
T1
T2
T3
Figure 21.14a
A1 A2 A3 A4 A5
A6 A7
A8
Wild-type larva
Tail
Tail
A8
(a)
A7
Mutant larva (bicoid)
A8
A6
A7
Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation in the
mother’s bicoid gene leads to tail structures at both ends (bottom larva).
The numbers
toInc.
the
thoracic
andCummings
abdominal segments that are present.
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© 2005 Pearsonrefer
Education,
publishing
as Benjamin
Maternal effect gene
Egg cell
Nurse cells
Animation:
21ceembryo
1 Developing
egg cell
bicoid mRNA
2 Bicoid mRNA
in mature
unfertilized egg
Fertilization
Translation of bicoid mRNA
100 µm
3 Bicoid protein in
early embryo
Anterior end
Figure 21.14b
(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.
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Segmentation Pattern
• Segmentation genes
– Produce proteins that direct formation of
segments after the embryo’s major body axes
are formed
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• Sequential activation of three sets of
segmentation genes
1.Gap genes
2.Pair-rule genes
3.Segment polarity genes
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• Gap genes map out the basic subdivisions along
the anterior-posterior axis.
– Mutations cause “gaps” in segmentation.
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• Pair-rule genes define the modular pattern in
terms of pairs of segments.
• Mutations result in embryos with half the normal
segment number.
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• Segment polarity genes set the anteriorposterior axis of each segment.
• Mutations produce embryos with the normal segment
number, but with part of each segment replaced by a
mirror-image repetition of some other part.
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Identity of Body Parts
• The anatomical identity of Drosophila segments is set by
master regulatory genes called homeotic genes
• Mutations to homeotic genes Structures
characteristic of a particular part of the animal arise in the
wrong place.
Eye
Antenna
Leg
Wild type
Figure 21.13
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Mutant
• A summary of gene activity during Drosophila
development
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
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Segmentation genes
of the embryo
C. elegans: The Role of Cell Signaling
• The complete cell lineage  Of each cell in the
Time after fertilization (hours)
nematode roundworm C. elegans is known
Zygote
0
First cell division
Nervous
system,
outer skin,
musculature
10
Musculature,
gonads
Outer skin,
nervous system
Germ line
(future
gametes)
Musculature
Hatching
Intestine
Intestine
Eggs
ANTERIOR
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Vulva
POSTERIOR
1.2 mm
Figure 21.15
Induction
• As early as the
four-cell stage in
C. elegans
2
Anterior
Posterior
1
4
3
Receptor
Signal
protein
EMBRYO
4
3
 Cell signaling helps
direct daughter cells
down the appropriate
pathways, a process
called induction
Signal
Anterior
daughter
cell of 3
Will go on to
form muscle
and gonads
Figure 21.16a
Posterior
daughter
cell of 3
Will go on to
form adult
intestine
(a) Induction of the intestinal precursor cell at the four-cell stage.
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Epidermis
• Induction is also
critical later in
nematode
development
 As the embryo
passes through three
larval stages prior to
becoming an adult
Larva
Gonad
Signal
Anchor cell protein
Vulval precursor cells
ADULT
Outer vulva
Inner vulva
Epidermis
Figure 21.16b
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(b)
Induction of vulval cell types during larval
development.
Programmed Cell Death (Apoptosis)
生時燦如夏花
死時美若秋葉
----泰格爾
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• In apoptosis
– Cell signaling is involved in programmed cell death
normal
Figure 21.17. Apoptosis of human white blood cells.
2 µm
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outer mitochondrial membrane
Ced-9
protein (active)
inhibits Ced-4
activity
Ced-9 master regulator
of apoptosis
Ced-3  chief caspase,
the main proteases of
apoptosis
Ced-4 Ced-3
Death
signal
receptor
(a) No death signal
• Apoptosis is regulated
not at the level of
transcription or
translation, but through
changes in the activity
of proteins that are
continually present in the
cell.
Mitochondrion
Inactive proteins
Cell
forms
blebs
Ced-9
(inactive)
Death
signal
Active Active
Ced-4 Ced-3
Activation
cascade
(b) Death signal
Figure 21.18a, b
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Other
proteases
Nucleases
• In vertebrates
– Apoptosis is essential for normal
morphogenesis of hands and feet in humans
and paws in other animals
Interdigital tissue
1 mm
Figure 21.19
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Plant Development: Cell Signaling and
Transcriptional Regulation
• Thanks to DNA technology and clues from
animal research
– Plant research is now progressing rapidly
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Mechanisms of Plant Development
• In general, cell lineage
– Is much less important for pattern formation
in plants than in animals
• The embryonic development of most plants
– Occurs inside the seed
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Pattern Formation in Flowers
• Floral meristems
– Contain three cell types (L1-3) that affect
flower development
Stamen
Carpel
Petal
Cell
layers
L1
L2
L3
Sepal
Floral meristem
Figure 21.20
Anatomy of a flower
flower with
four types of organs
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Tomato flower
• Tomato plants with a mutant allele
– Have been studied in order to understand the
genetic mechanisms behind flower development
Graft
Chimeras
EXPERIMENT
Tomato plants with the fasciated (ff )
mutation develop extra floral organs.
Sepal
Petal
Carpel
Stamen
Wild-type
normal
Fasciated (ff)
extra organs
For each chimera, researchers recorded the
flower phenotype: wild-type or fasciated.
Researchers grafted stems from mutant
Analysis using other genetic markers identified
plants onto wild-type plants. They then
the parental source for each of the three cell
planted the shoots that emerged near the
layers of the floral meristem (L1–L3) in the
graft site, many of which were chimeras.
chimeras.
Figure 21.21
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RESULTS
The flowers of the chimeric plants had the fasciated phenotype only
when the L3 layer came from the fasciated parent.
L1 L2
Key
Wild-type
Fasciated (ff)
Plant
Flower
L3
Floral
meristem
Phenotype
Wild-type
parent
Wild-type
Fasciated (ff)
parent
Fasciated
Chimera 1
Fasciated
Chimera 2
Fasciated
Floral Meristem
參考
The number of organs
per flower depends
on genes of the L3
CONCLUSION Cells in the L3 layer induce the L1 and L2 layers to
layer organ
form flowers with a particular number of organs.
identity genes
Chimera 3
Wild-type
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• Organ identity genes
– Determine the type of structure that will grow
from a meristem
– Are analogous to homeotic genes in animals
Figure 21.22
Wild type
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Mutant
• Concept 21.4: Comparative studies help
explain how the evolution of development leads
to morphological diversity
• Biologists in the field of evolutionary
developmental biology, or “evo-devo,” as it is often
called
– Compare developmental processes of
different multicellular organisms
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Widespread Conservation of Developmental Genes
Among Animals
• All homeotic genes of Drosophila include a
180-nucleotide sequence called the homeobox,
which specifies a 60-amino-acid homeodomain,
part of a transcription factor.
• Homeobox-containing genes often called Hox
genes, especially in mammals conserved in
animals for hundreds of millions of years.
• Related sequences are present in yeast, prokaryotes, human …etc.
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• An identical or very
similar nucleotide
sequence in the
homeotic genes of
both vertebrates
and invertebrates
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Figure 21.23
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• Not all, homeobox-containing genes are
homeotic genes that are associated with
development. some don’t directly control the
identity of body parts.
• For example, in Drosophila, homeoboxes are present not
only in the homeotic genes but also in the egg-polarity
gene bicoid, in several segmentation genes, and in the
master regulatory gene for eye development.
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Effect of differences in Hox gene expression
• In some cases, small changes in regulatory
sequences of particular genes can lead to
major changes in body form.
Thorax
Genital
segments
Abdomen
crustaceans
Thorax
Abdomen
insects
Figure 21.24
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• In other cases
– Genes with conserved sequences play
different roles in the development of different
species
• In plants
– Homeobox-containing genes do not function in
pattern formation as they do in animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Comparison of Animal and Plant Development
• In both plants and animals
– Development relies on a cascade of
transcriptional regulators turning genes on or
off in a finely tuned series
• But the genes that direct analogous
developmental processes
– Differ considerably in sequence in plants and
animals, as a result of their remote ancestry
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings