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Chapter 21
The Genetic Basis
of Development
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: From Single Cell to Multicellular
Organism
• Genetic analysis and DNA technology have
revolutionized the study of development
• Researchers use mutations to deduce
developmental pathways
• They apply concepts and tools of molecular
genetics to the study of developmental biology
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• When the primary research goal is to understand
broad biological principles, the organism chosen
for study is called a model organism
• Researchers select model organisms that are
representative of a larger group, suitable for the
questions under investigation, and easy to grow in
the lab
Video: C. elegans Crawling
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 21.1: Embryonic development involves cell
division, cell differentiation, and morphogenesis
• In 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
• Development involves three processes: cell
division, cell differentiation, and morphogenesis
(“creation of form”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-3
Fertilized egg of a frog
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 encompasses the processes that
give shape to the organism and its various parts
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-4
Animal development
Cell
movement
Zygote
(fertilized egg)
Eight cells
Gut
Blastula
Gastrula
Adult animal
(cross section) (cross section)
(sea star)
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Plant development
Zygote
(fertilized egg)
Two cells
Shoot
apical
meristem
Root
apical
meristem
Embryo
inside seed
Plant
Concept 21.2: Different cell types result from
differential gene expression in cells with the same DNA
• Differences between cells in a multicellular
organism come almost entirely from gene
expression, not differences in the cells’ genomes
• These differences arise during development, as
regulatory mechanisms turn genes off and on
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Evidence for Genomic Equivalence
• Many experiments support the conclusion that
nearly all cells of an organism have genomic
equivalence (the same genes)
• A key question that emerges is whether genes are
irreversibly inactivated during differentiation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Totipotency in Plants
• One experimental approach for testing genomic
equivalence is to see whether a differentiated cell
can generate a whole organism
• A totipotent cell is one that can generate a
complete new organism
• Cloning is using one or more somatic cells from a
multicellular organism to make a genetically
identical individual
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-5
Transverse
section of
carrot root
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.
A single somatic (nonreproductive)
carrot cell developed into a
mature carrot plant. The new
plant was a genetic duplicate
(clone) of the parent plant.
Adult plant
Plantlet is cultured on agar
medium. Later
it is planted
in soil.
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
• Experiments with frog embryos have shown that a
transplanted nucleus can often support normal
development of the egg
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-6
Frog embryo
Frog egg cell
Frog tadpole
UV
Fully differentiated
(intestinal) cell
Less differentiated cell
Donor
nucleus
transplanted
Most develop
into tadpoles
Enucleated
egg cell
Donor
nucleus
transplanted
<2% develop
into tadpoles
Reproductive Cloning of Mammals
• In 1997, Scottish researchers announced the birth
of Dolly, a lamb cloned from an adult sheep by
nuclear transplantation from a differentiated
mammary cell
• Dolly’s premature death in 2003, as well as her
arthritis, led to speculation that her cells were
“older” than those of a normal sheep, possibly
reflecting incomplete reprogramming of the
original transplanted nucleus
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-7
Mammary
cell donor
Egg cell
donor
Egg cell
from ovary
Cultured
mammary cells
are semistarved,
arresting the cell
cycle and causing
dedifferentiation
Nucleus
removed
Cells fused
Nucleus from
mammary cell
Grown in culture
Early embryo
Implanted in uterus
of a third sheep
Surrogate
mother
Embryonic
development
Lamb (“Dolly”) genetically identical
to mammary cell donor
• Since 1997, cloning has been demonstrated in
many mammals, including mice, cats, cows,
horses, and pigs
• “Copy Cat” was the first cat cloned
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Problems Associated with Animal Cloning
• In most nuclear transplantation studies, only a
small percentage of cloned embryos have
developed normally to birth
• Many epigenetic changes, such as acetylation of
histones or methylation of DNA, must be reversed
in the nucleus from a donor animal in order for
genes to be expressed or repressed appropriately
for early stages of development
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Stem Cells of Animals
• A stem cell is a relatively unspecialized cell that
can reproduce itself indefinitely and differentiate
into specialized cells of one or more types
• Stem cells isolated from early embryos at the
blastocyst stage are called embryonic stem cells
• The adult body also has stem cells, which replace
nonreproducing specialized cells
• Embryonic stem cells are totipotent, able to
differentiate into all cell types
• Adult stem cells are pluripotent, able to give rise to
multiple but not all cell types
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-9
Embryonic stem cells
Totipotent
cells
Adult stem cells
Pluripotent
cells
Cultured
stem cells
Different
culture
conditions
Different
Liver cells
types of
differentiated
cells
Nerve cells
Blood cells
Transcriptional Regulation of Gene Expression
During Development
• Cell determination precedes differentiation and
involves expression of genes for tissue-specific
proteins
• Tissue-specific proteins enable differentiated cells to
carry out their specific tasks
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-10_1
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
OFF
OFF
LE 21-10_2
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
OFF
OFF
mRNA
OFF
Determination
Myoblast
(determined)
MyoD protein
(transcription
factor)
LE 21-10_3
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
OFF
OFF
mRNA
OFF
Determination
MyoD protein
(transcription
factor)
Myoblast
(determined)
Differentiation
mRNA
MyoD
Muscle cell
(fully differentiated)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Cytoplasmic Determinants and Cell-Cell Signals in
Cell Differentiation
• Maternal substances that influence early
development are called cytoplasmic determinants
• These substances regulate expression of genes
that affect the cell’s developmental fate
Animation: Cell Signaling
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-11a
Unfertilized egg cell
Sperm
Molecules of another
cytoplasmic determinant
Molecules of a
Nucleus
cytoplasmic
Fertilization
determinant
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
Cytoplasmic determinants in the egg
• The other important source of developmental
information is the environment around the cell,
especially signals from nearby embryonic cells
• In the process called induction, signal molecules
from embryonic cells cause transcriptional
changes in nearby target cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-11b
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
Induction by nearby cells
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
• It occurs continually in plants, but it is mostly
limited to embryos and juveniles in animals
• Positional information, the molecular cues that
control pattern formation, tells a cell its location
relative to the body axes and to neighboring cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Drosophila Development: A Cascade of Gene
Activations
• Pattern formation has been extensively studied in
the fruit fly Drosophila melanogaster
• Combining anatomical, genetic, and biochemical
approaches, researchers have discovered
developmental principles common to many other
species, including humans
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Life Cycle of Drosophila
• After fertilization, positional information specifies
the body segments in Drosophila
• Positional information triggers the formation of
each segment’s characteristic structures
• Sequential gene expression produces regional
differences in the formation of the segments
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-12
Follicle cell
Egg cell
developing within
Nucleus
ovarian follicle
Egg cell
Nurse
cell
Fertilization
Laying of egg
Fertilized egg
Egg shell
Nucleus
Embryo
Multinucleate
single cell
Early blastoderm
Plasma
membrane
formation
Yolk
Late blastoderm
Body
segments
Cells of
embryo
Segmented
embryo
0.1 mm
Hatching
Larval stages (3)
Pupa
Metamorphosis
Adult fly
Head Thorax
Abdomen
0.5 mm
Dorsal
BODY
AXES
Anterior
Posterior
Ventral
Genetic Analysis of Early Development: Scientific
Inquiry
• Study of developmental mutants laid the
groundwork for understanding the mechanisms of
development
• Mutations that affect segmentation are likely to be
embryonic lethals, leading to death at the
embryonic or larval stage
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-13
Eye
Leg
Antenna
Wild type
Mutant
Axis Establishment
• Maternal effect genes encode for cytoplasmic
determinants that initially establish the axes of the
body of Drosophila
• These maternal effect genes are also called eggpolarity genes because they control orientation of
the egg and consequently the fly
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• One maternal effect gene, the bicoid gene, affects
the front half of the body
• An embryo whose mother has a mutant bicoid
gene lacks the front half of its body and has
duplicate posterior structures at both ends
• This phenotype suggests that the product of the
mother’s bicoid gene is concentrated at the future
anterior end
• This hypothesis is an example of the gradient
hypothesis, in which gradients of substances
called morphogens establish an embryo’s axes
and other features
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-14a
Tail
Head
Wild-type larva
Tail
Tail
Mutant larva (bicoid)
Drosophila larvae with wild-type and bicoid mutant
phenotypes
LE 21-14b
Nurse cells
Egg cell
Developing
egg cell
bicoid mRNA
Bicoid mRNA
in mature
unfertilized
egg
Fertilization
Translation of bicoid mRNA
100 m m
Bicoid protein
in early
embryo
Anterior end
Gradients of bicoid mRNA and Bicoid protein in normal
egg and early embryo
Animation: Development of Head-Tail Axis in Fruit Flies
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The bicoid research is important for three reasons:
1. It identified a specific protein required for some
early steps in pattern formation
2. It increased understanding of the mother’s role
in embryo development
3. It demonstrated a key developmental principle
that a gradient of molecules can determine
polarity and position in the embryo
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Segmentation Pattern
• Segmentation genes produce proteins that direct
formation of segments after the embryo’s major
body axes are formed
• Positional information is provided by sequential
activation of three sets of segmentation genes:
gap genes, pair-rule genes, and segment-polarity
genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Identity of Body Parts
• The anatomical identity of Drosophila segments is
set by master regulatory genes called homeotic
genes
• Mutations to homeotic genes produce flies with
strange traits, such as legs growing from the head
in place of antennae
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
C. elegans: The Role of Cell Signaling
• The nematode C. elegans is a very useful model
organism for investigating the roles of cell
signaling, induction, and programmed cell death in
development
• Researchers know the entire ancestry of every cell
of an adult C. elegans—the organism’s complete
cell lineage
Video: C. elegans Embryo Development (time lapse)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-15
Zygote
Time after fertilization (hours)
0
First cell division
Nervous
system,
outer
skin, musculature
Musculature,
gonads
Outer skin,
nervous system
Germ line
(future
gametes)
Musculature
10
Hatching
Intestine
Intestine
Eggs
Vulva
ANTERIOR
POSTERIOR
1.2 mm
Induction
• As early as the four-cell stage in C. elegans, cell
signaling helps direct daughter cells down the
appropriate pathways, a process called induction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-16a
2
Anterior
1
4
Posterior
Signal
Receptor protein
3
EMBRYO
4
3
Signal
Anterior
daughter
cell of 3
Will go on to
form muscle
and gonads
Posterior
daughter
cell of 3
Will go on to
form adult
intestine
Induction of the intestinal precursor cell at the four-cell stage
• Induction is also critical later in nematode
development, as the embryo passes through three
larval stages prior to becoming an adult
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-16b
Epidermis
LARVA
Gonad
Anchor
cell
Signal
protein
Vulval precursor cells
ADULT
Inner vulva
Outer vulva
Epidermis
Induction of vulval cell types during larval development
• A number of important concepts apply to C.
elegans and many other animals:
– In the developing embryo, sequential
inductions drive organ formation
– The effect of an inducer can depend on its
concentration
– Inducers produce their effects via signal
transduction pathways, as in adult cells
– The induced cell often responds by activating
genes that establish a pattern of gene activity
characteristic of a particular kind of cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Programmed Cell Death (Apoptosis)
• Cell signaling is involved in apoptosis,
programmed cell death
• During apoptosis, a cell shrinks and becomes
lobed (called “blebbing”); the nucleus condenses;
and the DNA is fragmented
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-17
2 mm
• In C. elegans, a protein in the outer mitochondrial
membrane is a master regulator of apoptosis
• Research on mammals has revealed a prominent
role for mitochondria in apoptosis
• A built-in cell suicide mechanism is essential to
development in all animals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-18
Ced-9 protein (active)
inhibits Ced-4 activity
Mitochondrion
Death
signal
receptor
Ced-4 Ced-3
Inactive proteins
No death signal
Cell
forms
blebs
Ced-9
(inactive)
Death
signal
Active Active
Ced-4 Ced-3
Activation
cascade
Death signal
Other
proteases
Nucleases
• The timely activation of apoptosis proteins in some
cells functions during normal development and
growth in both embryos and adult
• In vertebrates, apoptosis is part of normal
development of the nervous system, operation of
the immune system, and morphogenesis of hands
and feet in humans and paws in other mammals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-19
Interdigital tissue
1 mm
Plant Development: Cell Signaling and
Transcriptional Regulation
• Genetic analysis of plant development, using
model organisms such as Arabidopsis, has lagged
behind that of animal models because fewer
researchers work on plants
• Plant research is now progressing rapidly, thanks
to DNA technology and clues from animal
research
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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, and thus is relatively inaccessible
to study
• However, other important aspects of plant
development are observable in plant meristems,
particularly apical meristems at the tips of shoots
• In meristems, cell division, morphogenesis, and
differentiation give rise to new organs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Pattern Formation in Flowers
• A floral meristem has three layers of cells (L1-L3),
all of which participate in forming a flower with four
types of organs:
carpels (containing egg cells)
stamens (containing sperm-bearing pollen)
petals
sepals (leaflike structures outside the petals)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-20
Carpel
Stamen
Cell
layers
Petal
L1
L2
L3
Sepal
Floral meristem
Anatomy of a flower
Tomato flower
• To examine induction of the floral meristem,
researchers grafted stems from a mutant tomato
plant onto a wild-type plant and then grew new
plants from the shoots at the graft sites
• The new plants were chimeras, organisms with a
mixture of genetically different cells
• The number of organs per flower depended on
genes of the L3 (innermost) cell layer
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-21
Sepal
Petal
Carpel
Stamen
Fasciated (ff),
extra organs
Wild-type,
normal
Graft
Chimeras
L1
Key
Wild-type (FF)
Fasciated (ff)
Plant
Flower
L2
L3
Floral
meristem
Phenotype
Wild-type
parent
Wild-type
Fasciated (ff)
parent
Fasciated
Chimera 1
Fasciated
Chimera 2
Fasciated
Chimera 3
Wild-type
Floral Meristem
• In contrast to genes controlling organ number in
flowers, genes controlling organ identity (organ
identity genes) determine the types of structures
that will grow from a meristem
• Organ identity genes are analogous to homeotic
genes in animals
• Mutations cause plant structures to grow in
unusual places, such as carpels in place of sepals
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-22
Wild type
Mutant
Concept 21.4: Comparative studies help explain how the
evolution of development leads to morphological diversity
• Evolutionary developmental biology (“evo-devo”)
compares developmental processes of different
multicellular organisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Widespread Conservation of Developmental Genes
Among Animals
• Molecular analysis of the homeotic genes in
Drosophila has shown that they all include a
sequence called a homeobox
• An identical or very similar nucleotide sequence
has been discovered in the homeotic genes of
both vertebrates and invertebrates
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-23
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
• Related genetic sequences have been found in
regulatory genes of yeasts, plants, and even
prokaryotes
• In addition to developmental genes, many other
genes are highly conserved from species to
species
• Sometimes small changes in regulatory
sequences of certain genes lead to major changes
in body form, as in crustaceans and insects
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 21-24
Thorax
Thorax
Genital
segments
Abdomen
Abdomen
• In other cases, genes with conserved sequences
play different roles in 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 genes that direct analogous developmental
processes differ considerably in sequence in
plants and animals, as a result of ancestry
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings