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Ch. 21: The Genetic Basis of
Development
The development of an organism
from a zygote to an embryo to an
adult is a delicate process that
involves much genetic control!
Chapter 21: The Genetic Basis of Development
1. How do we study development in the genetics-based lab?
-Model organisms
-fruit fly, nematode worm, mouse, etc.
Figure 21.2 Model Organisms for Genetic Studies of Development
DROSOPHILA MELANOGASTER
(FRUIT FLY)
Drosophila
- small, easy & cheap to culture
- 2 week generation time
- 4 chromosomes
- LARGE literature of info
CAENORHABDITIS ELEGANS
(NEMATODE)
C elegans
- easy to culture
- transparent body with few cell types
- zygote to mature adult in 3 days
0.25 mm
Chapter 21: The Genetic Basis of Development
-The complete cell lineage of the C. elegans nematode is known…
Zygote
0
Time after fertilization (hours)
First cell division
Nervous
system,
outer
skin, musculature
Outer skin,
nervous system
Musculature,
gonads
Germ line
(future
gametes)
Musculature
10
Hatching
Intestine
Intestine
Eggs
Vulva
ANTERIOR
POSTERIOR
1.2 mm
ARABIDOPSIS THAMANA
(COMMON WALL CRESS)
MUS MUSCULUS
(MOUSE)
Mouse
- vertebrate
- LARGE literature
- transgenics & knock-outs
DANIO RERIO
(ZEBRAFISH)
Chapter 21: The Genetic Basis of Development
2. How does a zygote transform into an organism?
1) Cell division
2) Cell differentiation
3) Morphogenesis—”creation of form/shape”
Figure 21.3a, b
(a) Fertilized eggs
of a frog
(b) Tadpole hatching
from egg
(a) Animal development. Most
animals go through some
variation of the blastula and
gastrula stages. The blastula is
a sphere of cells surrounding a
fluid-filled cavity. The gastrula
forms when a region of the blastula
folds inward, creating a
tube—a rudimentary gut. Once
the animal is mature,
differentiation occurs in only a
limited way—for the replacement
of damaged or lost cells.
Cell
movement
Zygote
(fertilized egg)
Eight cells
Blastula
(cross section)
Gut
Gastrula
(cross section)
Adult animal
(sea star)
Cell division
Morphogenesis
(b) Plant development. In plants
with seeds, a complete embryo
develops within the seed.
Morphogenesis, which involves
cell division and cell wall
expansion rather than cell or
tissue movement, occurs
throughout the plant’s lifetime.
Apical meristems (purple)
continuously arise and develop
into the various plant organs as
the plant grows to an
indeterminate size.
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Zygote
(fertilized egg)
Root
apical
meristem
Two cells
Figure 21.4a, b
Embryo
inside seed
Plant
Chapter 21: The Genetic Basis of Development
1. How do we study development in the genetics-based lab?
2. How does a zygote transform into an organism?
3. How do cells become differentiated?
-All cells have the same DNA, so differential gene expression
must be the explanation!
• -Once a cell is differentiated, it’s difficult to “de-differentiate.”
EXPERIMENT Researchers enucleated frog egg cells by exposing them to ultraviolet light, which
destroyed the nucleus. Nuclei from cells of embryos up to the tadpole stage were transplanted into the
enucleated egg cells.
Frog embryo
Frog egg cell
Fully differentiated
(intestinal) cell
Less differentiated cell
Donor
nucleus
transplanted
Figure 21.6
Most develop
into tadpoles
Frog tadpole
Enucleated
egg cell
Donor
nucleus
transplanted
<2% develop
into tadpoles
However, plants behave differently…
Transverse
section of
carrot root
EXPERIMENT
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.
A single
RESULTS
Somatic (nonreproductive) carrot
cell developed into a mature carrot
plant. The new plant was a genetic
duplicate(clone) of the parent plant.
Adult plant
CONCLUSION At least some differentiated (somatic) cells in plants are totipotent, able
to reverse their differentiation and then give rise to all the cell types in a mature plant.
Figure 21.5
APPLICATION This method is used to produce cloned
animals whose nuclear genes are identical to the donor
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.
The cloned animal is identical in appearance
RESULTS
and genetic makeup to the donor animal supplying the nucleus,
but differs from the egg cell donor and surrogate mother.
Egg cell
donor
Mammary
cell donor
1
2
Egg cell
from ovary Nucleus
Nucleus
removed
Cells
fused
3
removed
Cultured
mammary cells
are semistarved,
arresting the cell
cycle and causing
dedifferentiation
4 Grown in culture
Nucleus from
mammary cell
Early embryo
5 Implanted in uterus
of a third sheep
6 Embryonic
development
Figure 21.7
Surrogate
mother
Lamb (“Dolly”)
genetically identical to
mammary cell donor
Chapter 21: The Genetic Basis of Development
4. What is a stem cell?
-a relatively unspecialized cell
-can differentiate into cells of different types under specific conditions
-Embryonic = totipotent
-Adult = pluripotent (can produce some, but not all, cell types)
Embryonic stem cells
Early human embryo
at blastocyst stage
(mammalian equivalent of blastula)
Adult stem cells
From bone marrow
in this example
Totipotent
cells
Pluripotent
cells
Cultured
stem cells
Different
culture
conditions
Different
types of
differentiated
cells
Figure 21.9
Liver cells
Nerve cells
Blood cells
Chapter 21: The Genetic Basis of Development
5. What type of genetic signal leads to cell differentiation?
-Step 1: Cell receives signals from other cells
-Step 2: A regulatory gene is turned “on”, and a protein is made that
activates other genes. (“point of no return”)
-Step 3: Activated genes make proteins that determine cell type/
structure/behavior.
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
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
Myoblast
(determined) muscle cell.
OFF
1
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.
OFF
mRNA
MyoD protein
(transcription
factor)
mRNA
MyoD
Muscle cell
(fully differentiated)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Chapter 21: The Genetic Basis of Development
6. What is induction?
-signal molecules (proteins or hormones) from embryonic cells cause
trancriptional changes in nearby target cells.
2
Anterior
Posterior
1
4
3
Receptor
Signal
protein
EMBRYO
4
(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.
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
(a) Induction of the intestinal precursor cell at the four-cell stage.
Epidermis
Gonad
Anchor cell
Signal
protein
Vulval precursor cells
Outer vulva
ADULT
Inner vulva
Epidermis
Figure 21.16b
(b) Induction of vulval cell types during larval
development.
EXPERIMENT
Spemann and Mangold transplanted a piece of the dorsal lip of a pigmented newt gastrula to the
ventral side of the early gastrula of a nonpigmented newt.
Pigmented gastrula
(donor embryo)
Dorsal lip of
blastopore
Nonpigmented gastrula
(recipient embryo)
RESULTS
During subsequent development, the recipient embryo formed a second notochord and neural tube in
the region of the transplant, and eventually most of a second embryo. Examination of the interior of the double embryo
revealed that the secondary structures were formed in part from host tissue.
Primary embryo
Primary
structures:
Secondary
structures:
Notochord (pigmented cells)
Secondary (induced) embryo
Neural tube
Notochord
Neural tube (mostly nonpigmented cells)
CONCLUSION
Figure 47.25
The transplanted dorsal lip was able to induce cells in a different region of the recipient to form
structures different from their normal fate. In effect, the dorsal lip “organized” the later development of an entire embryo.
Chapter 21: The Genetic Basis of Development
-cytoplasmic determinants in the unfertilized egg regulate gene
expression in the zygote that affects differentiation/development
Unfertilized egg cell
Molecules of a
a cytoplasmic
determinant
Fertilization
Nucleus
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
(a)
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.
Figure 21.11a
Chapter 21: The Genetic Basis of Development
-Cytoplasmic determinants from mother’s egg initially establish the axes of
the body of Drosophila.
-bicoid gene
Tail
Head
T1 T2
T3
A1 A2 A3 A4 A5
A6 A7
A8
Wild-type larva
Tail
Tail
A8
A7
Mutant larva (bicoid)
A8
A6
A7
(a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation
Figure 21.14a
in the mother’s bicoid gene leads to tail structures at both ends (bottom larva).
The numbers refer to the thoracic and abdominal segments that are present.
Egg cell
Nurse cells
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
(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo.
Figure 21.14b
Chapter 21: The Genetic Basis of Development
-7. How does morphogenesis (pattern formation) occur in animals?
After the body’s axes are determined (by cytoplasmic determinants)…
-Segmentation genes produce proteins that direct formation of
body segments.
-Then, the development of specific features of the body segments is directed
by HOMEOTIC GENES (Hox genes.)
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segmentation genes
of the embryo
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
Chapter 21: The Genetic Basis of Development
8. What is the relationship among the genetic basis of development
across organisms?
-Molecular analysis of the homeotic genes in Drosophila has shown that they
all include a sequence called a homeobox
-An identical (or very similar) DNA sequence has been discovered in the
homeotic genes of vertebrates and invertebrates
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Figure 21.23
Chapter 21: The Genetic Basis of Development
9. What is apoptosis?
-programmed cell death (cell suicide)
Ced-9
protein (active)
inhibits Ced-4
activity
Death
signal
receptor
Mitochondrion
Ced-4 Ced-3
Inactive proteins
Cell
forms
blebs
(a) No death signal
Ced-9
(inactive)
Death
signal
Active Active
Ced-4 Ced-3
Activation
cascade
(b) Death signal
Other
proteases
Nucleases
Figure 21.18 Molecular
basis of apoptosis in C.
elegans
Chapter 21: The Genetic Basis of Development
8. What is apoptosis?
-programmed cell death (cell suicide)
-necessary for development of hands/feet in vertebrates
Interdigital tissue
1 mm
Figure 21.19