Chapter 21
The Genetic Basis of
Development
Model Organisms
Embryonic Development
Cell Division
Cell differentiation
Morphogenesis
– Body plan and body axes
– Animals: Movements of cells and tissues
– Plants: Apical meristems
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
Differential Gene Expression
Genomic equivalence?
Totipotency in plants – Fig, 21.5
– Cells dedifferentiated
Dedifferentiation in animal cells?
– Cloning
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.
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
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
Stem Cells in Animals
Stem cells
Embryonic
– Totipotent
Adult
– Pluripotent stem cells
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
Differentiation
Transcriptional regulation of gene
expression during development
– Determination and tissue-specific proteins
– Master regulatory genes
Cytoplasmic determinants and cell-cell
signals
– Cytoplasmic determinants
– Induction
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
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
LE 21-11b
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
Induction by nearby cells
Pattern Formation
Pattern formation – the body plan
Positional information
– Cytoplasmic determinants
– Induction
LE 21-12a
Follicle cell
Egg cell
developing within
ovarian follicle
Nurse
cell
Fertilized egg
Nucleus
Nucleus
Egg cell
Fertilization
Laying of egg
Egg shell
Embryo
Multinucleate
single cell
Early blastoderm
Plasma
membrane
formation
Yolk
Late blastoderm
Cells of
embryo
Segmented
embryo
Body
segments
0.1 mm
LE 21-12b
Hatching
Larval stages (3)
Pupa
Metamorphosis
Head Thorax
Abdomen
Adult fly
0.5 mm
Dorsal
BODY
AXES
Anterior
Posterior
Ventral
LE 21-13
Eye
Leg
Antenna
Wild type
Mutant
Cell Signaling: Body Plan
Axis establishment
– Maternal effect genes (aka, egg-polarity
genes)
– Morphogens
Segmentation pattern – genes
Identity of body parts
– Homeotic genes (Hox genes)
Apoptosis
LE 21-14
Tail
Head
Wild-type larva
Tail
Tail
Mutant larva (bicoid)
Drosophila larvae with wild-type and bicoid
mutant phenotypes
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
Table 21-UN425
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
Morphological Diversity
Widespread conservation of
developmental genes among animals
– Homeobox  homeodomain
– Not only present in Hox genes
LE 21-23a
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
LE 21-23b
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse