The Genetic Basis of Development

1
Chapter 21

• The Genetic Basis of Development

2
Model Organisms
3
Embryonic Development

• Cell Division
• Cell differentiation
• Morphogenesis
• Body plan and body axes
• Animals Movements of cells and tissues
• Plants Apical meristems

4
LE 21-4
Animal development
Gut
Cell movement
Zygote (fertilized egg)
Eight cells
Blastula (cross section)
Gastrula (cross section)
Adult animal (sea star)
Cell division
Morphogenesis
Observable cell differentiation
Seed leaves
Plant development
Shoot apical meristem
Root apical meristem
Two cells
Zygote (fertilized egg)
Embryo inside seed
Plant
5
Differential Gene Expression

• Genomic equivalence?
• Totipotency in plants Fig, 21.5
• Cells dedifferentiated
• Dedifferentiation in animal cells?
• Cloning

6
LE 21-5
Transverse section of carrot root
2-mg fragments
Fragments cul- tured in nutrient medium
stir- ring 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 cul- tured on agar medium. Later it
is planted in soil.
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
7
LE 21-6
Frog embryo
Frog egg cell
Frog tadpole
UV
Fully differ- entiated (intestinal) cell
Less differ- entiated cell
Donor nucleus trans- planted
Donor nucleus trans- planted
Enucleated egg cell
Most develop into tadpoles
lt2 develop into tadpoles
8
LE 21-7
Mammary cell donor
Egg cell donor
Egg cell from ovary
Nucleus removed
Cells fused
Cultured mammary cells are semistarved, arresting
the cell cycle and causing dedifferentiation
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
9
Stem Cells in Animals

• Stem cells
• Embryonic
• Totipotent
• Adult
• Pluripotent stem cells

10
LE 21-9
Embryonic stem cells
Adult stem cells
Pluripotent cells
Totipotent cells
Cultured stem cells
Different culture conditions
Different types of differentiated cells
Liver cells
Nerve cells
Blood cells
11
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

12
LE 21-10_3
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic precursor cell
OFF
OFF
Determination
mRNA
OFF
MyoD protein (transcription factor)
Myoblast (determined)
Differentiation
mRNA
mRNA
mRNA
mRNA
Myosin, other muscle proteins, and
cell-cycle blocking proteins
MyoD
Another transcription factor
Muscle cell (fully differentiated)
13
LE 21-11a
Unfertilized egg cell
Sperm
Molecules of another cytoplasmic determinant
Molecules of a cytoplasmic determinant
Nucleus
Fertilization
Zygote (fertilized egg)
Mitotic cell division
Two-celled embryo
Cytoplasmic determinants in the egg
14
LE 21-11b
Early embryo (32 cells)
Signal transduction pathway
NUCLEUS
Signal receptor
Signal molecule (inducer)
Induction by nearby cells
15
Pattern Formation

• Pattern formation the body plan
• Positional information
• Cytoplasmic determinants
• Induction

16
LE 21-12a
Follicle cell
Egg cell developing within ovarian follicle
Nucleus
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
17
LE 21-12b
Hatching
Larval stages (3)
Pupa
Metamorphosis
Thorax
Head
Abdomen
Adult fly
0.5 mm
Dorsal
BODY AXES
Posterior
Anterior
Ventral
18
LE 21-13
Eye
Leg
Antenna
Wild type
Mutant
19
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

20
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
21
Table 21-UN425
22
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 Ced-4
Active Ced-3
Other proteases
Nucleases
Activation cascade
Death signal
23
Morphological Diversity

• Widespread conservation of developmental genes
  among animals
• Homeobox ? homeodomain
• Not only present in Hox genes

24
LE 21-23a
Adult fruit fly
Fruit fly embryo (10 hours)
Fly chromosome
25
LE 21-23b
Mouse chromosomes
Mouse embryo (12 days)
Adult mouse