1. Embryonic Development
Timing and coordination of gene activation

2. Genetic Basis of Development
Figure 21.8
Agenda
Turn in your take-home quiz (Place on music stand)
Timing and coordination of development
Cytoplasmic Determinants and the maternal effect
Induction through cell signaling
Homeotic Genes and segment determination
Apoptosis
Science Skills Practice (Homework if we don’t get to it)
“Copy Cat”

3. Cell Division - Morphogenesis –Differentiation
Figure 21.4a, b
Animal development. Formation of three germ layers, body cavity, gut, and nervous system.
Cells actively migrate during development.
Certain cells in each developed tissue remain as partially differentiated stem cells to replace cells that are old or damaged.
Plant development. Morphogenesis involves cell division and selective cell expansion. Cells cannot move.
The apical meristems located in the roots and shoots remain undifferentiated throughout the plants life for growth.
Zygote
(fertilized egg)
Eight cells
Blastula
(cross section)
Gastrula
Adult animal
(sea star)
Cell
movement
Gut
Cell division
Morphogenesis
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Root
Plant
Embryo
inside seed
Two cells
(a)
(b)

4. Evidence that all cells in a developed organism have all the genetic material
Figure 21.6
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
Frog tadpole
Less differentiated cell
Donor nucleus
Transplanted
Enucleated
egg cell
Fully differentiated
(intestinal) cell
transplanted
Most develop
into tadpoles
<2% develop
Why are the fully differentiated cells less successful in developing into tadpoles?

5. Timing and coordination of development
Cells specialize by activating master control genes
What is the function of master control genes?
DNA
OFF
mRNA
Another transcription factor
MyoD
Muscle cell (fully differentiated)
MyoD protein (transcription factor)
Myoblast (determined)
Embryonic precursor cell
Myosin, other muscle proteins, and cell-cycle blocking proteins
Other muscle-specific genes
Master control gene myoD
Nucleus 1 2

6. How do cells know which master control genes to activate during differentiation and morphogenesis?
Cytoplasmic Determinants and the maternal effect
Induction by cell signaling

7. Induction: Cell signaling changes gene expression
Figure 21.11b
Signal
transduction
pathway
receptor
molecule
(inducer)
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.
NUCLEUS
Early embryo
(32 cells)
Cell signals made by cells early on in in differentiation
Affect transcription factors in the nucleus of cells near by
Change gene expression of target cell

8. Pattern Formation in Drosophila
Translation of bicoid mRNA
Fertilization
Nurse cells
Egg cell
bicoid mRNA
Developing
egg cell
Bicoid mRNA
in mature
unfertilized egg
100 µm
Bicoid protein in
early embryo
Anterior end
(b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo. 1 2 3
Figure 21.14b
Bicoid mRNA is placed in the egg cell by nurse cells (maternal effect)
There is a gradient of Bicoid mRNA and Protein
What experimental evidence suggests that high Bicoid protein concentration causes Anterior (head) segments develop?

9. C. elegans- a model of induction
What experimental evidence showed that cell to cell signaling or cell to cell contact between adjacent cells was essential for correct differentiation and morphogenesis? 4
Anterior
EMBRYO
Posterior
Receptor
Signal
protein
daughter
cell of 3
Will go on to
form muscle
and gonads
form adult
intestine 1 2 3
Induction of the intestinal precursor cell at the four-cell stage.
(a)

10. Cytoplasmic Determinants
Figure 21.11a
Unfertilized egg cell
Molecules of a
a cytoplasmic
determinant
Fertilization
Zygote
(fertilized egg)
Mitotic cell division
Two-celled
embryo
Nucleus
mRNA, protein or other signaling molecules in the cytoplasm of the unfertilized egg
Unevenly distributed
Mitosis creates cells with different sets of cytoplasmic determinants
How might these cytoplasmic determinants regulate gene expression?
Molecules of
another cyto-
plasmic deter-
minant

11. Homeotic Genes
Homeotic genes are regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis.
These seem to be the master genes of development
Mutant with legs growing out of head
Normal
What are the functional (protein) products of Homeotic genes that enable them to determine cell fate?
What part of the DNA would they interact with?

12. Four general phases for body formation
Organize body along major axes
Organize into smaller regions (organs, legs)
Cells organize to produce body parts
Cells themselves change morphologies and become differentiated

13. Programmed Cell Death (Apoptosis)
Cell signaling is involved in programmed cell death
Is essential for normal development.
2 µm
Figure 21.17

14. Apoptosis is essential for morphogenesis of hands and feet
Figure 21.19
Interdigital tissue
1 mm

16. Homeotic Genes and Evolution 
What is the evidence that homeotic genes are evolutionarily conserved?
What does this figure mean? 
Adult
fruit fly
Fruit fly embryo
(10 hours)
Fly
chromosome
Mouse
chromosomes
Mouse embryo
(12 days)
Adult mouse
Figure 21.23

17. A. Drosophila's eight Hox genes in a single cluster and 39 HOX genes in humans. B. Expression patterns of Hox and HOX genes along the anterior-posterior axis in invertebrates and vertebrates.

18. Hox genes in the Animal Kingdom: increasing numbers and types of Hox genes (animal homeotic genes), increased body plan complexity

19. Hox genes determine the number and types of vertebrae in animals
How does homeotic gene regulation help organisms evolve different body plans?
Hox genes determine the number and types of vertebrae in animals
Hoxc-6 determines that in the chicken the 7 vertebrae will develop into ribs
Snake: Hoxc-6 is expanded dramatically toward the head and toward the rear so all these vertebrae develop ribs.

20. Start homework!
Science Skills Practice: how do we know enhancers regulate gene expression
Science Skills practice: Hox genes and segment development