Embryonic Development Timing and coordination of gene activation
Embryonic Development
Timing and coordination of gene activation
Genetic Basis of Development
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)
Figure 21.8
“Copy Cat”
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(cross section)
Adult animal(sea star)
Cellmovement
Gut
Cell division
Morphogenesis
Observable cell differentiation
Seedleaves
Shootapicalmeristem
Rootapicalmeristem
PlantEmbryoinside seed
Two cells Zygote
(fertilized egg)
(a)
(b)
• 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 nucleusTransplanted
Enucleatedegg cell
Fully differentiated(intestinal) cell
Donor nucleustransplanted
Most developinto tadpoles
<2% developinto tadpoles
Why are the fully differentiated cells less successful in developing into tadpoles?
Timing and coordination of development
• Cells specialize by activating master control genes
• What is the function of master control genes?
DNAOFF OFF
OFFmRNA
mRNA mRNA mRNA mRNA
Anothertranscriptionfactor
MyoDMuscle cell(fully differentiated)
MyoD protein(transcriptionfactor)
Myoblast (determined)
Embryonicprecursor cell
Myosin, othermuscle proteins,and cell-cycleblocking proteins
Other muscle-specific genesMaster control gene myoDNucleus
1
2
How do cells know which master control genes to activate during differentiation and morphogenesis?
• Cytoplasmic Determinants and the maternal effect• Induction by cell signaling
Induction: Cell signaling changes gene expression
Figure 21.11b
Signaltransductionpathway
Signalreceptor
Signalmolecule(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
Pattern Formation in Drosophila
Translation of bicoid mRNAFertilization
Nurse cells Egg cell
bicoid mRNA
Developing egg cell
Bicoid mRNA in mature unfertilized egg
100 µm
Bicoid protein inearly 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?
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
ReceptorSignalprotein
Signal
Anteriordaughtercell of 3
Posteriordaughtercell of 3
Will go on toform muscle and gonads
Will go on toform adultintestine
12
43
3
Induction of the intestinal precursor cell at the four-cell stage.(a)
Figure 21.11a
Unfertilized egg cell
Molecules of a a cytoplasmicdeterminant Fertilization
Zygote(fertilized egg)
Mitotic cell division
Two-celledembryo
Nucleus
Cytoplasmic Determinants• 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
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
Normal Mutant with legs growing out of head
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?
Four general phases for body formation
1. Organize body along major axes2. Organize into smaller regions (organs,
legs)3. Cells organize to produce body parts4. Cells themselves change
morphologies and become differentiated
Programmed Cell Death (Apoptosis)• Cell signaling is involved in programmed cell death• Is essential for normal development.
2 µmFigure 21.17
Apoptosis is essential for morphogenesis of hands and feet
Figure 21.19
Interdigital tissue
1 mm
Homeotic Genes and Evolution • What is the evidence that homeotic
genes are evolutionarily conserved?
• What does this figure mean?
Adultfruit fly
Fruit fly embryo(10 hours)
Flychromosome
Mouse chromosomes
Mouse embryo(12 days)
Adult mouse
Figure 21.23
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.
Hox genes in the Animal Kingdom: increasing numbers and types of Hox genes (animal homeotic genes), increased body plan complexity
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.
How does homeotic gene regulation help organisms evolve different body plans?
Start homework!
• Science Skills Practice: how do we know enhancers regulate gene expression
• Science Skills practice: Hox genes and segment development