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Embryonic Development

Embryonic Development. Timing and coordination of gene activation. Figure 21.8. 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

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Embryonic Development

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  1. Embryonic Development Timing and coordination of gene activation

  2. Figure 21.8 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) “Copy Cat”

  3. 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. (a) Gut Cell movement Zygote (fertilized egg) Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Eight cells Cell division Morphogenesis Observable cell differentiation 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. (b) Seed leaves Shoot apical meristem Root apical meristem Zygote (fertilized egg) Two cells Embryo inside seed Plant Figure 21.4a, b Cell Division - Morphogenesis –Differentiation

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

  5. 2 1 Nucleus Other muscle-specific genes Master control gene myoD DNA OFF OFF Embryonicprecursor cell OFF mRNA MyoD protein(transcriptionfactor) Myoblast (determined) mRNA mRNA mRNA mRNA Myosin, othermuscle proteins,and cell-cycleblocking proteins Anothertranscriptionfactor MyoD Muscle cell(fully differentiated) Timing and coordination of development • Cells specialize by activating master control genes • What is the function of master control genes?

  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 • 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 Early embryo (32 cells) Signal transduction pathway NUCLEUS Signal receptor Signal 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. Figure 21.11b

  8. Egg cell Nurse cells Developing egg cell 1 bicoid mRNA Bicoid mRNA in mature unfertilized egg 2 Fertilization 100 µm Translation of bicoid mRNA Bicoid protein in early embryo 3 Anterior end (b) Gradients of bicoid mRNA and Bicoid protein in normal egg and early embryo. Figure 21.14b Pattern Formation in Drosophila • 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. 2 Posterior 1 Anterior Signal protein 4 3 Receptor EMBRYO 3 4 Signal Anterior daughter cell of 3 Posterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine (a) Induction of the intestinal precursor cell at the four-cell stage. 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?

  10. Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Nucleus Zygote (fertilized egg) Mitotic cell division Two-celled embryo Figure 21.11a 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

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

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

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

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

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

  18. 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? Hox genes determine the number and types of vertebrae in animals

  19. Start homework! • Science Skills Practice: how do we know enhancers regulate gene expression • Science Skills practice: Hoxgenes and segment development

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