Embryonic Development Timing and coordination of gene activation

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”

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)

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?

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

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 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

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?

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)

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

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?

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

Programmed Cell Death (Apoptosis) Cell signaling is involved in programmed cell death Is essential for normal development. 2 µm Figure 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?  Adult fruit fly Fruit fly embryo (10 hours) Fly chromosome 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 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.

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