Chapter 21: The Genetic Basis of Development Model organisms for study of development

Embryonic Development Cell division Cell division Zygote divides by mitosis  more cells Zygote divides by mitosis  more cells Morphogenesis Morphogenesis Cells organize into tissues & organs Cells organize into tissues & organs Cells establish a basic body plan/axes (head to tail, back to belly) Cells establish a basic body plan/axes (head to tail, back to belly) Animals: movement of cells & tissues  establishes form Animals: movement of cells & tissues  establishes form Plants: morphogenesis & growth occurs throughout the life of the plant (apical meristems) Plants: morphogenesis & growth occurs throughout the life of the plant (apical meristems) Cell differentiation Cell differentiation Cells specialize in structure & function Cells specialize in structure & function

Evidence for genomic equivalence All cells = genomically equivalent; differentiated cells inactivate unused genes All cells = genomically equivalent; differentiated cells inactivate unused genes Evidence : Evidence : Totipotency in plants Totipotency in plants Mature cells in plant can de-differentiate, giving rise to new organism Mature cells in plant can de-differentiate, giving rise to new organism Cloning Cloning Somatic cell is used to create another genetically identical individual Somatic cell is used to create another genetically identical individual Nuclear transplantation in animals Nuclear transplantation in animals Nucleus from a mature cell when implanted into egg or zygote can give rise to all tissues & organs Nucleus from a mature cell when implanted into egg or zygote can give rise to all tissues & organs Stem cells of animals Stem cells of animals Embryonic stem cells=totipotent Embryonic stem cells=totipotent Adult stem cells=pluripotent Adult stem cells=pluripotent

Differential Gene expression during development Transcriptional regulation Transcriptional regulation Regulatory genes code for proteins  enhance target genes= more expression Regulatory genes code for proteins  enhance target genes= more expression Cytoplasmic determinants Cytoplasmic determinants mRNA, proteins, & organelles in egg unequally distributed mRNA, proteins, & organelles in egg unequally distributed New cells from mitosis  differing amounts of cytoplasmic materials  determines developmental fate New cells from mitosis  differing amounts of cytoplasmic materials  determines developmental fate Cell-cell signals Cell-cell signals Interaction among neighboring embryonic cells causes changes in gene expression Interaction among neighboring embryonic cells causes changes in gene expression

Pattern formation Development of a spatial organization for tissues & organs in an organism Development of a spatial organization for tissues & organs in an organism Positional information Positional information Molecular cues provided by cytoplasmic determinants & inductive signals which control pattern formation Molecular cues provided by cytoplasmic determinants & inductive signals which control pattern formation

Model for pattern formation: Drosophila melanogaster Cytoplasmic determinants in unfertilized egg provide positional information for body axes Cytoplasmic determinants in unfertilized egg provide positional information for body axes After fertilization  mitosis= multinucleated embryo  mRNA determinants begin translation After fertilization  mitosis= multinucleated embryo  mRNA determinants begin translation Location of the protein products cause nuclei migration to the periphery  & cytokinesis establishes body axes Location of the protein products cause nuclei migration to the periphery  & cytokinesis establishes body axes Further segmentation based on position & location of protein products in embryo Further segmentation based on position & location of protein products in embryo

Model for pattern formation: C. elegans Cell-cell signals play a key role in cell differentiation Cell-cell signals play a key role in cell differentiation Proteins on neighboring cells cause cell specialization Proteins on neighboring cells cause cell specialization Apoptosis Apoptosis Programmed cell death Programmed cell death Signals trigger “suicide” proteins to activate in cells destined to die Signals trigger “suicide” proteins to activate in cells destined to die Neighboring cells intake & digest dead cells Neighboring cells intake & digest dead cells

Cell signaling & transcriptional regulation in plants Plant cells totipotent  cell fate dependant on position Plant cells totipotent  cell fate dependant on position Regulating mechanisms= cell-signaling (induction) & transcriptional regulation Regulating mechanisms= cell-signaling (induction) & transcriptional regulation Environmental factors (i.e. temp., amount of daylight) trigger cell-signaling pathways  development of organs Environmental factors (i.e. temp., amount of daylight) trigger cell-signaling pathways  development of organs Organ identity determined by regulatory genes  code for proteins that enhance target genes Organ identity determined by regulatory genes  code for proteins that enhance target genes

Evolution of development & Morphological diversity Homeobox Homeobox 180 nucleotide sequence of homeotic & developmental genes  widely conserved across species 180 nucleotide sequence of homeotic & developmental genes  widely conserved across species