Download Chapter 21: The Genetic Basis of Development

Chapter 21: The Genetic Basis
of Development
Model organisms for study of development
Embryonic Development
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Cell division
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Zygote divides by mitosis more cells
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Morphogenesis
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Cells organize into tissues & organs
Cells establish a basic body plan/axes (head to tail, back to
belly)
Animals: movement of cells & tissues establishes form
Plants: morphogenesis & growth occurs throughout the life
of the plant (apical meristems)
Cell differentiation
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Cells specialize in structure & function
Evidence for genomic equivalence
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All cells = genomically equivalent; differentiated cells
inactivate unused genes
Evidence:
 Totipotency in plants
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Cloning
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Somatic cell is used to create another genetically identical
individual
Nuclear transplantation in animals
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Mature cells in plant can de-differentiate, giving rise to new
organism
Nucleus from a mature cell when implanted into egg or
zygote can give rise to all tissues & organs
Stem cells of animals
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Embryonic stem cells=totipotent
Adult stem cells=pluripotent
Differential Gene expression during
development
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Transcriptional regulation
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Cytoplasmic determinants
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Regulatory genes code for proteins enhance target
genes= more expression
mRNA, proteins, & organelles in egg unequally
distributed
New cells from mitosis differing amounts of
cytoplasmic materials determines developmental
fate
Cell-cell signals
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Interaction among neighboring embryonic cells
causes changes in gene expression
Pattern formation
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Development of a spatial organization for
tissues & organs in an organism
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Positional information
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Molecular cues provided by cytoplasmic
determinants & inductive signals which control
pattern formation
Model for pattern formation:
Drosophila melanogaster
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Cytoplasmic determinants in unfertilized egg
provide positional information for body axes
After fertilization mitosis= multinucleated
embryo mRNA determinants begin translation
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
Model for pattern formation: C.
elegans

Cell-cell signals play a key role in cell
differentiation
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Proteins on neighboring cells cause cell
specialization
Apoptosis
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Programmed cell death
Signals trigger “suicide” proteins to activate in
cells destined to die
Neighboring cells intake & digest dead cells
Cell signaling & transcriptional
regulation in plants
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Plant cells totipotent cell fate dependant on
position
Regulating mechanisms= cell-signaling
(induction) & transcriptional regulation
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
Evolution of development &
Morphological diversity

Homeobox

180 nucleotide sequence of homeotic &
developmental genes  widely conserved
across species