The Genetic Basis of Development

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The Genetic Basis of Development

• CHAPTER 21

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Question

• How does a complex multicellular organism develop
  from a single cell?

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Model Organisms used to study this question

• fruit fly (Drosophila melanogastor)
• nematode (C. elegans)
• mouse (Mus musculus)
• zebrafish (Danio rerio)
• Arabidopsis thaliana (small flowering
  plant in the mustard family)

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Embryonic Development

• 3 processes
• cell division
• cell differentiation
• process by which cells become specialized in
  structure function
• morphogenesis
• physical processes that give an organism its
  shape
• 2 ways animal plant development differs
• animals movements of cells tissues are
  necessary for transformation
• plants morphogenesis growth are not limited to
  embryonic and juvenile periods because they have
  perpetually embryonic regions called apical
  meristems

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Cell Differentiation

• arises primarily from differences in gene
  expression not from differences in the cells
  genomes
• evidence for genomic equivalence
• totipotency in plants
• mature cells have the potential to
  dedifferentiate and give rise to all the
  specialized cell types of the mature organism
• nuclear transplantation in animals
• transplantation of a nucleus from a
  differentiated cell into an enucleated egg (or
  egg that has had nucleus removed) of the same
  species can support normal development
  (reproductive cloning)
• however, the older the donor nucleus the lower
  the of normal development so something in the
  animal nucleus does change as animal cells
  differentiate
• stem cells of animals (totipotent)
• unspecialized cells isolated from early embryos
  that can be stimulated to differentiate into
  various cell types (therapeutic cloning)

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Determination

• the events that lead to the observable
  differentiation of a cell
• at the end of this process, an embryonic cell is
  irreversibly committed to its final fate
  (determined)
• marked by the expression of genes for tissue
  specific proteins, which act as transcription
  factors for genes that help define cell type

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Sources of Developmental InfoCytoplasmic
Determinants

• maternal substances in the egg that influence the
  course of early development
• distributed unevenly to new cells produced by
  mitotic division of the zygote
• the set of cytoplasmic determinants a cell
  receives helps regulate gene expression

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Sources of Developmental InfoInduction

• communication between cells can induce
  differentiation
• in animals, contact with neighboring cells the
  binding of growth factors secreted by neighboring
  cells
• in plants, cell-cell junctions (plasmodesmata)
  allow signal molecules to pass from one cell to
  another

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Pattern Formation

• development of a spatial organization in which
  the tissues and organs of an organism are all in
  their characteristic places
• begins in early embryo when the major axes of the
  organism are established
• molecular cues (positional information) that
  control pattern formation are provided by
  cytoplasmic determinants inductive signals

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Axis Establishment

• the cytoplasmic determinants in the egg are
  encoded by genes of the mother called maternal
  effect genes
• when mutant in the mother, results in a mutant
  phenotype in the offspring
• also called egg-polarity genes because they
  control the orientation (polarity) of the egg
• in animals, one group of these genes sets up the
  anterior-posterior axis another group sets up
  the dorsal-ventral axis
• localized concentrations or gradients of the
  molecules they code for determine polarity

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Segmentation Pattern

• proteins encoded by egg-polarity genes regulate
  the expression of some of the embryos own genes
• gradients of proteins produced from these genes
  bring about regional differences in the
  expression of segmentation genes
• 3 sets gap genes, pair-rule genes, segment
  polarity genes (each type activates the next)

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Identity of Body Parts

• controlled by homeotic genes
• turned on my segment-polarotity gene products
• specify the types of appendages and other
  structures that each segment will form

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Summary of Cascade Gene Activity
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more about Induction

• roles of induction
• drives formation of organs
• leads to programmed cell death (apoptosis)
• effect of inducers
• activation or inactivation of genes in induced
  cell
• mechanism
• triggers signal transduction pathways
• effect can depend on inducers concentration

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Apoptosis

• triggered by signals that activate a cascade of
  suicide proteins in the cells destined to die
• what happens
• cell shrinks, nucleus condenses, DNA is
  fragmented
• neighboring cells engulf digest remains
• in animals, essential for
• normal development of the nervous system
• normal operation of the immune system
• normal morphogenesis of hands feet in humans
  and paws of other mammals

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Mechanisms of Plant Development

• many plant cells are totipotent and their fates
  depend more on positional information than on
  cell lineage
• ? major mechanisms regulating development
• cell-signaling (induction)
• transcriptional regulation

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Pattern Formation in Plants

• environmental signals (ie day length,
  temperature) trigger signal transduction pathways
  that convert ordinary shoot meristems to floral
  meristems, causing a plant to flower
• organ identity genes determine the structure that
  will grow from a floral meristem (analogous to
  homeotic genes in animals)

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Comparative Studies

• comparison of development processes in different
  multicellular organisms helps us
• understand how developmental processes have
  evolved
• how changes in these processes can modify
  existing organisms or lead to new ones
• conservation of developmental genes in animals
• homeobox (180-nucleotide sequence) region found
  in homeotic genes other developmental genes of
  many invertebrates and vertebrates is
  similar/identical
• many developmental genes are highly conserved
  among species but may play different
  developmental roles in different species
• genes that direct analogous developmental
  processes in plants animals differ greatly