Pattern Formation and Cell Differentiation

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Pattern Formation andCell Differentiation

• Chapter 22

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Patterns of Development

• Patterns of development are established early in
  embryogenesis
• Fate of a single cell depends on its position
  within the embryo
• Fate is realized in a progression of stepseach
  mediated by specific regulatory proteins or
  cell-to-cell signals

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Pattern Formation in Drosophila

• Establishment of the spatial organization of an
  embryo
• Determines the fate of the cell
• Christiane Nüsslein-Volhard and Eric Wieschaus
  studied pattern formation mutants
• Found more than 100 genes
• Found bicoid mutant that had two posteriors and
  no anterior end

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Bicoid Mutants

• Bicoid gene is shown to have maternal effect
  inheritance
• First appears at fertilization and diffuses away
  from the anterior
• Produces a steep concentration gradient from the
  anterior to the posterior end

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Bicoid Gene
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Bicoid Is a Regulatory Transcription Factor

• A protein that binds to a regulatory sequence in
  DNA and controls the transcription of genes
• Controls segmentation genes
• Segmentation genes organize cells and tissues
  into distinct segments
• Some are gap genes, pair-rule genes, and segment
  polarity genes

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Segmentation Genes

• Gap genes produce mutants that lack several
  consecutive segments
• Expressed along the head-to-tail axis
• Pair-rule genes produce mutants that lack
  alternate segments and have half the normal
  number of segments
• expressed in alternate segments
• Segment polarity genes produce mutants that lack
  portions of each segment
• Expressed in restricted regions of each segment

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Nature of Developmental Signals

• Genes that regulate development deliver two types
  of information to target cells
• The type of signals
• Their concentration
• Vary with respect to a cells position in the
  embryo

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Developmental Signals in Animals

• Example Drosophyla

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Homeotic Genes

• Homeosis is the replacement of one structure by
  another
• Mutants that have replacement or transformation
  have mutation in homeotic genes
• Homeotic genes specify the proper location and
  development of various body structures

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Homeotic Mutants
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Homeotic Genes

• The group of homeotic genes is known as the
  homeotic complex (HOM-C )
• Found in two clusters on the same chromosome
• Five genes for the anterior form the Antennapedia
  complex
• Three genes for the posterior form the Bithorax
  complex
• Expression of the HOM-C genes is sequential
• In order along the chromosome and from anterior
  to posterior of the embryo

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HOM-C and Hox Genes and Regulation

• The Hox gene complex of frogs, crustaceans,
  birds, worms, mice, and humans is homologous to
  the homeotic complex of Drosophila
• Varies in the number of Hox loci among species
• HOM-C and Hox genes encode regulatory
  transcription factors
• Bind DNA and regulate the activity of other genes
• HOM-C and Hox genes have a DNA sequence called
  the homeobox that codes for a DNA-binding domain

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Developmental Signals in Plants

• Example Arabadopsis

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Pattern Formation in Plants

• Pattern formation does not stop in early
  development like in animals
• Plant parts are produced throughout a plants
  life
• Mutants display abnormal development of the
  root-to-shoot axis
• apical mutants lacking cotyledons,
• central mutants lacking hypocotyl structures
• basal mutants lacking hypocotyl and root
  structures

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Pattern Formation in Plants
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Differentiation
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Differentiation

• Differential gene expression leads to
  differentiation
• All Adult cells have the genes necessary to
  become another type of cell
• Cell fate is a four-step process pattern
  formation, morphogenesis, determination, and
  differentiation.
• Morphogenesis is the process by which cells
  become assembled into recognizable tissues and
  organs

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Differentiation

• Determination indicates a cells irreversible
  commitment to becoming a particular cell type
• Differentiation involves turning on/off certain
  genes to transform a cell

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Early Development
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Cleavage

• After a sperm fertilizes an egg
• The zygote undergoes cleavage, leading to the
  formation of a blastula
• The blastula undergoes gastrulation
• Resulting in the formation of embryonic tissue
  layers and a gastrula

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Early embryonic development in animals
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Tissues

• Animal embryos
• Form germ layers,
• embryonic tissues,
• including ectoderm,
• endoderm, and mesoderm
• Diploblastic animals have two germ layers,
  missing mesoderm layer (jellyfish and coral)
• Triploblastic animals have three germ layers
  (worms , arthros and other animals)

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Characteristics of Early Development

• In protostomes splitting of the initially solid
  masses of mesoderm to form the coelomic cavity is
  called schizocoelous development
• In deuterostomes formation of the body cavity is
  described as enterocoelous development

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Organizing Tissues

• The endoderm forms the digestive tract
• Notochord forms from the mesoderm
• Ectoderm folds to form the neural tubethe
  precursor of the brain and spinal

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Organizing Tissues
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Characteristics of Early Development

• In protostomes splitting of the initially solid
  masses of mesoderm to form the coelomic cavity is
  called schizocoelous development
• In deuterostomes formation of the body cavity is
  described as enterocoelous development

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Animal Phylogeny
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Animal Phylogeny

• Leading hypotheses agree on major features of the
  animal phylogenetic tree
• Zoologists currently recognize about 35 animal
  phyla
• The current debate in animal systematics
• Has led to the development of two phylogenetic
  hypotheses, but others exist as well

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Animal Phylogeny

• One hypothesis of animal phylogeny based mainly
  on morphological and developmental comparisons

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Animal Phylogeny

• One hypothesis of animal phylogeny based mainly
  on molecular data