Chapter 20 - Learning Objectives Developmental Patterning in Eukaryotes

Understand what it means that eukaryotic cells are genetically equivalent (except
gametes).

“Mature plant and animal cells are genetically equivalent—they contain the same genes” 
Cloning (e.g., Dolly the sheep) shows that all cells have the full genome, but express different parts of it based on their function.

Explain what it means that cells differentiate and what is controlling that on the
genetic/chromosomal level.

“Cell differentiation is the process by which cells become specialized in structure and function” 
“Differential gene expression results from genes being regulated differently in each cell type” 
It’s controlled by:

• Gene regulation

• Epigenetics

• Transcription factors

• Cytoplasmic determinants (maternal effect genes)

Genes like “master regulatory genes” regulate other genes by induction, promote
differentiation and morphogenesis, differential gene expression, and programmed
cell death/apoptosis. [Know what each of those things are/do.]

These genes trigger large gene expression programs:

• Induction:

“Signal to one another what they are” during development. 

• Differentiation: Specialization of cells (see above)

• Morphogenesis:

“The transformation from zygote to adult results from... morphogenesis” 

• Differential gene expression:
  Same DNA, different gene activity → different cell fates 

• Apoptosis (programmed cell death):

“Apoptosis is the most common type in animals… Helps tissues and organs take shape” 

Pattern formation in fruit fly; positional information; know the major axes.
Nusselhein-Volhard/Weischaus mutant screen identified genes controlling developmental patterning; know what maternal effect genes do, where they come from, and what happens to the embryo if maternal effect genes are mutated; problems with the deposition of maternal effect mRNA in an egg will affect all offspring equally; know what bicoid does.

“Pattern formation is the development of a spatial organization of tissues and organs” 
“Positional information: the molecular cues that control pattern formation.” 

• Major axes in animals:

  • Anterior ↔ Posterior

  • Dorsal ↔ Ventral

  • Left ↔ Right

Know what morphogens are and how researchers figured out that Bicoid was expressed as a morphogen, where it was found in the embryo, and what type of protein it is.

 

Understand what homeotic genes are (determine organ formation)

Definition:

“initiate pattern formation (organs development) … in late embryo, larva, and adult stages” 

Mutation causes homeosis:

“Transformation of one organ into another via mutation”

• Ex: Antennapedia (legs instead of antennae), Bithorax (extrathoracic segment) 

 

Hox Genes

“Guide pattern formation in embryos—tell the cells of the body how to differentiate.” 

• Found in prokaryotes, yeast, plants, animals. 

• Highly conserved across species

• Organized in chromosomal clusters, and:

“The order of the genes along the chromosome corresponds to the order of where the genes are expressed in the embryo.” 

Protein function:

are transcription factors that regulate many other genes

 Loss of Hox6/8 overlap:

“In the area where ____ expressed by itself, the forelimb forms”
“In snakes... ____ are always expressed together, so no forelimbs form.” 

Loss of Sonic Hedgehog (Shh):

“ signaling in the pelvic region of whales led to the disappearance of hindlimbs.” 

Almost all animals contain related sets of

with conserved order and function

Homeobox (Hox) genes

reveal the shared evolutionary history of life