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Ch. 21
The Genetic Basis Of
Development
• I. Embryonic development
•
A. Three processes make embryo
development possible.
• Cell division - this would only produce identical cells
• Cell differentiation - cells become specialized in
structure and function
• Morphogenesis - "creation of form" the physical process
that gives an organism shape(this is like Power
Rangers).
• B. All three process overlap
• Morphogenesis allows for the basic body plan. Cell
division and cell differentiation help to direct
morphogensis.
• 2. Animals and plants have different morphogenesis
» Animals complete morphogenesis during embryonic
development, and only grow for a certain juvenile
period
• Plants have areas in which growth occurs throughout the
life of the plant called meristems - perpetual embryonic
regions. These occur at the tips of shoots and roots.
• C. Model organisms used to study development
• 1. Frogs
– They have large eggs that are easy to see
– Fertilization and development happen outside the body.
– 2. Drosophilia, C.elegans, mouse, zebra fish (Decker has
some of these) and for plants Arabidopsis.
– 3. Drosophilia has a 2 week generation time
• Caenorhabditis elegans can be grown easily
in petri dishes and grows from zygote to adult
in 3.5 days. Its genome has been sequenced.
They are also hermaphrodites
• Mus musculus - mouse, there are now transgenic mice
and mice who have had genes mutated, however embryo
development is difficult to see.
• Danio rerio - zebrafish - easy to breed and generation
time is longer than other models (2-4 months) but early
development is quick with a fish hatching two days after
fertilization.
• Arabdopsis thaliana - a weed can produce 1000s of
progeny in 8-10 weeks. They are easily transformed
• Differential gene expression - we know that all cells have
the same genes so how do certain genes get turned on
and others not?
– Carrot cells taken from the root grew into normal carrot
plants (a clone). Plant cells are totipotent- they have the
potential to become any type of cell.
– Animal cells do not behave like this. Many can not even
divide in culture at all.
• Attempts have been made to remove the
nuclei from a frog egg cell and replace it with
the nuclei from various stage tadpole cells.
This has been marginally successful.
• As embryonic development progresses potency
of cells in animals changes.
– Dolly - The nucleus from an udder cell into a
unfertilized egg cell ( the udder cells were stopped at
the G1 checkpoint), then the eggs were implanted
into surrogates. Her mitochondrial DNA is from the
egg donor.
– Many clones do not develop normally because of
DNA being methylated in the wrong places and
disrupting gene expression.
• Stem cells- unspecialized, continually
reproducing, can differentiate into specialized
cells of different types.
– Bone marrow- stem cells for different blood cells
– Recently discovered brain stem cells
– Pluripotent- cells that can become different types of
cells.
– Embryos have stem cells that can be made immortal.
• Different cell types make different proteins
– Specific cells make proteins that will direct their
specific roles in the body.
– Cytoplasmic determinants from the mother direct the
embryo development
– 3. Induction - signal molecules cause changes in
developing cells by causing changes in gene
expression.
Transcription factors
Animations of development
Fetal development