Ch. 21The Genetic Basis Of

Development

Eye on antennae

• 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• 1. Morphogenesis allows for the basic body plan. Cell

division and cell differentiation help to direct morphogenesis.

• 2. Animals and plants have different morphogenesisAnimals 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. Drosophila, C.elegans, mouse, zebra fish (Decker has some of these) and for plants Arabidopsis.

• 3. Drosophila has a 2 week generation time• 4.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.

Stem cell, Brain stem cell

• 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

Apoptosis

Evolution of homologous genes

Mice