1. The Genetic Basis of Development
CHAPTER 21

2. Question
How does a complex multicellular organism develop from a single cell?

3. 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)

4. 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

5. 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)

6. 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

7. Sources of Developmental Info: Cytoplasmic 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

8. Sources of Developmental Info: Induction
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

9. 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

10. 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

12. Segmentation Pattern
proteins encoded by egg-polarity genes regulate the expression of some of the embryo’s 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)

13. Identity of Body Parts controlled by homeotic genes
turned on by segment-polarity gene products
specify the types of appendages and other structures that each segment will form

14. Summary of Cascade Gene Activity

15. more about Induction roles of induction: effect of inducers:
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 inducer’s concentration

16. 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

17. 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

18. 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)

19. 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