The Genetic Basis of Development

CHAPTER 21

Question

How does a complex multicellular organism develop from a single cell?

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)

Embryonic Development3 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

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)

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

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

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

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

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

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)

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

Summary of Cascade Gene Activity

more about Induction

roles of induction: drives formation of organs leads to programmed cell death (apoptosis)

effect of inducers: activation or inactivation of genes in induced

cellmechanism:

triggers signal transduction pathways effect can depend on inducer’s concentration

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

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

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)

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