Download The Genetic Basis of Development

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 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
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
cell
mechanism:
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