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Chapter 21
The Genetic Basis of Development
How do we study
development?
 Developmentally biology used to simply be
observational studies to describe the sequence
of events in development (i.e. what is occurring
in development)
 Using concepts from molecular genetics,
scientists can now answer questions about the
mechanisms of development (i.e. how
development is occurring)
Model Organism
 The organism that is used to study a particular
question that is representative of a larger group
(the group in question)
 Criteria to be an effective model:





Easy to grow in lab
Short generation times
Develop outside of mother’s body
Small genome
Well understood genome
Examples of Model Organisms
Organism
Easy Short
to
Gen.
grow? time?
Develops
outside of
Mother’s
body?
Small
genome
?
Well understood
genome?
Yes!
2
weeks
Yes
180Mb
Yes, sequenced
Yes!
3.5
days
Yes
97Mb
Fairly well
understood
Yes!
9
weeks
No
2600Mb
Yes, can make
knock out
genes for
almost any trait
Yes!
2-4
Yes
1700Mb
In the process
of being
mapped and
sequenced
Yes
118Mb
Fairly well
understood
months
*
Yes!
8-10
weeks
Embryonic Development
 Involves three types of processes
 Cell division
 Cell differentiation
 Morphogenesis
 All of these processes can be happening at the
same time, but it all must begin with cell
division.
Cell Division
 Successions (series) of mitotic divisions
 Zygote (sperm + egg) gives rise to a
large number of cells
Cell Differentiation
 Cells become
specialized in
structure and
function
Morphogenesis
(morph-form) (genesis-creation)
 The physical processes that give an
organism its shape
Normal
Abnormal
Animals vs. Plants
In animals, but not plants, morphogenesis relies on movement of cells
Animals vs. Plants
In plants, but not animals, morphogenesis and growth occur throughout life.
Due to the presence of apical meristems (perpetually embryonic regions)
Genomic Equivalence
 Almost all cells in an organism have the
same genes
 If all of the cells have the same genes,
why do we have different cell types?
 Differential gene expression in cells with the
same DNA allows for the differentiation of
these cells.
Genomic Equivalence:
Evidence
 Totipotency in plants
 Toti- total, potent-power
 Differentiated cells from an
adult plant placed in a
culture medium grow into an
adult plant.
 Cloning- using one or more
somatic cells from a
multicellular organism to
make another genetically
identical individual
Genomic Equivalence:
Evidence
 Different approach used
for animals Nuclear
transplantation
 Nucleus from a
differentiated cell placed
in an enucleated* egg
cell
 Age of differentiated cell
seems to reduce the
percentage of success
Reproductive Cloning
 Goal: to produce new
individuals
 Specialized cell (e.g.
mammary cell) starved
of nutrients  cell
dedifferentiates fused
with an enucleated egg
cell grown in culture
implanted in uterus
 Let's watch
Problems:
 Small percentage of embryos develop
normally until birth
 Prone to difficulties that are generally
associated with aging
 Why? Donor nuclei are not always
completely reprogrammed
 Some still contain gene regulation mechanisms
such as methylation of the DNA which prevent
some genes from being full expressed when they
should
Therapeutic cloning
 Goal: to produce embryonic
stem cells to treat disease
 Stem cell:
 Unspecialized cell
 Can reproduce itself
indefinitely
 Can differentiate into
specialized cells
Stem Cells
 Embryonic stem cells
 Totipotent
 Isolated from developing
embryos at the blastula
stage
 Adult stem cells
 Pluripotent
 Replace non-reproducing
specialized cells
Why is this all possible?
 Transcriptional regulation of gene
expression
 Cell generates transcription factors that turn
on certain genes
 Results in cell determination
 The expression of genes for tissue specific
proteins
 Tissue-specific proteins are only found in a particular
type of cell (and therefore, tissue) that give it the
characteristic structures/functions
Why is this all possible?
What determines which cells
will express certain genes?
 Cytoplasmic determinants
 Maternal substances that
influence development
 Metotic division distribute the
heterogeneous cytoplasm
among the cells
 Exposure of these cells to
different determinants
regulates expression of the
cell’s genes
What determines which cells
will express certain genes?
 Induction
 Signaling from embryonic
cells that cause a change
in nearby target cells.
 More important as
number of embryonic
cells increases
Pattern Formation results
from similar genetic and
cellular mechanisms
 Pattern Formation
 Development of spatial organization
 In animals, this occurs mainly in embryos
and juveniles *
 In plants, this occurs continually at the apical
meristems
 Determined by positional information
(cytoplasmic determinants and inductive
signals)
Pattern Formation
 This can be difficult to study
 In order to study, researchers cause and
observe mutations. When a single mutation
occurs and a change is see, it can be
deduced that the mutation has caused the
change
 However, some mutations (especially those
in genes as important as pattern formation
genes) can be embryonic lethals
Pattern Formation
 1) Axis Development
 Result of cytoplasmic determinants
 cd’s are proteins encoded by maternal effect genes (also
called egg-polarity genes)
 Sets up the left/right axis and the ventral/dorsal axis
 2) Segmentation pattern
 Gradients of proteins in the different regions of the
body direct the expression of segmentation genes
Pattern Formation
 3. Identity of Body Parts
 Determined by homeotic genes
The role of Cell-Signaling
 Induction
 Sequential events that drive organ formation
 Produce effects via signal transduction pathways
 Response of the induced cell is generally activation
of transcription for genes that will characterize
(specialize) the cell
 Apoptosis
 Programmed cell death
1 mm
Figure 21.19
Plant Development
 Cell lineage is less important because
many cells are totipotent
 Most regulation is done with cell signaling
or transcriptional regulation
 Organ identity genes determine which
organ will grow from the apical meristem
at any given time
 Similar to homeotic genes of animals.
Homeotic Genes
 All contain a 180 nucleotide sequence called
homeobox
 This homeobox specifies (codes for) a 60 amino
acid homeodomain
 Homeotic genes are highly conserved across
the animal kingdom.
 Has led to the formation of a new field: Evo-devo
 While plants often have homeobox sequences,
they do not often play a major role in pattern
formation.
Rats.
 http://abcnews.go.com/video/playerIndex
?id=4134032