Development and Genes
Part 1
Characteristics of Development for Multicellular
Organisms
Development is the process of timed genetic controlled
changes that occurs in an organism’s life cycle.
• Mitosis
• Cell differentiation
• Pattern formation
• Morphogenesis
All four processes are anchored by differentiation with
regard to gene expression. Most cells in a multicellular
organism have the same genome or DNA. Genes must
be turned on and turned off during development.
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Cleavage
Cleavage is the time
of rapid mitosis
without significant
growth of daughter
cells. Cells become
increasing smaller.
Each cell is called a
blastomere. G1 and
G2 phases of cell
cycle is shortened or
eliminated.
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Nematode
Development
The embryonic
development and
fate of adult cells
has been mapped
with the
nematode.
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Gastrulation
After cleavage, development in animals is often
accompanied by mass movement of cells called
gastrulation.
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Animal Gastulation
The exact mechanism for gastrulation can vary from animal
species to animal species.
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Plant
Development
Plants do not
have mass
movement of
cells during
development due
to the cell walls.
Certain tissues
set aside for cell
division and this
tissue is called
the meristem or
meristematic
tissue.
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Cell, Genes and Development
Cells have two general classes of genes.
• Housekeeping genes which are necessary to go
about the “business” of life. For example genes
that code for the enzymes for cellular respiration
are housekeeping genes. Most cells have all of
these activated
• Specialized genes that produce unique gene
product important to the cells differentiation. For
example the activation of the crystallin gene that
produces product necessary for the development
of the lens of the eye.
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Genes and
Development
It would be wasteful for
lens cells to produce
albumin, and in the
same way it would be
wasteful for the liver
cells to produce
crystallin. These
specialized genes must
be regulated so that
they are only activated
when they are needed
and timing is critical.
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Determination comes before Differentiation.
Determination are those things or processes necessary to
commit a cell to a particular type of cell or fate. Most
often when a cell is committed to a particular fate it is
usually irreversible.
Differentiation is those changes that occur in a cell to
make it a certain cell type.
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12
The graph
shows the
percentage
of nuclear
transplants
embryos that
develop
normally in
relationship
to the age of
the donor.
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Determination
Determination – Events that lead to the observable
differentiation of a cell. Once determination has
occurred then the final fate of the cell is sealed.
If a determined cell is placed in another location in the
organism, it will still differentiate into the cell that was
its normal fate.
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Determination
If cells are placed in another location in the organism, and
the cells take on the identity of the surrounding tissue,
then the cells have not been determined. If the cells retain
their original identity then the cells have been determined
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Determination
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How Determination Occurs
.
There are two sources responsible
for determining the fate or
development of cells.
• Cytoplasmic Determinants
• Induction via signals secreted by
neighboring cells
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Initial Development
Governed by Cytoplasmic
Determinants
Development can
begin with fertilization
of an egg and
subsequent division of
cytoplasmic
determinants during
cytokinesis. There is
unequal distribution
of the cytoplasmic
determinants to the
daughter cells as
illustrated.
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Initial Development Governed by
Once there are a multitude
of cells,
Inductive Signals
neighboring cells may produce signal
molecules that can
interact with receptor
sites and receiving
cells. This causes the
activation of a signal
transduction pathway
for the receiving cell.
This can send the cell
down a specific
developmental
pathway.
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Initial Development Governed by Cytoplasmic
Determinants
Cell Differentiation
• Differentiation of activated genes and inactive genes
• Appearance of mRNA for cell specific proteins
• Changes in cellular structure
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Using Drosophila as a Model Organism for
Development
Drosophila and human development are homologous
processes.
They utilize closely related genes working in highly conserved
regulatory networks. Unlike humans, Drosophila is subject to
easy genetic manipulation. As a result, most of what we know
about the molecular basis of animal development has come
from studies of model systems such as Drosophila.
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Using Drosophila as a Model Organism for
Development and Determination of Axes
The Drosophila life cycle consists of a number of stages:
embryogenesis, three larval stages, a pupal stage, and (finally)
the adult stage!
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Pattern Formation or Setting Up the Body Plan
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Importance of Material in the Cytoplasm of an
Egg
Maternal effect genes are genes that when mutant in the
mother results in a mutant phenotype in the offspring,
regardless of the offspring’s own genotype.
In the fruit fly, mRNA or proteins of the maternal effect genes
are synthesized in the egg while it is still in the mother’s
ovary. A mutation in the maternal effect gene can cause
fertilized eggs to fail to develop normally.
Maternal effect genes control the polarity of the egg and
ultimately the fly and are also called egg-polarity genes. One
set of genes controls the anterior-posterior axis and another set
controls the ventral-dorsal axis. Mutations in these genes are
generally lethal.
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Using Drosophila as a Model Organism for
Devlopment
Two maternal effect genes
are called bicoid and a
nanos. Bicoid and nanos
genes are responsible for
the patterning of the
anterior and posterior ends respectively.
Nurse cells secrete maternally produced bicoid and nanos
mRNA into a maturing oocyte. They are differentially
transported along microtubules to opposite poles of the
oocyte due to the use of different motor proteins to
transport the two different mRNA.
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Using Drosophila as a Model Organism for
Devlopment
One the mRNA arrive at their respective ends, the mRNAs
become anchored in the cytoplasm where the mRNA are
translated.
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Using Drosophila as a Model Organism for
Development
After fertilization, the mRNAs are translated, creating
opposing gradients of bicoid and nanos proteins.
These proteins control the translation of two other
maternal genes, hunchback (needed for anterior
structures) and caudal (needed for posterior structures)
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Example Mutations in Bicoid The mRNA is translated
into bicoid protein at the
anterior end. The bicoid
protein then diffuses
toward the posterior end
forming a gradient.
Substances that form a
gradient in the zygote or
embryo and affects
development or
morphogenesis are called
morphogens. The bicoid
protein is classified as a
morphogen.
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These
bicoid and
Using Drosophila as a Model
Organism
for
Devlopmentnanos proteins
control the translation of two other
maternal genes,
hunchback (needed
for anterior
structures) and
caudal (needed for
posterior structures),
however the mRNAs
are evenly
distributed in the
oocyte.
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The hunchback
and caudalasproteins
will
form a gradient
Using Drosophila
a Model
Organism
for
because of interaction with
the bicoid and nanos protein.
Devlopment
The bicoid protein binds to and inhibits the translation of
the mRNA for caudal, and the nanos protein binds to and
inhibits the translation of hunchback. This interaction
causes a gradient for both.
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