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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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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
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been determined
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 Inductive
Signals
Once there are a multitude of
cells, 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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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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One the mRNA arrive at their respective ends, the mRNAs
become anchored in the cytoplasm where the mRNA are
translated.
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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 nanos
proteins control the
trans-lation 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 caudal proteins will form a gradient
because of interaction with the bicoid and nanos protein.
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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