Download Homeotic genes

Development and Genes Part II
Pattern Formation and Morphogenesis
Using Drosophila as a Model Organism for
Devlopment
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Using Drosophila as a Model Organism for
Devlopment
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Using Drosophila as a Model Organism for
Devlopment
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Example: Drosophila Pattern Formation for the
Posterior-Anterior Axes
Christane Nüsslein-Volhard and Eric
Wieschaus undertook a project to discover the
genetic mechanisms of how a fertilized
Drosophila egg became a segmented embryo.
Drosophila larvae begin segmentation shortly
after fertilization, as the cells organize into
fourteen distinct body segments.
To investigate this process, Nüsslein-Volhard and
Wieschaus exposed fly embryos to mutagens, and
systematically characterized their phenotypic mutations.
They screened mutated embryos that exhibited abnormal
development of the body axis or segmentation to identify
which genes had gone awry.
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Example: After Segmentation,
Segments are Specialized
In normal flies, structures like legs, wings, and
antennae develop on particular segments, and
this process requires the action of homeotic
genes. Ed Lewis (1940’s), who discovered
homeotic mutants - mutant flies in which
structures characteristic of one segment of the
embryo are found at some other segment.
In normal flies, structures like legs, wings, and
antennae develop on particular segments, and
this process requires the action of homeotic
genes.
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Example: Drosophila development of body
structures
Edward Lewis (1918-2004) discovered that the
genes that provided the code for the fly's body
were segmented and ordered, even in the
embryo stage. These genes dictated the
development of each segment of the body. By
causing mutations in certain genes, he found
that he could cause flies to grow extra body
parts or other abnormal features. Homeotic
genes are genes which regulate the
development of anatomical structures in
various organisms such as insects,
mammals, and plants.
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The homeotic genes encode
transcription factors that control the
expression of genes responsible for
particular anatomical structures, such as
wings, legs, and antennae. The homeotic
genes has a subsection which includes a
180 nucleotide sequence called the
homeobox (also called hox genes),
which is translated into a 60 amino acid
domain, called the homeodomain. The
homeodomain is involved in DNA
binding.
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A homeoprotein or HOX protein contains a
homoedomain (in red). It is the
homeodomain (60 amino acids long) that
interacts with the DNA as a transcription
factor for a particular gene needed in
development.
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Hox genes are switched on in different segments. Patterns
of Hox gene activity give each segment an identity, telling
it where it is in the body and what structures it should
grow. For instance, genes that are active in the head direct
the growth of mouth parts and antennae, while genes that
are active in the thorax direct the growth of legs and
wings.
Drosophila, like all insects, has eight Hox genes. These
are clustered into two complexes, both of which are
located on chromosome 3.
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Interestingly, Hox genes are arranged in clusters.
Typically, their order on the chromosome is the
same as the order in which they appear along the
body. In other words, the genes on the left
control patterning in the head, and the genes on
the right control patterning in the tail.
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Changes to Hox gene
expression change a
segment’s identity. For
example the first segment
of the thorax normally
grows legs, the second
grows legs and wings, and
the third grows legs and
halteres. When the Hox
gene activity in the third
segment is made the same
as that in the second, both
segments grow legs and
wings.
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On the right is
the normal
development,
and on the left
is the mutant.
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Evolution and
Importance of Hox
Genes
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Example of
Hox Gene
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Example of Hox Gene Developmental Mutations
Evolution of the
Hox genes
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Pattern Formation and
Morphogenesis
These are the hox genes that
control certain vertebrae and
their development.
Once the body pattern is
established, and then
morphogenesis can occur,
which is the formation of
various organs and systems
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Morphogenesis Can Involve Cell Death Occurs
Apoptosis occurs in the development of the digits of a
hand or paw. Shown is the development of a mouse paw.
The genes that regulate apoptosis are similar in both
vertebrates and invertebrates such as nematodes.
Fungi including yeast also have genes that regulate
apoptosis indicating these are ancient genes.
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During Development Cell Death Occurs
Apoptosis- Is a programed
cell death. Occurs in
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Infected cells
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Development
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Cells that are aged and no
longer functional
The cell below is a normal
leukocyte and the cell above
is a cell undergoing apoptosis
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Heterotrophy
Shown is the development of the hind limb of
chicken versus a duck. The duck retains the
webbing between the digits.
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Heterochrony
Heterochrony is
the regulation of
developmental
stages by
changing the
duration of the
developmental
process
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The one of the differences between the plain
zebra and Grevy zebra is that the Grevy zebra
has more narrow stripes. It is thought that the
genes responsible for the stripes are delayed in
the plain zebra, resulting in wider stripes.
Timing in development is very important.
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Neighboring Tissues
Can Effect
Morphogenesis
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Genes on the Y chromosome can
influence morphogenesis
There is primordial tissue that is
destined to become gonads. In
mammals the sex-determining
region Y (SRY) gene on the Y
chromosome will cause the
development of testis. Without
the presence of the SRY protein
the tissue will develop into
ovaries no matter the
chromosomal condition.
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Genes on the Y chromosome can influence
morphogenesis
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Effect of Testosterone on
Development
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Cloning using adult
differentiated cells
While it was thought
that differentiated adult
cells could not be used
to make a clone because
genes had been
permanently inactivated.
In 1997 researchers at
the Roslin Institute were
able to clone a lamb
from an adult
differentiated cell.
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