Download Ch. 21 TheGeneticBasisofDevelopment

Chapter 21
The Genetic Basis of Development
Zygote and Cell Division

When the zygote divides, it undergoes
3 major changes:
1. Cell division
2. Cell differentiation
3. Morphogenesis
Cell Signaling

Cell signaling is
largely responsible
for the
developmental
processes.
http://bcs.whfreeman.com/thelifewire/content/chp15/15020.html
1. Cell Division
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Cell division, mitosis
gives rise to
numerous cells.
2. Cell Differentiation
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Cell differentiation is
the process by
which cells become
specialized in form
and function.
These cells undergo
changes that
organize them into
tissues and organs.
3. Morphogenesis
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As the dividing cells begin to take form, they are
undergoing morphogenesis which means the
“creation of form.”
Morphogenetic events lay out the development very
early on in development.
These morphogenetic events “tell” the organism
where the head and tail are, which is the front and
back, and what is left and right.
As time progresses, later morphogenetic events will
give instructions as to where certain appendages will
be located.
Morphogenetic events, as well as cell division and
differentiation, take place in all multicellular
organisms.
Morphogenetic Events

Morphogenesis differs in 2 major ways in
plants and animals:
1. In animals, movements of cells and tissues are
required for the transformation of the early
embryo into the characteristic 3D form of the
organism.
2. In plants, morphogenesis and growth in overall
size are not limited to embryonic and juvenile
periods, they occur throughout the life of the
plant.
Apical Meristems
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Remember, apical meristems of plants
are responsible for a plant’s continued
growth and development and the
formation of new organs throughout the
plant’s life.
These are perpetually embryonic
regions in the tips of shoots and roots.
The Experiments of F.C. Steward
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In the 1950’s, Steward was working with carrot
plants.
He showed that cells taken from the root of the plant
would grow into an adult carrot when cultured in
growth medium. These plants were clones of the
original.
It demonstrated that differentiation doesn’t involve
irreversible changes in DNA; that cells can
dedifferentiate.
In plants, cells can remain totipotent: they retain the
potential to make all parts of the plant. (can become
any kind of cell) while other cells are pluripotent (can
become many cell types).
Tutorial 19.2 (E) Early Asymmetry in the Embryo
Animals: Stem Cells

Stem Cells: relatively unspecialized cells that have
two important properties:
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The adult body has various kinds of stem cells which
serve to replace nonreproducing specialized cells.
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They continually reproduce themselves.
They can differentiate into specialized cells of one or more
types.
Example stem cells in bone marrow give rise to all the
different kinds of blood cells.
Stem cells that can become multiple cell types are
called pluripotent.
Multicellular Organisms
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The cells of multicellular organisms come
almost entirely from differences in gene
expression.
Regulatory mechanisms turn certain
genes on and off during development.
These regulatory mechanisms are what
makes cells different because nearly all
cells have the same genetic complement.
Cloning

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Using the somatic (body) cells of a
multicellular organism to generate a new
organism is called cloning.
Each clone is genetically identical to the
parent plant.
Differentiated cells don’t usually divide in
culture, so researchers had to take a different
approach to decide if animal cells were
totipotent.
What Researchers Did…
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They removed the nucleus of
an unfertilized egg and
replaced it with one from a
differentiated cell.
The process is called nuclear
transplantation.
If the transplanted cell
retains all of its genetic
information, the recipient cell
should develop with all of
the necessary tissues and
organs.
Nuclear Transplantation

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As these experiments
were conducted on
frogs, it was determined
that something in the
DNA does change.
In tadpoles, normal
development
proceeded, but as the
age of the donor
nucleus increased, the
percentage of
organisms that
developed correctly
decreased.
Nuclear Transplantation
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Continued research showed that the DNA remains
the same for the most part, but the chromatin
changes in a way that problems arise.
Often times, the histones get modified or DNA is
methylated and these changes in the chromatin
prevent dedifferentiation.
Sometimes the process is reversible, but usually it
isn’t.
One thing is certain, most scientists agree that all
cells contain the necessary genetic information to
make an entire organism.
However, the different cell types exist because of the
variations in gene expression.
Nuclear Transplanting and Cloning
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In 1997, Scottish researchers cloned a
sheep named Dolly.
They used cells from mammary tissue
in an adult sheep, implanted the
nucleus from the cell into egg cells from
which the nucleus had been removed
and implanted into the uterus of a
lamb.
Nuclear Transplanting and Cloning
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Analysis of the DNA from Dolly showed
it was identical to that of the original
sheep, and its mitochondria matched
that of the mother lamb.
However, Dolly’s cells appeared older
than her age would indicate.
Dolly’s Problems
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She suffered from a lung disease seen
in older sheep.
She had arthritis.
These results indicate that not all of the
DNA had been reprogrammed.
Problems With Animal Cloning In General:
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Many of the animals exhibit a variety of
defects such as obesity and premature death.
Only a small percentage of the embryos
created develop correctly resulting in live
birth.
Possible reasons for these results include:

Epigenetic changes in chromatin (acetylation of
histones and/or methylation of DNA) result in only
a small number of genes being turned on while
the others remain suppressed.
Tutorial 19.1 Embryonic Stem Cells
Stem Cells
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The use of stem cells, especially embryonic
stem cells, has many obvious medical
applications.
There are obvious ethical dilemmas that arise
from the research.
There are moral issues on both sides:
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One is that it is immoral to tamper with human
embryos for medical purposes.
The other is that it is immoral not to because the
benefits outweigh the cost of doing nothing.
Cell Differentiation
There are 2 major things telling a cell when and
how to differentiate:

1.
The “stuff” found within the egg at the time of
conception.
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2.
The egg cell’s cytoplasm contains RNA and protein molecules
encoded by the mother’s DNA.
What has been received in the cytoplasm will determine the
developmental fate of each of the cells.
The environment in which the embryo develops.
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an embryo’s genes signal the expression of proteins that cause
changes in nearby target cells.
These signals send a cell down a specific developmental pathway-inducing further differentiation of the many specialized cells
within the new organism.
Pattern Formation
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Pattern formation is the development of
spatial organization in which the tissues and
organs of an organism are all in their
characteristic places.
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In plants, pattern formation occurs through the
life of the plant.
In animals, it is restricted to the embryonic or
juvenile stage.
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Pattern formation in animals begins in the embryo when
the major axes are determined.
Pattern Formation, An Example

Here is an
example of
pattern formation
and cell signaling
as seen in the
fruit fly.
Movie
Tutorial 19.3 Pattern Formation in the Drosophila Embryo
Apoptosis
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Apoptosis is the programmed cell death that occurs through
the normal course of development.
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It is usually triggered by signals that activate a cascade of signal
proteins in cells that are to die.
During the process, the cell shrinks, the nucleus breaks down and
the nearby cells quickly engulf and break down the contents of the
cell.
The process helps in the growth and development of the major
structures and systems of an organism.
It controls cell division helping to slow or stop division in certain
cells.
Homeotic Genes
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Looking across species, there are many similarities in
the genes controlling development.
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A homeobox is a DNA sequence found within genes that
are involved in the regulation of patterns of development
(morphogenesis). The homeobox-encoded region is part of
the protein that functions as a transcription regulator.
In this way, the homeobox genes work to switch certain
developmental genes on and off.
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The most studied the Hox genes, which control segmental
patterning during development.
Many distantly related eukaryotes such as plants and yeasts
also have these Hox genes (regulatory sequences).
Homeotic Genes
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Genes that have a
homeobox are called
homeobox genes and
form the homeobox gene
family.