Download Chapter 21 Presentation-The Genetic Basis of Development

Chapter 21
The Genetic Basis of Development
“Embryology is to me by far the strongest single class of
facts in favor of change of forms, and not one, I think, of
my reviewers has alluded to this.”
-Charles Darwin, 1860
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.
Movie
1. Cell Division
 Cell division gives
rise to numerous
cells.
2. Cell Differentiation
 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
 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 as cell division, cell
differentiation and morphogenesis
overlap.
3. Morphogenesis
 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
 Morphogenetic events, as well as cell division
and differentiation, take place in all
multicellular organisms.
 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
 For example, 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
 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; some cells are totipotent
while other cells are pluripotent.
Totipotent Cells
 Totipotency is the ability of a single cell
to divide and produce all the
differentiated cells in an organism,
including extraembryonic tissues.
 Totipotent cells are formed as a result
of sexual reproduction.
Pluripotent
 In cell biology, the definition of pluripotency
has come to refer to a stem cell that has the
potential to differentiate into any of the three
germ layers: endoderm, mesoderm, or
ectoderm.
 Pluripotent stem cells can give rise to any
fetal or adult cell type. However, alone they
cannot develop into a fetal or adult animal
because they lack the potential to contribute
to extraembryonic tissue, such as the placent.
Animals
 The ongoing development in adult
animals is normally restricted to the
generation of cells that need to be
continually replenished: blood cells,
skin cells and the cells lining the
intestine for example.
Multicellular Organisms
 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
 Using the somatic 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…
 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
 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
 Continued research showed that the
DNA remains the same for the most
part, but the chromatin changes in a
way that problems arise.
Nuclear Transplantation
 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
 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
 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
 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:
 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.
Stem Cells
 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:
 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.
2. The environment in which the embryo
develops.
1. The “Stuff” in the Egg
The egg cell’s cytoplasm contains RNA and
protein molecules encoded by the mother’s
DNA.
mRNA, proteins, organelles, and other
substances are scattered unevenly throughout
the cytoplasm of an unfertilized egg.
These maternal substances influence the
course of early development called cytoplasmic
determinants.
Cytoplasmic Determinants
 Following fertilization, mitotic divisions
distribute the zygote’s cytoplasm into
separate cells.
 The nuclei of these cells are subjected
to many different cytoplasmic
determinants.
 What has been received will determine
the developmental fate of each of the
cells.
Cytoplasmic Determinants
 Cytoplasmic determinants help to create
an animal’s 3D arrangement before
morphogenesis can shape the animal.
2. The Environment
 The environment in which the embryo
develops plays an important factor in
outcome of the developing organism.
 The surface contact of cell-to-cell interaction helps
to signal development.
 By the process of induction, 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
 Pattern formation is the development of
spatial organization in which the tissues
and organs of an organism are all in
their characteristic places.
In plants, pattern formation occurs through
the life of the plant.
In animals, it is restricted to the embryonic
or juvenile stage.
Pattern Formation
 Pattern formation in animals begins in the
embryo when the major axes are determined.
 Before tissues and organs within an animal
can be formed, the 3D arrangement must be
established. Recall that this occurs as a
result of cytoplasmic determinants.
 This process has been extensively studied in
many animals such as the fruit fly, sea urchin,
frog, nematode, and chicken.
Pattern Formation, An Example
 Here is an
example of
pattern formation
and cell signaling
as seen in the
fruit fly.
Movie
Apoptosis
 Apoptosis is the programmed cell death that occurs
through the normal course of development.
 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.
Apoptosis
 Apoptosis is essential to the
development of all cells. 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
 Looking across species, there are many
similarities in the genes controlling
development.
Homeotic Genes
 A homeobox is a DNA
sequence found within
genes that are involved in
the regulation of patterns
of development
(morphogenesis).
 Genes that have a
homeobox are called
homeobox genes and
form the homeobox gene
family.
Homeotic Genes
 The most studied and
the most conserved
group of homeodomain
proteins are the Hox
genes, which control
segmental patterning
during development.
 Not all homeodomain
proteins are Hox
proteins.
Homeotic Genes
 Many distantly related eukaryotes such
as plants and yeasts also have these
Hox genes (regulatory sequences).
 Such similarities indicate that the
homeobox sequence is very useful in
development and arose very early on in
evolution and has been conserved for
hundreds of millions of years.
Homeotic Genes
 Not all
Hox genes are homeotic genes--
not all of them control body parts.
However, most are involved in
development.
Homeotic Genes
 Research has revealed that the homeobox-
encoded region is part of the protein that
functions as a transcription regulator.
 The shape of the encoded region allows it to
bind to any DNA segment, but by itself, it
cannot select a specific sequence. The
variable regions within the whole protein
allow it to interact with other transcription
factors and enhancers within the DNA.
 In this way, the homeobox genes work to
switch certain developmental genes on and
off.
Homeotic Genes
 There are many other regions of DNA
that are highly conserved among
species.
 The common question is how can the
same genes code for different body
forms?
 It is likely that the small changes in the
regulatory sequences lead to major
changes in body form--the basis of the
next unit.