Transcript Ch 21 Lecture

Chapter 21: The
Genetic Basis
of Development
I. From single cell to multicellular organism:
A. Embryonic development =
cell determination  differentiation 
morphogenesis
One week
-apical meristems (stems and roots) in
plants are perpetual embryonic regions,
continually growing.
B.Researchers use the “model organisms” to
study development:
1. Mus musculus = mouse
2. Drosophila melanogaster = fruit fly
3. Caenorhabditis elegans = nematode
4. Danio rerio = zebrafish
5. Arabidopsis thaliana = common wall cress
These are model organisms because they have:
1. readily observable embryos
2. short generation times
3. relatively small genomes
4. preexisting knowledge of organism
C.The C. elegans have 959 somatic cells.
Researchers have mapped out exactly how
they develop into adulthood. These are
transparent worms.
This is a cell lineage; a fate map,
showing what cells are destined to become.
II. Cells differentiate based on which genes are
turned on and which genes are turned off.
A.Different types of cells in an organism have
the same DNA. This is called “genomic
equivalence.”
B.Can differentiated cells ever become a
whole new organism?
1.In many plant species, you can take
differentiated cells and create a new
plant.
This is called “Totipotency.”
The new organism becomes a clone of
the parent plant.
Differentiated
cells from
animals usually
fail to become a
new organism.
The ability of
the transplanted
nucleus to
support development depends
on the age of
the donor an
Embryo 
tadpole develops
Tadpole 
<2% develop
Nuclear Transplantation:
C.Cloning the first mammal:
1.Dolly, the sheep: In 1997, Ian Wilmut
cloned an adult sheep by nuclear transplantation:
1.Mammary cells
“starved” so that
cells go into G0
phase.
2.Mammary nuclei
are implanted
into egg that has
been denucleated.
3.Egg will grow in
culture and then
when an embryo
is formed, it will
be implanted
surgically into the
uterus of a sheep.
After 256 tries,
Dolly was born.
Dolly died after
6 years of life
due to a lung
disease; she
also had arthritis.
However, before
her death, she
was able to have
4 offspring.
D.Stem cells: Unspecialized cells that
that continually reproduce, and under specific
conditions, differentiate into specialized cells.
1.Stem cells are multipotent, or pluripotent,
which means they can become many types
of cells.
2.Stem cell research is of great interest
because with it, we may be able to repair
damaged organs by adding healthy, new
cells.
a. Parkinsons: Brain cells
b. Diabetics: Pancreatic cells
http://www.sumanasinc.com/webcontent/
anisamples/generalscience/stemcells.html
E.Determination leads up to differentiation in
cells.
1.As an organism develops, cells become
committed to its final state – it is
“determined.”
2.The first sign of differentiation is when
mRNA for specific proteins are made.
 code for that cell’s “tissue-specific
proteins.”
“Master Control/Regulatory Genes” commit
the cell.
F.How are the “master control gene” turned on?
 Maternal cytoplasm in the egg contains
maternal proteins, mRNA, and organelles,
not equally spread out in the egg:
1.These maternal
molecules are called
“cytoplasmic
determinants.”
Unevenly distributed,
they will turn on
certain genes.
2.Cells can synthesize signal molecules that
turn on genes in neighboring cells.
This is called induction.
Pattern formation:
the development of
a spatial
organization in
which the tissues
and organs of an
organism are all in
their characteristic
places.
III.Pattern formation in the fruit fly:
A.“Positional Information” tells a cell where it
is located relative to the body axes and to
neighboring cells. They are molecules that
determine how the cell will respond to
molecular signals.
Mitosis takes place
without cytokinesis
After the 10th division,
the nuclei migrate
outward. After the 13th
division, plasma
membrane forms around
each nuclei.
HIERARCHY OF GENE ACTIVITY IN DROSOPHILA
DEVELOPMENT
(A cascade of gene activations sets up the segmentation pattern)
(Basic
subdivisions
along the A-P axis)
(Segments
in pairs)
B.After segment polarity genes are turned on,
The homeotic genes direct the identity of
body parts (antennae, legs, and wings
develop on appropriate segments).
1.All homeotic genes of Drosophila include a
180-nucleotide sequence called the
homeobox, which specifies a
60-amino-acid homeodomain.
a.An identical or very similar sequence of
nucleotides (often called Hox genes) are
found in many other animals, including
humans.
Mutations in hox genes:
posterior
5´
anterior
3´
C.Programmed Cell Death: “Apoptosis”
Required for normal development. It is
controlled by apoptosis genes, “suicide genes.”
1.Examples:
-Hands and feet
-Neurons; surplus cells eliminated
-Endometrium at the start of menstrual cycle
Regulated by changes in the activity of proteins