Practice Exam
100 Points Total
20 pts.
5 pts.
5 pts.
1. Maternal and zygotic transcription factors play a role in patterning the fly and
frog embryos.
a. What are transcription factors and how do they act?
1) Transcription factors are sequence specific DNA binding proteins.
2) Transcription factors can either activate transcription (activators) or repress
transcription (repressors).
b. What is the difference between a maternal and zygotic patterning gene? Give
an example of each in both flies and frogs.
5 pts.
5 pts.
25 pts.
10 pts.
1) A maternally acting gene functions only in the mother to build the egg.
Examples: Dorsal in flies and VegT in frogs.
2) Both parental copies of zygotic genes are active in the embryo. Examples:
Snail, Twist, Sog, Rho, AS-C, Dpp in flies and Chordin, BMP4 in frogs.
2. Hilde Mangold provided the first clear evidence for the existence of a secreted
factor which could redirect the fates of cells in the frog embryo.
Draw a diagram of the Mangold experiment indicating how she performed her
transplantation experiment and what the result of that experiment was.
Distinguish host and donor derived tissues in your diagram. Indicate dorsal and
ventral and the relative positions of the mesoderm, neural ectoderm, and nonneural ectoderm.
Cut Out Small
Region
Graft Into
Host
Spemann Organizer
Emits Signals
Donor Derived
Spemann
Organizer
Donor Embryo
8 pts.
7 pts.
Host Embryo
Grafted Embryo
What critical feature of the experiment allowed Mangold to conclude that a
secreted factor was being liberated?
She used host and donor embryos from different amphibians that could be
distinguished in the transplantation experiments. This allowed her to show that
the secondary nervous system which formed was composed of host not donor
cells, indicating that the donor cells must have sent a signal to neighboring host
cells to redirect their developmental course.
What effect did this secreted factor have on neighboring ectodermal cells?
It caused them to follow their default preference to become neural.
15 pts.
4 pts.
3. In a classic experiment analyzing neural induction, animal cap cells (e.g.
ectodermal cell cut away from the marginal zone) were dissociated in a cell
culture medium, incubated for a period of several hours, and then reaggregated
by gentle centrifugation.
What cell type did these reaggregated cells give rise to?
The dissociated and then reaggregated cells gave rise to neural structures.
4 pts.
How did this result conflict with the prevailing of view of neural induction at the
time?
The previous view was that the default state of ectoderm was epidermal.
With the aid of a diagram describe a modern experiment that resolves this
apparent contradiction. (one short paragraph maximum).
7 pts.
See figure:
The Ectoderm Dissociation Experiment
Dissociate
Ectoderm ->
Single Cells
Dissociate
Ectoderm
+ BMP4
Dissociate
Ectoderm
+ BMP4
+ Chordin
Reaggregate cells by centrifugation
Allow reaggregated cells to develope into skin versus neural tissue
Neural
Skin
Neural
Conclusion: default ectodermal cell fate is neural
15 pts.
4pts.
4pts.
7 pts.
4. In fruit flies, what cell types do neural and non-neural ectoderm give rise to
respectively?
The non-neural ectoderm gives rise to epidermal cells
The neural ectoderm gives rise to both epidermal and neural cell types
Briefly describe experiments using fruit fly embryos which show that the default
state of ectoderm is neural (two sentences maximum).
In mutant embryos lacking dpp function cells in the dorsal region which otherwise
would give rise only to non-neural cells types give rise to neural precursors
instead. Conversely in sog- mutants, Dpp signaling spreads into the lateral
region of the embryo and suppresses neural development in these cells.
25 pts.
4 pts.
6 pts.
15 pts.
5. Lateral inhibition plays an important role in the neural versus epidermal cell fate
choice within the neuroectoderm.
Define the term lateral inhibition (one sentence) and then describe the Doe and
Goodman experiment and how it lead to the lateral inhibition hypothesis (two
sentences maximum).
Lateral Inhibition is the process by which cells in the neuroectoderm signal
reciprocally to inhibit each other from becoming neural precursor cells.
Doe and Goodman used a laser to destroy neuroblasts as they began to
delaminate from the neuroectoderm and observed that a neighboring cell, which
otherwise would have become an epidermal cell, delaminated and became a
replacement neuroblast [3 pts]. They inferred from this experiment that
neuroblasts must normally be emitting signals to their neighbors which
inhibited them from assuming the default neuronal cell fate [3 pts].
Draw a diagram of a cell pair engaged in lateral inhibitory signaling before and
after the neuronal versus epidermal cell fate decision has been made. Indicate
the expression patterns of Delta, AS-C, Notch, Su(H), and E(spl) in your
diagram.
[2 pts for indicating each gene correctly and 5 pts for drawing the correct
diagram].
Before Cell Fate Choice
is Decided
Notch
Su(H)
Delta
AS-C
Delta AS-C
Cell A
Nucleus Cell B
After Cell Fate Choice is Made
Decided Notch
Sending
Cell ->
(Neural
precursor)
Su(H)
Delta
AS-C
Receiving
Cell ->
Notch-ICD
(Epidermal
precursor)
E(spl)
Nucleus