Biology 52 Exam III, April 5, 2006


Feel free to use sketches when this is helpful to explain your answers. 


1) [16 points]  Frog oocytes are enucleated and given new nuclei from the sources indicated.  a) In each case, would the resulting zygotes develop into tadpoles or arrest at some earlier stage?

                        source of nucleus                      How it would develop

            i)          sperm

            ii)         blastomere

            iii)         skin cell

            iv)        blastomere from part iii


b) For any cases in which a tadpole fails to form, what limitation or biochemical modifications might have prevented the nucleus from being able to direct development to the tadpole stage?




c) The protocol for cloning sheep includes application of an electric pulse to co-cultured oocytes and donor cells.  What does the electric pulse accomplish? 




d) Cloning sheep works best if the donor cell is in the G0 phase of the cell cycle.  Why might this be?



e) An isoform of phospholipase C called PLC-zeta is activated during normal animal egg fertilization.  What signaling and cell biological events does activation of PLC-zeta initiate? 






3) [20 points] a) Using the following diagrams of frog embryos at different stages, indicate in which cells and at what stage(s) each of the following proteins is present.

i) Xnr (=nodal-related)

ii) noggin

iii) N-cadherin

iv) goosecoid








b) What biochemical activity (metabolic enzyme, receptor, etc.) does each of these proteins have?  i) Xnr (=nodal-related)

ii) noggin

iii) N-cadherin

iv) goosecoid


c) If frog embryos are irradiated on their vegetal sides with UV light just after fertilization, what immediate effect does this have on the zygote?  How would this affect development? 




e) Which of the proteins listed above can be used to overcome the effect of UV-irradiation?  Explain in each case how you would use the protein to restore normal development. 






4) [12 points] You are studying the organizer of the model vertebrate Xenopus tropicalis, which develops very similarly to Xenopus laevis but is convenient to transform with engineered genes and to breed.  a) Diagram a DNA construct that you could introduce into transgenic animals in order to express the GFP gene encoding Green Fluorescent Protein in the organizer. 





b) Outline experimental steps by which you could use these transgenic animals to determine which genes are expressed in cells of the organizer just before gastrulation.  Assume that microarrays having all the genes on the genome are available. 














c) What additional experiment would you do to determine whether these cells express different genes as gastrulation proceeds? 



5) [16 points] a) Indicate on the following diagram of a mouse blastocyst embryo:

i) the inner cell mass

ii) the trophectoderm


b) Does each of the following tissues arise from the inner cell mass or the trophectoderm?

            i) placenta

            ii) epiblast

            iii) hypoblast 


c) At what stage do the cells that will form the inner cell mass first appear different from the cells that will form the trophectoderm?   What morphological characteristic(s) of these cells differ at this stage? 



d) At what stage are distinct cells first specified (i.e. instructed) to form the trophectoderm?  At this stage, how do these cells differ from other cells of the embryo? 



e) Describe the experimental evidence that shows that trophectoderm specification occurs at the stage you indicated. 






f) How could you test experimentally whether these cells are determined (in the developmental sense) to become trophectoderm at this stage?





6) [9 points]  a) What effect would the following have on blood cell production in a mouse?  

                        i) extra copies of the gene encoding erythropoietin


                        ii) a dominant negative mutation in the gene encoding the erythropoietin receptor


iii) a mutation in the gene encoding the erythropoietin receptor that increased its

            responsiveness to erythropoietin


            b) Which cells express the gene encoding erythropoietin?


            c) Which cells express the gene encoding the erythropoietin receptor?


            d) How could you naturally increase the erythropoietin level in your own body?


            e) How does the treatment you indicated in part d work?

7) [26 points] a) The vertebrate colon is an epithelium similar to the small intestine, except that it lacks villi.  Sketch the wall of a normal colon.  On your diagram, indicate i) stem cells, ii) which cells are dividing most actively, and iii) which cells are most differentiated. 





b) Indicate which cells in the colon you expect would express each of the following genes.  Explain your answer in each case. 

            i) Oct4

            ii) Wnt

            iii) cyclin

            iv) APC


c) In the inherited predisposition to colon cancer Familial Adenomatous Polyposis (FAP), a mutation in the gene encoding the APC protein causes predisposition to cancer.  a) Does the mutation increase or decrease activity of the APC protein?


d) What biochemical or signaling event(s) does APC regulate in normal cells? 






e) If a man with this predisposition fathers 8 children, how many of them would you expect to have the same predisposition?  (Assume that the mother of the children does not have this predisposition.) 


f) What happens in colons of these individuals for cancer to develop?






g) Sketch a crypt from a colon in which adenomatous polyps are present.  i) Indicate which cells are different from the corresponding cells in part a.  ii) How are they different? 





h) Why do patients with FAP preferentially develop colon cancer instead of other cancers such as lung cancer?



Honor code pledge: I certify that I have neither given nor received help on this exam: