Problem Set 3

Problems are to be done in groups. The concept map you make for Problem 1 should be turned in at the end of the first hour of lab remaining answers are due Wed. midnight. Problem 3 is a puzzler, which will require some thinking outside the box 1. (3 points Review from last week). Working with your group, construct a concept map of the following eight components, to be handed in to your LA when youre finished. A) Transcription factors B) Wnt pathway signaling C) Ras and MAP kinase cascade D) (-catenin levels E) Cadherin activity F) Fibronectin receptors (integrins) G) ECM components H) Cell behavior (shape change, motility, adhesion to other cells, etc.) Ans Concept maps have no one right answer, but all the relationships possible should be included and all those included should be correct. See attached sheet for one possible configuration. 2. (4 points) GLP-1 is a C. elegans Notch homolog that is involved in two early embryonic signals from P2 to ABp, and later from MS (descended from E) to descendants of AB (see diagram from web notes). In the 4-cell embryo, in situ hybridization reveals that glp-1 mRNA is present at the same apparent concentration in all four cells however, antibody staining shows that the protein is present only in the two receiving cells for these signals, ABa and ABp. (Remember that there is virtually no transcription going on in the 4-cell embryo.) a) (2) Describe a mechanism of regulation that could make this possible. Is it transcriptional or post-transcriptional, and if the latter, could it be alternative splicing, translational, or post-translational regulation What kinds of proteins could be involved, what would these proteins bind to and where, and what would be the result of their binding Ans Regulation must be post-transcriptional (translational or post-translational) b) (2) Regulation, by whatever mechanism you have proposed above, appears to be occurring only in the two posterior cells, EMS and P2, implying that only those cells contain your regulatory protein(s). By what mechanism might the embryo manage to have these proteins present in only these cells (If you propose another cycle of regulation as above to accomplish this, then you must go on to explain how those new regulators are localized to the right cells, and so on. What has to be the primary event in causing this asymmetry) Ans This must ultimately be controlled by segregation of determinants, dictated by the anterior-posterior asymmetry caused by sperm entry. A plausible explanation would be that the P-granules contain a glp-1 mRNA translational inhibitor, which is segregated to the P1 cell at first cleavage and then distributed to P2 and EMS at second cleavage, so that GLP-1 protein is made only in the AB cells. 3. (Puzzler 2 points) You have isolated a new C. elegans mutation that causes embryonic lethality, and you have named the gene it identifies as dem-1, for dead embryos. To characterize it, you do the following selfing experiments and crosses, with the results shown Experiment Embryonic viability dem-1/ hermaphrodite selfing 100 dem-1/dem-1 hermaphrodite selfing 100 dem-1/dem-1 hermaphrodites x / males 100 dem-1/dem-1 hermaphrodites x dem-1/ males 100 dem-1/dem-1 hermaphrodites x dem-1/dem-1 males 0 dem-1/ hermaphrodites x dem-1/ males 100 dem-1/ hermaphrodites x dem-1/dem-1 males 0 / hermaphrodites x dem-1/dem-1 males 0 a) (1) Is the DEM-1 gene product most likely to function 1) very early in embryogenesis 2) later in embryogenesis but before gastrulation 3) after gastrulation Explain briefly. Ans Since the mutation shows a strict parental effect, it cant be supplied by transcription of the embryonic genome, hence must function early best answer is 1, but 2 could be possible. b) (1) When during development must the dem-1 gene be transcribed, and in what cells Ans This is a strict paternal effect mutation (rare, but a few have been found). Therefore, thegene must encode an essential gene product that is brought into the embryo as a sperm cytoplasmic component at fertilization, and the gene must be transcribed during spermatogenesis, in sperm or sperm cell precursors (spermatocytes). 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