Problem Set 2

Problems 2 (Classes 3,4) 1/24/06 Due Wed 1/25 by midnight on Homework site 1. (2) In an animal that has only a single myosin heavy chain (MHC) gene, striated skeletal muscle contains an isoform of MHC that is not found in smooth muscle, which contains a different isoform not found in skeletal muscle. (Isoforms: very similar proteins with the same general function but some difference in amino acid sequence.) List three different mechanisms that could account for this pattern of myosin expression. Ans: 1) alternative promoters used in the two tissues, leading to MHCs with different N-termini. 2) alternative internal splicing, leading to differences in the exons that end up in processed mRNAs. 3) alternative poly-A addition sites that lead to MHCs with different C-termini. 2. (1) Match each ligand or receptor from the list below with the type of signaling in which it participates: (1) juxtacrine, (2) paracrine, or (3) endocrine. a) Receptor tyrosine kinases Ans. 2, (1) b) Wnt Ans. 2 c) Notch Ans.1 d) Hedgehog Ans. 2 e) Testosterone Ans. 3 3. (2) The steroid hormone testosterone controls almost all aspects of male development in mammals. We know there is only a single type of testosterone receptor in all the cells that respond to it, because mutant XY individuals who lack the receptor are completely feminized in all the testosterone-controlled tissues.  However, testosterone clearly activates different genes in different cell types (e.g. muscle, gonadal tissues, etc.). Briefly explain how this is possible. Combinatorial control makes this possible. The same NHR can activate different genes in different tissues depending on what other transcription factors are present. 4. (4) Look at Figure 5.31 in the Gilbert 7th edition (in slides for 1/19 class). Briefly explain: a) (1) what the data indicate about the stability of casein mRNA. b) (2) what is the likely sequence of events that takes place in mammary gland cells when they are exposed to prolactin that can account for the observed difference in stability (mention all steps). c) (1) Note from the legend (in the Gilbert book) how and when this experiment was done. To measure amounts of labeled casein mRNA at various times it had to be isolated by hybridization to unlabeled casein DNA and then assayed for radioactivity. Could this experiment be done more simply today by using Northern blots to measure the amount of casein mRNA present at various times? Explain briefly why or why not. Answers: a) stability is increased in cells exposed to prolactin. b) Once they remember (or are told) that prolactin is a hormone, they should realize it can’t be directly affecting casein mRNA stability. They should be able to figure out that it must bind to a receptor, which activates a signaling pathway, which activates one or more transcription factors that cause expression of a protein that binds to the 3’ UTR of the mRNA to inhibit its degradation. Partial credit if they know something must be interacting with the 3’UTR to change the mRNA stability. c) No. Northern blotting would measure total casein mRNA present, which may be decreasing much less rapidly or not at all, because synthesis is still going on. To measure stability, a sub-population of the molecules must be pulse-labeled so that it can be followed independently of the new mRNA that is synthesized after the chase. 5. (1) What did you find most difficult to understand in the reading or problems for this week? If you had no difficulties, what did you find most interesting? (If one of you is turning in the answer to this for your group, please include answers or points of view from all group members.)