15-15 Activation maturation\') of frog oocytes is signaled through a MAP kinase
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Question
15-15 Activation maturation') of frog oocytes is signaled through a MAP kinase signaling module. An increase in the hormone progesterone triggers the module by stimulating the translation of Mos mRNA, which is the frog's MAP kinase kinase kinase (Figure Q15-2). Maturation is easy to score visually by the presence of a white spot in the middle of the brown surface of the oocyte (see Figure Q15-2). To mine the dose-response curve for progesterone-induced activation of MAP kinase, you place 16 es in each of six plastic dishes and add various concentrations of proges terone. After an overnight incubation, you crush the oocytes, prepare an extract, and determine the state of M kinase phosphorylation (hence, activation) by SDS poly- acrylamide-gel electrophoresis (Figure Q15-3A). This analy- sis shows a graded response of MAP kinase to increasing concentrations of progesterone. Before you crushed the oocytes you noticed that not all oocytes in individual dishes had white spots. Had some oocytes undergone partial activation and not yet reached the white-spot stage? To answer this question, you repeat the experiment, but this time you analyze MAP kinase acti- vation in individual oocytes. You are surprised to find that each oocyte has either a fully activated or a completely inac tive MAP kinase (Figure Q15-3B). How can an all-or-none m response in individual oocytes give rise to a graded response in the population2Explanation / Answer
If more than one effector molecule bind to the target molecules on then there is activation and response is sharpened depending on number of required effector molecules. At low concentration of effector molecule, most target proteins have single effector bound and are mostly inactive. The cellular response and activation increase as the concentration of effectors increase. An all or none law is basically a principle by which nerve or muscle respond to a stimulus but it is independent of the strength of the stimulus. Now, all or complete response is given when stimulus exceeds certain threshold but otherwise there will be no response.
In this case it is seen that maturation of oocyte is seen via MAP kinase signalling which in induced by the progesterone and this triggers activation of MAP kinase and oocyte maturation. Now on repeating the experiment it was found that all the oocytes matured because the concentration and strength of progesterone increased that exceeded the threshold limit. Thus, all or none response in individual oocyte give rise to graded response. The response to progesterone is all or one, with no oocytes having partially activated MAP kinase. The graded response arise from all or none response in individual oocytes, with different mixtures of either fully or immature oocytes giving rise to intermediate MAP Kinase activation. The action of all or none may be due to difference in age and size of the oocytes.
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