CN2011 Exam 1 Key - Cell Biology (20 pts) 1. (1/2 page...

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Cell Biology (20 pts) 1. (1/2 page maximum) In the axon, anterograde transport is accomplished via kinesin family plus-end motors and retrograde transport is mediated by the dynein minus-end direct motor. Things are more complicated in dendrites due to differences in the cytoskeleton. a. What is different (2 pts)? In axons microtubules are oriented with the (+) end distal and the (-) end soma proximal. By contrast, microtubules in the proximal dendrites are oriented in both orientations. In more distal portions of dendrites there are more microtubules with the (+) end distal to the soma. NOTE that microtubules aren’t actually + charged at the (+) end and negatively charged at the (-) end. (+) and (-) are just arbitrary designations to indicate that microtubules have a directionality to them. b. What problem(s) does this cause (2 pts)? Without oriented microtubules the problem is that both (+) or (-) end directed motors should transport cargo in both directions. Thus, it is difficult to provide a simple mechanism by which a net flux of cargo is transported towards distal end of dendrites. c. Provide a plausible explanation for why cargos are still able to reach distal dendrites (2 pts). Extra credit for providing more than one plausible explanation. Experimental evidence suggests that it is dynein that transports cargo out of the soma into dendrites. Models of this process suggest that because dynein is not able to transport cargo into axons that simple mass action will lead to next flux into the dendrites. In more distal dendrites where microtubules are oriented with the (+) end distal, kinesins take over transport. A second model would be that there are different ratios of (+) and (–) end soma-proximal oriented microtubules and thus net flux of cargo is feasible by one of the two motors. A third possibility is that (-) distal and (+) distal oriented microtubules would be decorated with different Microtubule Associated Proteins (MAPS) or other modifications (acetylation or tyrosination) and that this would change the affinity of a motor on the decorated MTs. 2. (1/3 page maximum) Describe the major differences between the cell biological life-cycle of a cholinergic synaptic vesicle and a Neuropepdtide-Y dense core vesicle (4 pts). Small clear cholinergic vesicles are synthesized as precursors in the trans Golgi, transported to synapses, filled with transmitter that is synthesized at the synapse, found at high abundance and clustered around active zone (AZ) release sites, found docked at AZs and released in response single action potentials that lead to very high very local Ca concentration near Ca channels at the AZ, and are recycled at the synapse via endocytosis to create new vesicles. NPY filled vesicles are synthesized as precursors in the trans-Golgi, but the NPY is loaded into
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CN2011 Exam 1 Key - Cell Biology (20 pts) 1. (1/2 page...

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