Fall_2010_Exam_2[1]

Fall_2010_Exam_2[1] - Cell Biology 3400 Quiz II NAME Answer...

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Cell Biology 3400 NAME_______________________ Quiz II Answer 1 & 7; 2,3 or 4; 5 or 6 1a) Describe the uptake of LDL particles by liver cells. Fig. 14-29. LDL receptors in the PM bind LDL particles via ApolipoproteinB at pH7.4 of extracellular media. A cytoplasmic sorting sequence on the Carboxy end of the receptor binds to the AP2 adaptor protein recruiting receptor to coated pits that are forming due to the association of Clathrin with the adaptor. Coated pit forms, uncoats and fuses with pH5.0 endosome where recept. and LDL uncouple sending recept,. back to PM and LDL to lysosome. b) If you wanted to block the uptake of LDL particles by cells, how would you do it? (Hint: what binds to receptors?) Many ways. IgG directed against receptor. Flood cells with excess ApolipoB which would block receptors. IgG directed at ApoB. Etc. c) Familial Hypercholesterolemia leads to high cholesterol levels in the blood which in turn leads to the development of atherosclerosis and death at an early age. Familial means that the disease is genetic: Give three possible causes for the disease. A mutation in ApoB which will not bind receptor. A mutation in the receptor so it will not bind ApoB. A mutation in receptor so it will not bind adaptor. A mutation in adaptor so it will not bind NPXY on carboxy end of receptor. Etc. All of these mutations have actually been identified in individuals with Hypercholesterolemia. d) What are microvesicular bodies (MVBs) ? Hint: Don’t forget what this question is about! MVPs are endosomes with vesicles included in their lumen. The vesicles form by invagination or inbudding of regions of the endosomal membrane that contain membrane proteins that are destined for degredation in lysosomes. Fig. 14-32 e) Describe the mechanism of MVB formation in as much detail as you can muster. Fig. 14-32 and 14-33. Hrs is a protein in the membrane of endosomes that becomes tagged with monomeric ubiquitin. The ubiquinated HRS recruits three ESCRT (endosomal sorting complexes required for transport) to the membrane which complete the inbudding process and pinch off a vesicle inside the endosome. An ATPase called Vps4 then hydrolyzes ATP and uses the energy to release ESCRT so it can be recycled and used again. 2a) Describe the composition and structure of 100A and 300A chromatin fibers. 100A fibers (10nm) are composed of DNA (10A) wrapped around nucleosomes (Octet of H3,H4,H2a and H2b dimmers). If Histone H1 is present the nucleosomes abut but in the absence of H1 linker DNA (~50nts) is seen between nucleosomes. The 300A fiber is derived from the coiling of the 100A fiber to form a “spring-like” structure. Fig. 6-30. b) What is a chromatin remodeling complex? Describe its function i.e. why does chromatin have to be remodeled? The CRC is a multiprotein complex that interacts with 300A and 100A chromatin fibers to facilitate access of transcription factors to regions of DNA. They appear to do this by “phasing” nucleosomes or moving them to different regions of the DNA thus exposing TF binding sites as well as by propagating and
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