28. In vitro, at a uniform tubulin concentration, the Duy ends of MTs grow faste

Question

28. In vitro, at a uniform tubulin concentration, the Duy ends of MTs grow faster than their Min ends. 29. The treadmilling of MTs is observed when the tubulin concentration is than the CC for the minus ends, but hun than the CC for the plus ends. 30. In vivo, the of MTs is caused by the alternation of growth and shrinkage phases at the plus ends. 31. During Dynamic instability (DI), the shrinkage is initiated when -bound tubulin subunits (dimers) are exposed at the plus ends of MTs. 32. During the growth phase of DI, the plus ends are protected from disassembly by a 33. Within the cell, MTs grow from a structure called mc Digunan 34. In the MTOC, nected by rings of-hsau 35. MTs are anchored to the MTOC via their man 36. An actin monomer is calledactin, the filament is called 37. Each F-actin filament is a helix made of wlinear strands of G-actin polymer 38. A growing F-actin filament has inherent plus/minus polarity, with the plus end predominantly made of cap. ends actin. -bound actin subunits. 39. In stress fiber, actin filaments are Collchl bd 40. In lamellipodia, actin filaments are pianks nu 41. In filopodia, actin filaments are Ql d 42. Intermediate filaments (Fs) called vimentin are mainly found in fonnGiv 43. The second step of IFs assembly is an) ¥doser -- 44. IFs' protofilaments are formed by tissues. lateral alignment of two dimers. cd to enbinding of tetramers

Explanation / Answer

During the Dynamic instability (DI), the shrinkage is initiated when GDP bound tubulin subunits are exposed at the plus end of microtubules.

Explanation:

Microtubules are made up of heterodimeric tubulin proteins. Being dynamic in nature, microtubules constantly undergo polymerization and depolymerization. This phase transition property of microtubules is called Dynamic instability. During dynamic instability, GTP binds with both alpha and beta subunit of tubulin dimer but only one GTP (beta subunit bound GTP) undergo hydrolysis while alpha bound GTP to remain as such. This process takes place every time when a new tubulin dimer added to the positive end of the chain.

GTP bound subunits are prone to polymerization and GDP bound subunits are prone to depolarization. Since the GTP containing tubulin adds onto positive end of microtubule there is the formation of a GTP cap at the tip of the microtubule protecting it from disassembly or shrinkage (polymerization phase). When the rate of hydrolysis surpasses the polymerization rate, the GTP cap undergoes hydrolysis converting beta bound GTP into GDP resulting in depolymerization and shrinkage of the growing chain (depolymerization phase). The switching from growth to shrinkage is called a catastrophe. Catastrophe is followed by rescue i.e addition of GTP bound tubulin to the chain And this process keeps on going.

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