• Exam%3%will%be%held%Friday,%Nov%13th%in%the%Tes:ng%Center%Arjona%110%• You%must%sign%up%for%an%available%Ex am%Slot%on%HuskyCT%Exam%Registra:on%Link!%!• Exam%slots:%12:00-1:30,%1:30-3:00,%3:00-4:30,%4:30-6:00;%there%is%no%regular%class%on%exam%day.%• If%you%have%another%class%conflict%w ith%the%available%slots%you%must%provide%a%scr eenshot%of%your%UConn%schedule,%and%contact%me%ASAP!%• HuskyCT%Materials%will%be%unavailable%online%on%Friday.%Be%sure%to%download%any%materials%you%want%if%you%plan%on%last%minute%studying!%%• There%will%be%2%Review%sessions%before%the%Exam.!• 1.!Wednesday,!11/11,!6-8pm!TLS!154!• 2.!Thursday,!11/12,!6-8pm!TLS!154!• Bring!your!Problem!Sets!!• The%early%exam%for%people%with%legi:mate%reasons%that%have%been%cleared%with%me%will%be%on%Thursday,%11/12%1:30-3:00pm,%BPB%201.%• If!you!need!to!take!the!early!exam!for!a!legiGmate!reason!and!have!NOT!contacted!me!yet,[email protected]!• Students%who%have%CSD%Exam%:me/place%accommoda:ons%will%be%taking%the%exam%at%the%CSD%office.!Please!get!any!Exam!AdministraGon!Forms!to!me!ASAP.!Mitosis%(Part%II)%and%Cell%Division:%Anaphase,%Telophase,%&%Cytokinesis%%Where%we%le_%off:%%Metaphase!Metaphase!is!defined!as!the!point!when!the!chromosomes!become!aligned!in!one!plane!halfway!between!the!two!spindle!poles.!Each!chromosome!is!held!under!tension!at!the!metaphase!plate!by!the!paired!kinetochores!and!the!kinetochore!MTs!aMached!to!opposite!spindle!poles.!4-%Anaphase!Anaphase!begins!as!the!sister!chromaGds!separate!and!are!pulled!toward!the!spindle!poles!by!the!kinetochore!MTs.!Two!categories!of!movement!can!be!disGnguished.!During!anaphase%A!the!kinetochore!MTs!shorten!and!the!chromosomes!move!toward!the!poles.!In!anaphase%B!the!polar!MTs!elongate!and!the!poles!move!farther!apart.!Anaphase!is!short,!lasGng!only!a!few!minutes.!Cells!in!mitosis!can!spend!a!long!Gme!in!metaphase!waiGng!for!a!signal!that!induces!the!iniGaGon!of!anaphase!and!the!separaGon!of!sister!chromaGds.!This!suggests!the!existence!of!a!spindle!assembly!checkpoint!that!arrests!cell!cycle!progress.!The!checkpoint!is!acGvated!by!!(1)!kinetochores!that!are!not!aMached!to!MTs!and/or!(2)!kinetochores!that!are!not!under!tension!because!their!sister!kinetochore!is!not!aMached!to!MTs!from!the!opposite!spindle!pole.!UnaMached!kinetochores!produce!a!signal!that!delays!anaphase!unGl!all!chromosomes!are!properly!aMached!to!the!spindle.!This!checkpoint!mechanism!therefore!helps!ensure!the!fidelity!of!chromosome!segregaGon.!!!Figure 17-39 Molecular Biology of the Cell (© Garland Science 2008) Spindle%assembly%checkpoint!D.Drubin!Once!all!kinetochores!are!properly!aMached!to!the!spindle,!anaphase!is!induced!by!a!protein!assembly!called!the!anaphase%promo:ng%complex%(APC)%or!cyclosome.!The!APC!acGvates!a!separase!enzyme!that!degrades!the!cohesins!connecGng!sister!chromaGds,!thereby!causing!sister!separaGon.!Anaphase!then!proceeds!in!two!phases:!anaphase!A!and!anaphase!B.!!Figure 17-44 Molecular Biology of the Cell (© Garland Science 2008) Spindle%assembly%checkpoint%and%the%APC!Anaphase%A%&%B!During!anaphase%A!chromosomes!move!towards!spindle!poles!based!on!MT!depolymerizaGon.!During!anaphase%B!the!spindle!poles!move!apart.!They!are!pushed!apart!by!kinesins!in!the!overlap!zone!and!pulled!apart!by!dyneins!near!the!cell!cortex.!!!Figure 17-46 Molecular Biology of the Cell (© Garland Science 2008)Forces%at%work%during%Anaphase!D.Drubin!MT!MT!MT!Anaphase%A%Poleward!movement!is!mediated!in!large!part,!by!depolymerizaGon!of!the!(+)!end!by!Kin-I!kinesins!(Kin-13)!that!can!depolymerize!both!ends,!and!by!a!Kin-N!protein!called!CENP-E!at!the!kinetochore!that!binds!to!the!MT!+!end.!%Anaphase%B%Kin-N%kinesins%(Kin-5)!bound!to!polar!microtubules!in!one!half!spindle!aMempt!to!walk!along!MTs!in!the!other!half!spindle!towards!the!(+)!end,!resulGng!in!sliding!of!the!two!half!spindles!such!that!the!two!poles!move!apart.!The!sliding!of!the!polar!MTs!relaGve!to!one!another!is!accompanied!by!their!elongaGon.!This!allows!for!a!greater!extent!of!spindle!elongaGon.!Dynein!motors!located!at!the!cell!cortex!may!also!parGcipate!by!pulling!the!poles!apart.!!NEWS AND VIEWSis critical for CENP-E tracking of depolymer-izing microtubule plusends.Previous work has implicated rapid biased diffusion in the ability of microtubule-asso-ciated proteins or protein complexes to tip-track depolymerizing microtubule plusends. Specifically, single yeast kinetochore Dam1 complexes, which bind to and diffuse along microtubules with a rapid stepping rate, were shown to diffuse away from a depolymer-izing microtubule tip, rather than dissociate with a departing tubulin subunit8. This was a result of the slow net tubulin dissociation rate from the microtubule tip relative to the rapid stepping rate of the diffusion of the Dam1 complex (a key kinetochore component) on the microtubule lattice9,10. This type of model predicts that non-walking CENP-E motors would tend to diffuse rapidly on the micro-tubule lattice to facilitate the experimentally observed CENP-E tip-tracking of depolym-erizing microtubule plusends. Furthermore, because Gudimchuk et al. found that the CENP-E motor tail domain was critical for the tip-tracking behaviour, the tail alone could possibly diffuse even more rapidly than the full-length motor. Consistent with this predic-tion, although CENP-E motion was typically characterized by unidirectionally walking mol-ecules, the diffusion coefficient for CENP-E molecules on the microtubule lattice was meas-ured as 0.16±0.01μm2sec−1 in the present study, which is even faster than the diffusion coefficient previously reported for individual Dam1 complexes (0.083±0.002μm2sec−1)8. Further, the diffusion coefficient for the puri-fied tail domain of CENP-E was an order of magnitude larger than for the full-length pro-tein (1.6μm2sec−1). However, the tail protein alone did not show processive association with either growing or shortening microtu-bule ends, probably because of its extremely rapid diffusive excursions both towards and away from the microtubule tip. Regardless, these results support the intriguing idea that diffusion of microtubule-associated proteins can promote tip-tracking of shortening micro-tubules through biased diffusion away from the depolymerizing tip10 (Fig.1). In general, an