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How chromatin structure is duplicated is not wellunderstood, however. During DNA synthesis, histone-modifying enz).rynesandvarious non-histone proteins are probably deposited onto the two new DNAstrands as they emerge from the replication fork, and these proteins are thoughtto help reproduce the local chromatin structure of the parent chromosome.CohesinsHelpHold5isterChromatidsTogetherAt the end of S phase, each replicated chromosome consists of a pair of identical sister chromatids glued together along their length.
This sister-chromatidcohesion sets the stage for a successfulmitosis because it greatly facilitates theattachment of the two sister chromatids in a pair to opposite poles of the mitoticspindle. Imagine how difficult it would be to achieve this bipolar attachment ifsister chromatids were allowed to drift apart after S phase. Indeed, defectsin sister-chromatid cohesion-in yeast mutants, for example-lead inevitably tomajor errors in chromosome segregation.Sister-chromatid cohesion depends on a large protein complex calledcohesin, which is deposited at many locations along the length of each sisterchromatid as the DNA is replicated in s phase.TWoof the subunits of cohesin aremembers of a large family of proteins called sMC proteins (for Structural Maintenance of chromosomes). cohesin forms giant ring-like structures, and it hasbeen proposed that these might form rings that surround the two sister chromatids (Figure 17-24).5 m cm o l e c u l ey'--'-}rhingeCN.ATPasedomain(A)(c)20"tFigure 17-24 Cohesin,Cohesinis aproteincomplexwith four subunits.Twosubunits,Smcl and Smc3,arecoiled-coilproteinswith an ATPasedomainat oneend; together,they form a largeV-shapedstructureas shown.Twoadditionalsubunits,Sccl and Scc3,connectthe ATPaseheaddomainsforminga ring structurethat mayencirclethe sisterchromatidsasshown.LLOLJo slueueSuBJJeeJpue edola^ue rEelf,nu aql Jo u^\op{Balq aq} sa}oruoJd oslE'saJn}3nJls e{ll-poJ '}JEdrrros olul spllPruoJqs Jelsrs'slla3{pl-WIEruruE uIpaurMualur eql Jo uorlBZruBSJoeJelef,s-e8JEla\l 'luo!rysuapuoJawosoutoJqJsre88u] osp tI 'alpuds aqt Jo alod elrsoddo eq] ol peqopnp sr rrcd p ur prleru-oJqrqcpepuegoJelsrs,tlqurasseernsuealpurdsJllolluraqtaq] eJnpurluql'srsolnu'urnrurunugo[1rearnocoesa8elsaqlur]snur Ipf,-WlV]eql slueru'{pO-I 'eseur{ ura}-a8ue.rreartr1ac xalduroc pue asJelrp aql Jo IIE lnoqe s8urrq-ord ay8ursp leq] sl IoJluoJ alcdc-gac Jo saJnleal alqe{Jeual tsotu eql Jo euosrsolrWolutr(rluf se^UOIp)-W'uaq] etErtseqrroc/ldv aql pue {pl-W,lt.or{pue srsolrru Jo sluale IuJrueqoaru ,{a1 aql equosap a,rr 'uorlJes sq} uI'srsauqog,b^,(qaq]Jo uorsrllp aq] pup 'alpurds cpotlx aqlyo l.1q1ac-Iuessuslp aql 'aseqdeue uorlalduroo eql Surpnlcur 'aseqd t{ elBI sluela3oJoIIBro; parmbar sr qf,rr{,t,r's1a8re11p3 go uoqeldroqdsoqdap eql pue uorlelrlcpur {pJot speal qJIq ^ 'sur1c,bJo uorlJnrtsap aq1 sra33rrl oslp 3/ldv eqJ 'spueluoJqcJalsrseqlJo uoperedas sal€qrur ^qaJeql pue urseqoJ seleelc ]eql asealord e 8ur-leJeqII'uunJas uorlJnJlsap aql sraSSrr]l/JdV aql uaqm 'uorlrsue4 asuqdeueJo-o1-aseqdelau eql lB su€aq srsolru;o ged roleu puocas eq; 'srrcd pl]EruoJr{Jralsrs eq] ot tueruqoptlp slr pup alpurds ollot1ru aqr go .{lqurasse eqt ot Surpeal'suralord;o rilarJel E alef i-roqdsoqdsaseuq uralord crlollu reqlopue {pO[BJeAas-y41'porrad s1q]Surrng '(asuqdelaurpuu 'aseqdeleruord 'eseqdord) srsolrurdlreaJo sluane aq] sraSSrrllurod4caqc W/zC aqr le.,br.qce {pl-hl ur eseeJJurldnrqeue 'lsJIC'ruals.{sIoJluor a1c[c-1acaq] Jo s]ueuodtuoc ]curlsrp dq paurano8 qcea'suud roferu o.tt] o]ur pep4lp eq UBJsrsolnu 'r,rar,r.go1urod,,{rolep8ar e ruorg'aseqd'snelf,nuW stuale rofeur aq] sezrJeru(unsI-ZI larredJo'sellpq oml olurIef,rluapr ue qtlm qoea61IIeo aql saprlrp-srseuqo{c-aseqd'pa]aldruoc sr srsolrrusV <WJJ> <JIVJ> 'adocsorcrurJo luala rofeu puocas aq]e uI uees sE Jorleqaq aruosoruoJql Jo srseq eql uo ,{preurrd peugep---?su2t4do7a1pue'asnqdnua 'asnqdnlatu 'asoqdnlauotd'asnqdotd-saBels ang olur paprnrp ,(1e-uonrpeJ] sr srsolrhl 'aruoua8 aql;o .{doc umo s}r q}lm qJEe 'ralJnu ralqBnup pcll-uepr rrcd u ot {pary&a8as)pa}nqrJtsrp pue paleredes erp sprtpruoJr.IJJelsrseqlJoqclqm Suunp 'srsolrur qlyrt sur8aq srq; 'aseqd yq;o pneaqdn cBprueJp aql sao8-Jepun IIaf, aq] '.zg q8noJql uonrsupJl pue aseqd g;o uorlaldurof, aql 3ura,ro11ogsrsolrw'a1c[cryau aqi Urun pasnar77acaq rcuuoJ puo asnr"ldS qJaa u! aJuo [1uo pua acuo patyf st utBuo qcaa 'snq1 'uru&uuotpct\datvNcl aruuru! oJ u!8tro ruqj monD o1 patlnbat arq jotlj sutajotd vlqlqu!osID 4pJ-S 'asoqd g Suunp pawnlpo sr utBrto uo4an1dat a a?uo 'sur&tto uotl.ac-tPaJpanocaqtutpuopurmunsallsuouDJrldatst!alDlltutaLItiDruLHsulajVNeVNe-otdsaJnnucauotlantlcapata33u1aqltqstuotlau\dnpawosotuoJ.qJryrum'4p)-Sto'uotl.cuntatuosotaotqcpun y111qstcadsosnounnutanoBaWajonosst)!.oq| sunjlorylm-otd uryawottlcaql uoryattldnpa4j sDqcoaw alnralow VNelolpm sD'alaosoraoJqJat\ua aq1to Surtdoc aruJnJOoaql sanlollurasnt4dg ur sawosoluolt\caql lo uollvcl\dnq{reuun5'Jel€l aqrJJsepalr,rse 'saxalduroc aseql;o uoqdnrsrp uo dpreurud spuadap eJoJalaqluorlrsueJl aseqdeue-o1-aseqduleruaqt ]e uorseqooJelsrsJo ssol aqJ 'saxaldruocurseqoJ uo dyreruud spuadep uors'pa^oruerueeq seq uoueualec eql ecuo '(tz-g aln8r{ ees)-eqo3 prlEruoJrlJ-Jelsrs'{perq eq} q8norqt reqlo eql Surssed,ayncaloruVNO ]nc aq] Suqeasaruaql puepue asuqd S ueemlaq sVNCIJelsrspelpueluJ eqlVNC euo 8uq1nc dq srsolrur,r(1ruasalSueluesrpdlpnper8 II asBJaurosrodolaruufuueaq;.srsaqluds y1qq Suunp laaur$lJoJ uorlecrydar om1ueq a sJnJco leql selnJalour VNC JelsrsJo Suruunlralur aql'uorlaualal y'lred urlseel le 'sllnseJosle uorsaqoopqeuorqo-JalsrsIO ruo4stsolttalpuldsaqi ]o salod alrsoddoo l s p r l e | .
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