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16.1. ADN l vt cht di truyn16.2. Nhiu protein phi hp vi nhau trong qu trnh

sao chp v sa cha ADN

16.3. Mi nhim sc th gm mt phn t ADN cng gi cng vi cc protein

o nm 1953, James Watson v Francis Crick gychn ng cng ng khoa hc bng vic cng b mhnh chui xon kp v cu trc phn t ca axit

deoxyribonucleic, c gi tt l ADN. Hnh 16.1 cho thyWatson (tri) v Crick ang say sa ngm nhn m hnh ADNc dng bng v hp v dy thp ca h. Tri qua hn 50

nm, m hnh ny xut thn ch l mt xut mi dn trthnh biu tng ca sinh hc hin i. ADN, hp cht ditruyn, chnh l phn t ng ngng m nht trong thi ica chng ta. Trong thc t, cc yu t di truyn ca Mendelcng nh cc gen nm trn nhim sc th ca Morgan u chaADN. Trn quan im ha hc, c th ni ti sn di truyn mmi ngi chng ta li cho th h sau chnh l cc phn tADN c trn 46 nhim sc th m chng ta c tha hng tcc thn sinh (b, m) v ADN c trong cc ti th c truynli theo dng m.Trong tt c cc phn t c trong t nhin, cc axit nucleic lc nht, v nh v kh nng t sao chp (ti bn) t cc nphn thnh phn. Trong thc t, c im con ci ging b, m

l kt qu ca qu trnh sao chp chnh xc ADN v s ditruyn ca n qua cc th h. Thng tin di truyn c m habng ngn ng ha hc ca ADN v c ti bn mi t botrong c th ca mi ngi chng ta. Chnh ngn ng lp trnhca ADN iu khin qu trnh pht trin cc tnh trng vha sinh, gii phu, sinh l v mt mc nht nh l tptnh mi c th sinh vt. Chng ny cp n vic bngcch no cc nh khoa hc chng minh c ADN l vt chtdi truyn v bng cch no Watson v Crick pht hin ra cutrc phn t ca n. ng thi, chng ta cng s thy bngcch no ADN c th sao chp (c s phn t ca di truyn) vc sa cha. Cui cng, chng ta s kho st xem ADN cngvi protein ng gi nh th no trong nhim sc th.

Ngy nay, ngay c hc sinh tiu hc cng c nghe ni vADN, trong khi cc nh khoa hc thng xuyn thao tc viADN trong phng th nghim nhm lm thay i tnh trng ditruyn ca cc t bo v c th. Tuy vy, n u th k XX,phn t no lm nhim v di truyn vn cn cha r v lc cu hi ny l mt trong nhng thch thc ln nht vi cc nhsinh hc.

Tm kim vt cht di truyn:

Qu trnh tm hiu khoa hc

Khi nhm nghin cu ca T. H. Morgan cho thy cc gen nmdc theo cc nhim sc th (xem m t Chng 15), hai thnhphn ha hc ca nhim sc th - ADN v protein - c coi lhai hp cht ng vin cho vai tr vt cht di truyn. Cho nnhng nm 1940, cc bng chng "ng h" protein dng nhu th hn; c bit, mt s nh ha sinh xp protein vonhm cc i phn t va c tnh c hiu chc nng cao vac tnh a dng vn l nhng yu cu thit yu ca vt cht ditruyn. Ngoi ra, lc hiu bit v cc axit nucleic cn hnch; dng nh cc thuc tnh vt l v ha hc ca chng

khng tng ng vi s a dng phong ph ca cc tnh trngdi truyn biu hin c th mi c th sinh vt khc nhau.Quan im ny sau c thay i dn t kt qu ca mts nghin cu vi sinh vt. Ging nh cc nghin cu caMendel v Morgan, mt trong nhng yu t quan trng nht xc nh c tnh nht qun ca vt cht di truyn chnh lvic la chn c loi sinh vt th nghim ph hp. Vai tr ditruyn ca ADN c pht hin u tin vi khun v virut;chng c c im n gin hn so vi u H lan, rui gimv ngi. phn tip theo ca chng ny, chng ta s ln theoqu trnh tm hiu khoa hc t cc nh khoa hc tm rav xc nh c vai tr l vt cht di truyn ca ADN.

V

Cc khi nim chnh

Hnh 16.1 Cu trc ADN c xc nh nh th no?

C s phn tca di truyn

Tng quan

Bn ch dn vn hnhs sng

Khi nim

ADN l vt cht di truyn16.1


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Chng 16 C s di truyn hc phn t 307

bacteriophag(ngha l th n khun hay thc khun th)hoc c gi tt l phag. So vi cc t bo, cc virut c cuto n gin hn nhiu. Mt virut thng ch bao gm ADN(hoc i khi l ARN) c bao bc bi mt lp v protein. c th sinh sn, virut phi ly nhim vo trong mt t bo riginh ly b my trao i cht ca t bo.

Cc phag v ang c s dng rng ri lm cng cnghin cu di truyn hc phn t. Nm 1952, Alfred Hersheyv Martha Chase tin hnh th nghim cho thy ADN l vtcht di truyn ca phag c tn l T2. y l mt trong nhiu

loi virut ly nhim Escherichia coli (E. coli), mt loi vikhun thng sng trong rut ng vt c v. Thi k , cc

nh sinh hc bit r l: phag T2, ging vi nhiu phagkhc, c thnh phn cu to hu nh ch gm ADN v protein.H cng ng thi bit rng phag T2 c th nhanh chngchuyn t bo E. coli thnh mt nh my sn xut T2 dnn s gii phng nhiu bn sao phag cng s phn r t bo.Bng mt cch no , T2 c th ti lp trnh t bo ch ca n sn sinh cc virut. Nhng, cu hi l: thnh phn no cavirut - protein hay ADN - chu trch nhim cho qu trnh ?

Hershey v Chase tr li cu hi ny bng vic thit kmt th nghim cho thy ch mt trong hai thnh phn ca

phag T2 xm nhp c vo trong t bo E. coli trong qutrnh ly nhim (Hnh 16.4). Trong th nghim ca mnh, cc

Protein hay ADN l vt cht di truyn ca phag T2?Alfred Hershey v Martha Chase s dng cc ng v phng x 35S v 32P nhm tng ng xc nh "s phn"

bin i ca cc protein v ADN c ngun gc phag T2 sau khi chng ly nhim vo t bo vi khun. H mun xc nh phn tno trong cc phn t ny i vo t bo v ti lp trnh hot ng ca vi khun gip chng c th sn sinh ra nhiu virut th h con.

Khi protein c nh du (l th nghim 1), hot tnh phng x c gi bn ngoi t bo; nhng khi ADN c nhdu phng x (l th nghim 2), hot tnh phng x c tm thy bn trong t bo. Cc t bo vi khun mang ADN ca phag nhdu phng x gii phng ra cc virut th h con mang ng v phng x 32P.

ADN ca phag i vo t bo vi khun, nhng protein ca phag th khng. Hershey v Chase kt lun rng: ADN,ch khng phi protein, c chc nng l vt cht di truyn phag T2.

A.D. Hershey and M. Chase, Independent functions of viral protein and nucleic acid in growth of bacteriophage, Journal of GeneralPhysiology36: 39 - 56 (1952)

Kt qu th nghim s khc bit nh th no nu nh protein l vt cht mang thng tin di truyn?

Hnh 16.4Nghin cu pht hi n

Th nghim

Kt qu

Kt lun

Ngun

iu g Nu ?

Phag nh du phng xc trn vi vi khun. Phag

ly nhim cc t bo vi khun.

Khuy mnh hn hp bngmy xay lm tung phn

phag bn ngoi t bo rakhi t bo.

Ly tm cc t bo vikhun dnh kt vi nhau

thnh cn ly tm yng nghim; phn bnngoi ca phag v phagt do nh hn nn dngphn tn trong dch ly tm.

o hot phng x

trong phncn ly tm vdch ly tm.

Hot phng x(protein phag) ctrong dch ly tm.

Phag

T bo vi khun

Protein cnh duphng x

V protein

ADN phag

ADN

Ly tm

Cn ly tm (gm t bo vi khunv cc thnh phn ca n)

Hot phng x(ADN phag) c

trong cn ly tm.

ADN cnh duphng x

Ly tm

Cn ly tm

L th nghim 1: Phagc nui trong mitrng cha ng vphng x lu hunh (35S) nh du protein caphag (mu hng).

L th nghim 2: Phagc nui trong mitrng cha ng vphng x phospho (32P) nh du ADN caphag (mu xanh dng).


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S hp tc ngn ngi nhng rt ni ting ca h gip lmsng t cu trc b n ca ADN ngay sau khi Watson c chuynthm i hc Cambridge l ni m Crick ang nghin cu cccu trc protein bng mt k thut gi l tinh th hc tia X(xem Hnh 5.25). Khi thm phng th nghim ca MauriceWilkins, Watson nhn thy hnh nh nhiu x tia X ca ADNdo mt ng nghip qu c ca Wilkins l Rosalin Franklinchp c (Hnh 16.6a). Cc bc nh c to ra bng k thuttinh th hc tia X cho thy chng khng phi cc hnh nh ca

cc phn t thc s. Cc im chm v vt nhe nh trn Hnh16.6b c to ra l do tia X b nhiu x (khc x) khi chng iqua cc si ADN tinh sch xp thng hng. Cc nh khoa hcv tinh th hc thng dng cc cng thc ton hc chuynti cc thng tin t cc hnh nh nh vy thnh hnh dng bachiu ca cc phn t; ring Watson th quen thuc vinhng hnh nh c to ra bi cc phn t dng chui xon.Khi nghin cu k nh nhiu x tia X ca ADN do Franklinchp, Watson khng ch tm ra ADN c dng chui xon mng cn c lng c chiu rng ca chui xon v khongcch gia hai baz nit lin k dc trc chui xon. Chnhchiu rng ca chui xon ch ra n c to nn t haimch, khng ging vi cng b ngay trc ca Linus

Pauling v mt m hnh phn t gm 3 mch. S pht hin rahai mch gii thch cho vic thut ng thng c dng hinnay m t ADN l chui xon kp (Hnh 16.7).

Watson v Crick bt u xy dng cc m hnh ca chuixon kp sao cho ph hp vi cc s liu o c qua cc hnh

nh nhiu x tia X, t tm ra cu trc ha hc ca ADN.ng thi sau khi c mt bn bo co thng nin cha ccng b v nghin cu ca Franklin, hai nh khoa hc bit rngFranklin tng kt lun rng khung ng - phosphate nmbn ngoi chui xon kp. S sp xp ny rt thuyt phc v n

(a) Rosalind Franklin (b) nh nhiu x tia X ca ADNdo Franklin chp

Hnh 16.6 Rosalind Franklin v nh nhiu x tia Xca ADN. Franklin, mt nh khoa hc li lc v tinh th hc tia X, thc hin mt th nghim quan trng, t chp c bc nh gipWatson v Crick lun ra cu trc xon kp ca ADN. Franklin qua inm 1958 do bnh ung th khi c mi 38 tui. ng nghip ca c lMaurice Wilkins c nhn ng gii thng Nobel nm 1962 cng viWatson v Crick.

(a) Cu trc c bn ca ADN (b) Cu trc ha hc mt on ngn ca ADN (c) M hnh lp kn khng gian

Hnh 16.7 Chui xon kp. (a) Di "ruy bng" trong hnh v biu din khung ng - phosphate ca hai mch n ADN. Chuixon ny theo "chiu phi", ngha l vn v pha phi theo hng i ln. Hai mch chui xon c gi li vi nhau qua cc lin kthydro (ng nt chm mu ) hnh thnh gia cc baz nit kt thnh tng cp bn trong chui xon. (b) thy cu trc ha hcr hn, hai mch ADN c v thng hng nh mt on ngn ca chui xon. Lin kt cng ha tr mnh ni cc tiu n v(nucleotide) trn mt mch vi nhau, trong khi cc lin kt hydro yu hn gi hai mch n vi nhau. iu ng lu l hai mch nny l i song song, ngha l chy song song nhng theo 2 chiu ngc nhau. (c) S xp chng ln nhau ca cc cp baz nit mtcch cht ch c thy r trong m hnh c v bi my tnh ny. Lc hp dn Van de Waals gia tng cp baz xp chng lnnhau chnh l yu t chnh gip gi ton b phn t vi nhau (xem Chng 2).

u 5'

u 3'

u 3'

u 5'

Lin kt hydro


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310 khi kin thc 3 Di truyn hc

s y cc baz nit c tnh k nc tng i vo trong v rixa khi cc phn t nc trong phn dung dch bao quanh.Watson xy dng c mt m hnh vi cc baz nit hngvo pha trong ca chui xon kp. Trong m hnh ny, haikhung ng - phosphate chy i song song vi nhau, ngha lcc tiu n v ca chng chy song song nhng ngc chiunhau (xem Hnh 16.7). Chng ta c th tng tng s sp xptng th ging nh mt chic thang dy vi cc thanh thangvng chc. Hai dy ca thang hai bn tng ng vi hai

khung ng - phosphate, cn mi thanh thang tng ng vimt cp baz nit. Lc ny, chng ta c th tng tng mtu ca thang dy c c nh, cn u kia c vn xon,to nn cu trc xon l xo u n. Cc s liu ca Franklinch ra rng chiu di mi vng xon trn vn dc trc chuixon di 3,4 nm. Vi khong cch gia hai lp cp baz k tipxp chng ln nhau l 0,34nm, s c 10 lp cp baz nit(tng ng vi 10 thanh thang) trong mi vng ca chui xon.

Cc baz nit trong chui xon kp kt cp vi nhau theo thp c hiu: adenine (A) vi thymine (T), cn guanine (G) vicytosine (C). Th nghim theo m hnh "th v sai", Watson vCrick pht hin c c im quan trng ny ca ADN.

u tin, Watson tng tng cc baz kt cp theo tng loi,ngha l A vi A, C vi C. Nhng m hnh ny khng ph hpvi cc s liu tia X vn cho bit ng knh dc chui xon lng nht. V sao s sp xp ny khng ph hp vi kiu ktcp theo tng loi? Adenine v guanine l cc purine; cc bazca chng c cu trc hai vng hu c. Ngc li, cytosine vthymine thuc mt h cc baz khc gi l pyrimidine vn chc cu trc mt vng. Do vy, cc purine (A v G) s c chiurng gp khong 2 ln so vi cc pyrimidine (C v T). Mt cppurine - purine s l qu rng, trong khi mt cp pyrimidine -pyrimidine s l qu hp, so vi ng knh khong 2 nm cachui xon kp. Nhng, nu s kt cp lun l mt purine vi

mt pyrimidine th ng knh chui xon s ng nht.

Watson v Crick t suy lun rng: hn phi c mt kiukt cp c hiu b sung no c to ra bi cu trc cacc baz nit. Mi baz nit u c cc gc ha hc mt bnc th to lin kt hydro vi cc "i tc" ca chng: Adeninec th to hai lin kt hydro vi thymine v ch vi thymine;trong khi guanine c th to ba lin kt hydro vi cytosine vch vi cytosine. Ni cch khc, A kt cp vi T, cn G kt cpvi C (Hnh 16.8).

M hnh ca Watson v Crick ng thi gii thch c chonguyn tc Chargaff. Bt c khi no mt mch ca phn tADN si kp c A, th mch i din s l T; v G c mt trnmt mch s lun lun kt cp vi C trn mch b sung tng

ng. Do vy, trn ADN ca tt c cc sinh vt (ch tr cc virutc vt cht di truyn khng phi ADN si kp), s lngadenine lun bng thymine, cn s lng guanine lun bngcytosine. Mc d nguyn tc kt cp ca cc baz nit l c sto nn cc thanh thang ca chui xon kp, nhng n khnghn ch trnh t ca cc nucleotide dc theo chui xon. Trt tca bn loi baz nit dc theo chui xon c th bin i bt

k, nhng mi gen ch c mt trt t duy nht ca cc chng;trt t ny c gi l trnh t cc baz nit.Thng T nm 1953, Watson v Crick lm sng st cng

ng khoa hc bng vic cng b mt bi bo ngn ch di 1trang trn tp chNature*. Bi bo m t m hnh ADN m htm ra: chui xon kp, ri t n dn tr thnh biu tngca sinh hc phn t. V p ca m hnh ny chnh l ch:cu trc ADN ch cho chng ta thy c ch sao chp ca n.

*J.D. Watson and F.H.C.Crick, Molecular structure of nucleic acids: a

structure for deoxyribose nucleic acids,Nature 171: 737-738 (1953).

Purine + Purine: qu rng

Pyrimidine + Pyrimidine: qu hp

Purine + Pyrimidine: chiurng ph hp vi s liu tia X

Hnh 16.8 S kt cp cc baz nit trong ADN.Cc cp baz nit trong mt chui xon kp ADN c gi livi nhau bi cc lin kt hydro (trn hnh y c v bngng nt chm mu ).

ng

ng

ng

ng

16.1

1. Mt loi rui c t l cc nucleotide trong ADN nhsau: 27,3% A, 27,6% T, 22,5% G v 22,5% C.

Nguyn tc Chargaff c chng minh bi cc sliu trn nh th no?

2. M hnh ca Watson v Crick gip gii thchnguyn tc Chargaff nh th no?

3. Nu bin np khng xy ra trongth nghim ca Griffith, th kt qu th nghim s cth khc bit nh th no? Gii thch.

Xem gi tr li Ph lc A.

Kim tra khi nim

iu g Nu ?


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Mi quan h gia cu trc v chc nng bc l r nt chui xon kp. Chnh suy ngh cho rng c s kt cp c hiugia cc baz nit trong phn t ADN lm le ln tnga Watson v Crick n vi m hnh cu trc chnh xc cachui xon kp. Cng lc , h tm thy ngha v mt chcnng ca nguyn tc kt cp cc baz. H kt thc bi boca mnh bng mt cu pht biu hi hc nh sau: iu mnh chng ti l nguyn tc kt cp c hiu m chng ti coinh nh ngay lp tc ch ra mt c ch sao chp vt chtdi truyn c th tn ti. Trong phn ny, chng ta s cpn nguyn l c bn ca s sao chp (ti bn) ADN cng nhmt s c im chi tit quan trng ca qu trnh .

Nguyn l c bn: kt cp baz vi mchlm khun

Trong mt bi bo th hai, Watson v Crick pht biu githit ca h v c ch sao chp ADN nh sau:

Lc ny, trong thc t m hnh v axit deoxyribonucleic ca chng tichnh l mt cp cc mch lm khun, m mi mch b sung vimch cn li. Chng ti tng tng ra rng: trc khi s sao chpdin ra, cc lin kt hydro b t gy, sau hai mch gin xon vt ch khi nhau. Mi mch sau c dng lm khun hnh thnhnn mt mch mi i km vi n; nh vy, cui cng chng ta s nhnc hai cp chui xut ph t t mt cp chui ban u. Ngoi ra,trnh t ca cc cp chui baz c sao chp mt cch chnh xc. *

Hnh 16.9 m t tng c bn ca Watson v Crick. dtng tng, trn hnh ch minh ha mt on chui xon kpngn dng dui thng. iu ng lu l nu chng ta chbit trnh t ca mt trong hai mch ADN c v trn Hnh16.9a, th chng ta c th xc nh c trnh t baz nit camch cn li trn c s p dng nguyn tc kt cp b sung cacc baz. Ni cch khc, hai mch ADN b sung cho nhau;mi mch mang thng tin cn thit ti thit mch cn li.Khi mt t bo sao chp mt phn t ADN, mi mch ca phn

t ADN c dng lm khun sp xp cc nucleotide vomch mi b sung vi n. Cc nucleotide xp hng dc theomch lm khun tun th nguyn tc kt cp ca cc baz ngthi lin kt vi nhau hnh thnh nn mt mch mi. Nu c mt phn t ADN si kp vo u qu trnh sao chp, th snhanh chng thu c mt bn sao chnh xc ca phn t m.C ch sao chp ging nh vic dng phim m bn to nnnh dng bn; sau nh dng bn li c dng to nnmt phim m bn mi ri qu trnh c lp i lp li.

M hnh sao chp ADN nh vy khng c kim chngtrong nhiu nm k t khi cu trc ca ADN c cng b. Thnghim nhm chng minh cho m hnh ny v nguyn tc th

d tng tng, nhng v thc nghim th kh thc hin. Mhnh ca Watson v Crick d on rng khi chui xon kp saochp, mi phn t con s mang mt mch c c ngun gc tphn t m cn mt mch c tng hp mi. M hnh bnbo ton ny nh vy khng ging vi m hnh bo ton vncho rng sau qu trnh sao chp bng mt cch no ccmch ca chui xon kp kt cp tr li vi nhau (tc l phnt ADN m c bo ton nguyn vn). M hnh ny ng thicng khc vi m hnh phn tn l m hnh cho rng c bnmch ADN sau qu trnh sao chp l s t hp li ca cc phnon ADN c xen ln cc phn on ADN mi tng hp (Hnh16.10 trang sau). Mc d c ch ADN c th sao chp theo

kiu bo ton hoc phn tn l kh gii thch, nhng trong thct khng th loi b nhng m hnh ny nu thiu bng chng

16.2Khi nimNhiu protein phi hp trongsao chp v sa cha ADN

(a) Phn t ADN m c hai mch ADNb sung vi nhau. Mi loi baz ktcp c hiu vi mt loi baztng ng ca n qua cc lin kthydro; trong A lin kt vi T, vG lin kt vi C.

(b) Bc u tin trong sao chp l haimch n ADN tch nhau ra. Mimch ca phn t ADN m lc nyc dng lm khun xc nhtrnh t cc nucleotide dc theomch ADN mi, b sung vi n.

(c) Theo nguyn tc b sung, ccnucleotide "xp hng" dc mchmi v lin kt vi nhau to nnkhung ng - phosphate. Miphn t "con" s c mt mch cca phn t ADN m (xanh sm) vmt mch ADN mi (xanh nht).

Hnh 16.9 M hnh sao chp ADN c bn. Trong m hnh gin lc ny, mt phn on ADN si kp ngn cgin xon thnh dng cu trc ging nh mt chic "thang dy". Trong "dy thang" hai bn l khung ng - phosphatetrn hai mch ADN; cn mi "thanh thang" l mt cp baz nit trn hai mch lin kt hydro vi nhau theo nguyn tc bsung (nguyn tc Chargaff). Cc hnh n gin biu din bn loi baz. Trong , mu xanh sm biu din cc mch ADNc ngun gc t phn t ADN m; cn mu xanh nht biu din cc mch ADN c tng hp mi.

*

F.H.C.Crick and J.D. Watson, The complementary structure of deoxyri-bonuleic acid, Proceedings of the Royal Society of London A 223: 80 (1954).


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ADN ny v phn chia thnh hai t bo con c vt cht ditruyn ging ht nhau trong vng cha n 1 gi. Mi t botrong c th bn c 46 nhim sc th trong nhn t bo; trong, mi nhim sc th l mt phn t ADN xon kp di, mchthng. Tnh tng cng, h gen nhn ngi cha khong 6 tcp baz, tc l ln hn h gen ca t bo vi khun. Nu chngta in ton b trnh t h gen ngi di dng cc k t A, T, Gv C kch c nh trnh by y, th thng tin di truyn trong6 t cp nucleotide trong mt t bo lng bi s tng ng vi

khong 1200 cun sch c kch thc nh cun sch ny. Vym, mi t bo ch cn vi gi c th sao chp ton b lngthng tin vi mc sai st rt thp (tn s sai st ch vokhong 10-9, ngha l c 1 t nucleotide mi c mt nucleotidesao chp sai). Ni cch khc, s sao chp ADN l mt qu trnhdin ra vi tc cc k nhanh v chnh xc.

C hng chc enzym v protein tham gia vo qu trnh saochp ADN. Nhng hiu bit n nay v b my sao chp vi

khun (chng hn nhE. coli) l y hn so vi sinh vtnhn tht (eukaryote). Nu khng c ch gii g thm, cc bcc m t y l qu trnh xy ra E. coli. Mc d vy, ccnghin cu cho n nay nhn chung cho thy phn ln ccnguyn tc sao chp ADN c bn l ging nhau sinh vt nhns (prokaryote) v sinh vt nhn tht (eukaryote).

S khi u sao chpQu trnh sao chp ADN lun bt u ti nhng v tr c

th trn phn t ADN c gi l im khi u sao chp. l nhng on ADN ngn c trnh t nucleotide xc nh.Ging nhiu vi khun ni chung, nhim sc th ca E. coli cdng vng v ch c mt im khi u sao chp. Cc proteinkhi u sao chp nhn ra v tr ny v gn vo ADN; chngtch hai mch ADN ra khi nhau v to nn bng sao chp.T im khi u sao chp, qu trnh sao chp ADN tin v chai pha (Hnh 16.12a) cho n khi ton b phn t ADN c

Hnh 16.12 Cc im khi u sao chp E. coliv sinh vtnhn tht (eukaryote). Mi tn mu ch s dch chuyn ca chc saochp, ngha l chiu sao chp ADN din ra mi bng sao chp.

im khiu saochp

(a) Nhim sc th dng vng E. coliv nhiu vi khun khcch c mt im khi u sao chp. Ti im khi u saochp, hai mch n ADN tch khi nhau hnh thnh nnbng sao chp gm hai chc sao chp. Qu trnh sao chptin v c hai pha cho n khi cc chc sao chp hai ugp nhau; kt qu to nn hai phn t ADN con. Hnh nhchp bng knh hin vi in t truyn qua (TEM) trn chothy nhim sc th vi khun ch c mt bng sao chp.

(b) Nhim sc th eukaryote l nhng phn t ADN dng mchthng, kch thc ln. Qu trnh sao chp ADN bt u khibng sao chp hnh thnh ti nhiu im dc phn t ADN.Bng sao chp m rng khi hai chc sao chp ca n tindn v hai pha. Cui cng, cc bng sao chp dung hp vinhau v s tng hp cc mch ADN con kt thc. Hnh nhchp bng TEM trn cho thy on nhim sc th trn cat bo chut Hamster dng Chinese c ba bng sao chp.

V tip Trn nh TEM hnh (b), hy vthm mi tn bng sao chp th ba.

Phn tADN sixon kp

Hai phn t ADN con

Mch lm khun (m)Mch mitng hp

(con)

Chc sao chp

Bng sao chp

im khi usao chp

Phn t ADNsi xon kp

Mch lm khun (m)

Mch mi tng hp (con)

Bng sao chp Chc sao chp

Hai phn t ADN con


10/20


sao chp xong. Khng ging vi khun, nhim sc th sinhvt nhn tht c th c hng trm, thm ch hng nghn imkhi u sao chp. Kt qu l nhiu bng sao chp c hnhthnh; cui cng chng dung hp vi nhau to nn nhngbn sao hon chnh ca cc phn t ADN c kch thc rt ln(Hnh 16.12b). Bng cch ny, tc sao chp ADN sinh vtnhn tht tng ln. Cng ging vi khun, qu trnh sao chpADN sinh vt nhn tht tin v c hai pha ca bng sao chp.

hai u ca bng sao chp c chc sao chp. l vng

c dng ch Y l ni hai mch n ADN ca chui xon kptch nhau ra. C mt s protein tham gia vo hot ng thoxon ny (Hnh 16.13). Helicase l nhm cc enzym c vai trgin xon v tch hai mch n ra khi nhau. Mi mch nsau c s dng lm khun (mch m) tng hp nnmch ADN mi. Sau khi cc mch lm khun tch nhau ra, ccprotein lin kt mch n, gi tt l SSB, s nh kt vomch ADN lm khun v gip chng tr nn n nh. S thoxon ca chui xon kp trong vng bng sao chp s lm chocc u chc sao chp tr nn b vn xon cht hn v hnhthnh lc cng. Topoisomerase l nhm enzym gip gii talc cng ny bng kh nng lm t gy tm thi cc lin ktcng ha tr trn mch n ADN, xoay cc mch n quanh

nhau tho xon, ri ni chng tr li vi nhau.Cc mch ADN m sau khi gin xon c dng lm

khun tng hp cc mch ADN mi. Tuy vy, enzym trctip tng hp ADN khng c kh nng khi u qu trnh tnghp chui polynuclotide mi; chng ch c th b sung ccnucleotide vo u 3 ca mt chui c sn nu nucleotide bsung kt cp ph hp vi nucleotide trn mch ADN lmkhun. V l do ny, chui nucleotide u tin c to ratrong tng hp ADN lun l mt on ngn ARN, ch khngphi ADN. on ARN ngn ny c gi l on mi v cxc tc tng hp bi enzym primase (xem Hnh 16.13).Primase bt u tng hp on mi t mt ribonucleotide (haynucleotide ARN) duy nht; sau , mi ln phn ng n gnthm mt ribonucleotide theo nguyn tc b sung vi mchADN lm khun. Mt on mi hon chnh, gm khong 5 - 10nucleotide, lc ny s bt cp vi mch khun. Mch ADNc tng hp mi s bt u t u 3 ca on mi ARN.

Tng hp mch ADN miCc enzym c tn gi l ADN polymerase chnh l cc

enzym trc tip xc tc tng hp mch ADN mi bng vic b

sung cc nucleotide vo mt chui sn c. E. coli, c mt sloi ADN polymerase khc nhau; tuy vy, hai loi c vai trchnh trong sao chp ADN l ADN polymerase III v ADNpolymerase I. c im ny sinh vt nhn tht l phc tphn. n nay, c t nht 11 loi ADN polymerase khc nhau c xc nh; tuy vy, nguyn l hot ng chung cachng v c bn l ging nhau.

Phn ln cc enzym ADN polymerase u cn mt onmi v mt mch ADN lm khun. Trn c s trnh t camch lm khun, chng b sung cc nucleotide mi dc theo

mch c tng hp mi theo nguyn tc b sung. E. coli,ADN polymerase III (vit tt l ADN pol III) b sung ccnucleotide ADN vo on mi ri tip tc ko di chui ADNtheo nguyn tc b sung vi mch lm khun cho n khi ktthc chui. Tc ko di chui ADN vo khong 500nucleotide mi giy vi khun, v vo khong 50 nucleotidemi giy ngi.

Mi nucleotide khi c b sung vo chui ADN ang ko

di u dng nucleotide triphosphate; l mt nucleoside(gm ng pentose v baz nit) lin kt vi ba nhmphosphate. Chng ta cp n mt phn t nh vy l ATP(adenosine triphosphate; xem Hnh 8.8). S khc bit duy nhtgia phn t ATP (c vai tr trong trao i nng lng) vidATP (l tin cht ca adenine trong ADN) l thnh phnng. Nu nh ng trong ADN l deoxyribose th ngtrong ATP l ribose. Cng ging nh ATP, cc nucleosidetriphosphate c dng tng hp nn ADN l nhng chtha hc phn ng mnh; mt phn bi chng cha uitriphosphate vn tch in m v km bn. Mi ln mtnucleotide gn thm vo chui ang ko di, hai nhm

phosphate (k hiu l -i v cn c gi l pyrophosphate)

s t khi phn t nucleoside triphosphate tin cht. S thyphn din ra ngay sau ca nhm pyrophosphate thnh hai

phn t phosphate v ci i lin vi cc phn ng sinh nhitl ng lc phn ng trng hp ADN din ra (Hnh 16.14).

Hnh 16.13 Mt s protein lin quann khi u sao chp ADN. Cc loiprotein ging nhau hot ng c hai chc saochp ca cng mt bng sao chp. ginlc, y ch minh ha mt chc sao chp.

Topoisomerase lm t gycc mch n, tho xon rini chng tr li vi nhau.

Cc protein lin kt mchn (SSB) gip lm n nhmch ADN lm khun.

Primase tng hpon mi ARN ctrnh t b sung vimch khun.

Helicase lm gin xon v tchhai mch n ADN khi nhau.

onmi ARN


11/20


Ko di chui kiu i song song

Nh nu trn, hai u ca mt mch ADN l khc nhau,to cho mi mch ADN c tnh phn cc, ging nh ng mtchiu vy (xem Hnh 16.5). Ngoi ra, hai mch ADN trongchui xon kp l i song song, ngha l chng phn cc theochiu i din nhau, cng ging nh hai ln ng mt chiutrn xa l theo hng ngc nhau vy (xem Hnh 16.14). Rrng l hai mch mi c tng hp trong qu trnh sao chpADN phi i song song so vi cc mch khun ca chng.

S sp xp i song song ca chui xon kp nh hng thno n qu trnh sao chp? Do c im cu trc, cc enzymADN polymerase ch c th b sung cc nucleotide vo phau 3 t do ca mt on mi hoc ca mt mch ADN angko di, ch khng bao gi b sung c cc nucleotide vo

pha u 5 (xem Hnh 16.14). V vy, mt mch ADN mi chc th ko di theo chiu 5 3. Vi nguyn tc , hay xems sao chp din ra th no ti mt chc sao chp (Hnh 16.15).Dc theo mt mch khun ADN, ADN polymerase III c thtng hp mch mi mt cch lin tc theo nguyn tc b sungbng vic ko di mch mi theo chiu bt buc 5 3. ADNpol III mt cch n gin lch vo chc sao chp trn mchkhun ri b sung lin tc cc nucleotide vo mch mi cngvi vic chc sao chp tin v pha trc. Mch ADN mi ctng hp theo kiu ny c gi l mch dn u. tnghp mch dn u, ADN pol III ch cn mt on mi duy nht(xem Hnh 16.15).

c th ko di mch ADN mi cn li theo ng chiu5 3, ADN pol III phi hot ng dc theo mch khun cnli theo chiu ngc hngvi chiu dch chuyn ca chc saochp. Mch ADN mi c tng hp theo chiu ngc hngny c gi l mch ra chm hay mch theo sau *. Khngging mch dn u c tng hp lin tc, mch ra chmc tng hp gin on thnh cc on nh. Cc on ca

*Qu trnh tng hp mch dn u v mch theo sau din ra ng thi vi tc

tng ng. S d gi l mch ra (hay chm mch theo sau) l do s tnghp mch ny din ra chm hn cht t so vi mch dn u; mi phn onmi ca mch ra chm ch c khi u tng hp khi mt on mch khunADN ti chc sao chp bc l di.

u 5'

u 3'

u 5'

u 3'

ADN polymerase

u 5'

u 3'

u 5'

u 3'

ng

Phosphate

Nucleosidetriphosphate

Mch mi Mch khun

Pyrophosphate

Hnh 16.14 S kt hpnucleotide vo mch ADN.Enzym ADN polymerase xc tc vicb sung mt nucleoside triphosphatevo u 3 ca mt mch ADN angko di, vi s gii phng hai nhmphosphate.

S dng s ny gii thchti sao chng ta ni mi mch

ADN c tnh phn cc.

?

Hnh 16.15 Tng hp mch dn u trong saochp ADN. S ny tp trung vo chc sao chp bn trica mt bng sao chp. ADN polymerase III (ADN pol III), cv ging nh bn tay khum hnh chn, nh kt cht ch vi mtprotein c gi l kp trt, c v ging nh mt chitbnh vng. Protein kp trt y ADN pol III trt dc mchADN lm khun.

Bng sao chp ADN tng qut

Mch dn u Mch ra chmim khi u sao chp

Mi

Mch dn uMch ra chmChi u sao

chp chung

au khi on m i ARNc to ra, ADN pol III btu tng hp mch dn u

Mch dn u cko di lin tc theo chiu5 3 cng vi chiu dichuyn ca chc sao chp

im khi usao chp

Mi ARN

Proteinkp trt

ADN pol IIIADN m


12/20


mch ra chm nh vy c gi l cc on Okazaki theo

tn nh khoa hc Nht bn pht hin ra chng. E. coli ccon Okazaki di khong 1000 n 2000 nucleotide, trong khi sinh vt nhn tht chng di khong 100 n 200 nucleotide.

Hnh 16.16 minh ha cc bc ca qu trnh tng hp mchra chm. Nu nh tng hp mch dn u ch cn mt onmi duy nht, th mi on Okazaki trn mch ra chm u cnring mt on mi. Mt loi ADN polymerase khc, gi l

ADN polymerase I (ADN pol I) s thay th cc nucleotideARN ca on mi bng cc nucleotide ADN tng ng bngvic b sung tng nucleotide vo u 3 ca on Okazaki link (on s 2 trn Hnh 16.16). Tuy vy, ADN pol I khng thni nucleotide cui cng thuc on ADN va c thay thvi nucleotide u tin ca on Okazaki lin k (on s 1trn Hnh 16.16). Lc ny, mt enzym khc, gi l ADN ligase,s thc hin nhim v ny; n xc tc phn ng ni khungng - phosphate ca tt c cc on Okazaki vi nhau tonn mch ADN ra chm lin tc.

Hnh 16.16 v Bng 16.1 m t tm tt qu trnh sao chpADN. Hy c v quan st trc khi tip tc cc phn di y.

Phc h sao chp ADNTrc y, d tng tng, cc phn t ADN polymerasec m t ging nh cc u xe la di chuyn dc ngray ADN; tuy vy, hnh nh khng chnh xc hai imquan trng. Th nht, b my sao chp ADN trong thc t lmt phc h ln gm nhiu protein khc nhau. Cc tng tcprotein - protein qui nh hiu qu v chc nng ca phc hny. Chng hn nh, bng s tng tc vi cc protein khc tichc sao chp, primase r rng hot ng ging nh mt chicphanh phn t, lm chm s m rng chc sao chp v iuphi tc sao chp tng ng gia mch dn u v mchra chm. Th hai, phc h sao chp ADN khng di chuyn dc

ADN; thay vo , chui ADN c tut qua phc h trongqu trnh sao chp. Trong cc t bo sinh vt nhn tht, nhiuphc h sao chp c l kt nhm vi nhau hnh thnh nn ccnh my; dng cu trc ny, chng c c nh vo mngli nhn (mng li ny c hnh thnh t cc si dn rngqua phn cht nhn ca t bo). Cc nghin cu gn y ng hcho m hnh trong hai phn t ADN polymerase lin kt vohai mch ADN lm khun (mi phn t enzym lin kt trnmt mch); ri mch ADN lm khun c ko qua enzymging nh xe ch, kt qu l hai phn t ADN con c hnhthnh v c y ra ngoi. Mt s bng chng b sung chothy mch ra chm hnh thnh nn cu trc ging thng lngquanh phc h sao chp; v vy, khi ADN polymerase honthnh vic sao chp mt on Okazaki v ri ra th n khngcch xa ng k so vi on mi ca on Okazaki k tip. Cutrc thng lng ca mch ra chm cho php t bo c thtng hp nhiu on Okazaki trong thi gian ngn.

c sa v sa cha ADNS chnh xc trong sao chp ADN khng n thun ph thucvo tnh c hiu trong nguyn tc kt cp ca cc baz. Mcd li sao chp ADN xut hin vi tn xut chung vo khongmt trong 10 t nucleotit (10-10), nhng trong thc t cc li ktcp nucleotit ban u vo mch ADN ang m rng bi hotng ca enzym ADN polymerase thng cao hn khong

Hnh 16.16 Tng hp mch ra chm.


Mch dn u Mch ra chmim khi u sao chp

Mch dn u

Mch ra chm

Chiu sao

chp chung

Primase ni nucleotideARN vo on mi

Khi tip cnon ARN mi ktip, ADN pol III rikhi chc sao chp

Mch khun ADN pol III b sungcc nucleotide ADNvo mi, hnh thnhon Okazaki 1

onOkazaki

ADN pol III b sungnucleotide vo on mi 2cho n khi n tip cn

on mi 1, ri ri ra.

ADN pol I thay th ARNbng ADN, bng cch kodi u 3 ca on 2.

ADN ligaseni hai onOkazaki vi nhau

y, mch rachm c saochp hon chnh

Chiu sao chp chung


13/20


100.000 ln - tc l, khong mt nucleotit sai trong c 100.000nucleotit ca mch lm khun. Trong qu trnh sao chp, ccenzym ADN polymerase c sa tng nucleotit da trn trnht mch lm khun ngay khi chng b sung thm nucleotit mivo chui ang ko di. Nu tm ra nucleotit kt cp sai, enzympolymerase s ct b nucleotit ny ri tng hp li bngnucleotit kt cp ng. (Hot ng ny ging nh khi chng tadng phm "BackSpace" trn bn phm my tnh xa mt kt sai, ri nhp li mt k t ng).

Tuy vy, i khi cc nucleotit kt cp sai c th thot khihot ng c sa ca cc ADN polymerase. Trong c ch sacha kt cp sai, cc enzym s tin hnh loi b v thay thcc nucleotit sai do cc li ca qu trnh sao chp. Cc nhkhoa hc n tm quan trng ca nhng enzym ny khitm thy mt sai hng di truyn mt trong nhng gen m hacc enzym nh vy lin quan trc tip n s pht sinh mtdng ung th rut kt. R rng l, sai hng di truyn ny cho


Mch dn u Mch ra chmi m kh i u sao chp

Mi

Mch dn uMch ra chm Chiu sao

chp chungHelicase

tho xon sixon kp "m"

Protein lin ktmch n gip mchkhun sau khi thoxon tr nn n nh

Mch dn uc tng hp lin tctheo chiu 5' 3' biADN polymerasse

Mch dn u

ADN pol III

Mi Primase

ADN pol III Mch ra chm

ADN pol I ADN ligase

Phn tADN "m"

Primase bt u tnghp mi ARN cho onOkazaki th 5

ADN pol III ang hon thnh tng hpon Okazaki th 4. Khi n tip cn mica on Okazaki th 3, n s tch ra, ridi chuyn n chc sao chp v tip tcchc nng b sung nucleotit vo u 3'ca on mi thuc on Okazaki th 5.

ADN pol I loi b mi u 5' ca onOkazaki th 2, thay th n bng cc nucleotitADN bng vic b sung tng nucleotit vo u 3'ca on Okazaki th 3. S thay th nucleotitARN cui cng bng nucleotit ADN s li mtkhung ng - phosphate c u 3' t do.

ADN ligase niu 3' ca onOkazaki th 2 viu 5' ca onOkazaki th nht.

Hnh 16.17 Tm tt qu trnh sao chp ADN vi khun.S ny minh ha mt chc sao chp; nhng nh minh ha trn

s tng qut (pha trn bn phi), qu trnh sao chp thngdin ra ng thi c hai chc ca mi "bng" sao chp. Timi chc sao chp, chng ta d dng nhn thy, mt mchADN mi c tng hp lin tc v c gi l mch dn u;trong khi mch cn li c tng hp thnh tng on ngn vc gi l mch ra chm.

Bng 16.1 Cc protein sao chp ADN vi khunv chc nng ca chng

Protein Chc nngHelicase Tho xon chui xon kp ti v tr chc sao

chp

Protein linkt mch n

Lin kt v lm n nh cc mch n ADNcho n khi cc mch ny c dng lmkhun cho qu trnh sao chp

Topoisomerase Lm gim lc cng pha trc chc sao chpbng cch lm t tm thi cc mch ADN,lun chng qua nhau, ri ni li.

Primase Tng hp on mi ARN ti u 5 camch dn u v ti mi on Okazaki camch ra chm

ADN pol III S dng mch ADN "m" lm khun, tnghp mch ADN mi bng vic b sung ccnucleotit vo u 3 ca mch ADN sn choc on mi ARN qua lin kt cng ha tr

ADN pol I Loi b cc nucleotit ARN thuc on mibt u t u 5, ri thay th chng bngcc nucleotit ARN

ADN ligase Ni u 3 ca on ADN c loi bon mi vi phn cn li ca mch dnu, hoc ni gia cc on Okazaki camch ra chm


14/20


php cc li dn n pht sinh ung th c th tch ly trn phnt ADN vi tc nhanh hn so vi bnh thng.

Cc nucleotit kt cp sai hoc bin i cng c th xuthin sau qu trnh sao chp. Trong thc t, duy tr chnh xcthng tin di truyn, cc t bo cn thng xuyn sa cha ccsai hng khc nhau xy ra vi ADN. Cc phn t ADN dngnh lun trng thi bc l vi nhiu nhn t vt l v ha hcnguy hi (s c chng ta cp k hn Chng 17). Cchp cht phn ng mnh (c mt trong mi trng sng hocxut hin t nhin trong cc t bo), cc tia phng x, nh sng

cc tm v mt s phn t nht nh trong khi thuc l c thgy nn s bin i ca cc nucleotit v nh hng n thngtin di truyn c m ha trn cc phn t ADN. Ngoi ra, bnthn cc baz trong ADN cng c th bin i t pht trongiu kin sinh l bnh thng ca t bo. Tuy nhin, thng thnhng bin i ny s c sa cha cho ng trc khi chngc th tr thnh cc t bin di truyn n nh qua cc th h.Mi t bo u lin tc theo di v sa cha vt cht di truynca chng. Do hot ng sa cha ADN c tm quan trngsng cn i vi c th, nn khng c g l ngc nhin khi cnhiu enzym sa cha ADN xut hin trong qu trnh tinha. E. coli, c khong 100 enzym sa cha ADN cbit n; trong khi , con s ny ngi l khong 130.

Phn ln cc h thng sa cha ADN ca t bo, bt k ivi cc sai hng do li sao chp hay cc sai hng khc v cutrc ADN, u da trn nguyn tc b sung gia cc baz trnhai mch ca phn t ADN. Thng thng, mt on trn mchADN mang nucleotit sai hng c ct b bi mt enzym ctADN - nuclease - ri on trng hnh thnh s c lp y trli bng cc nucleotit kt cp ng da trn mch ADN khngb sai hng lm khun. Cc enzym lin quan n vic lp yon trng gm ADN polymerase v ADN ligase. Mt hthng sa cha ADN nh vy c gi l sa cha bng ctb nucleotit (Hnh 16.18).

Mt chc nng quan trng ca cc enzym sa cha ADNtrong t bo da ca chng ta l sa cha cc sai hng di truyn

gy ra do tia cc tm n t nh sng mt tri. Mt loi saihng nh vy c minh ha trn Hnh 16.18; trong , ccbaz thymine lin k vi nhau trn mch ADN hnh thnh linkt cng ha tr vi nhau. Lin kt kp thymine nh vylmbin dng cu trc ADN bnh thng v nh hng n qutrnh sao chp. Tm quan trng ca sa cha ADN i vinhng sai hng ny c nhn thy qua bnh kh b sc t; yl bnh gy ra bi sai hng di truyn lin quan n gen m haenzym sa cha ADN theo c ch ct b nucleotit. Nhng cth ri lon v enzym ny thng rt mn cm vi nh sngmt tri; cc t bin trong t bao da ca h do tia cc tm gyra vn khng c sa cha, tch ly qua thi gian v c nguyc gy nn bnh ung th da.

Sao chp u tn cng ca phn t ADNMc d kh nng sao chp ca cc enzym ADN polymerase l"n tng", nhng trong t bo lun c mt t l nh trnh tADN m cc ADN polymerase khng th sao chp v sa chac. i vi cc phn t ADN mch thng, chng hn nh nhim sc th sinh vt nhn tht, cc ADN polymerase ch cth b sung cc nucleotit vo u 3 ca mt chuipolynucleotit ang ko di; iu ny dn n vn l: b mysao chp khng c cch no c th sao chp hon chnhphn u 5 ca cc mch ADN con. Ngay c mt onOkazaki c th bt u bng mt on mi ARN lin kt viu tn cng ca mch ADN lm khun cng khng th thay

th bng ADN bi khng c sn u 3 pha trc phnng b sung cc nucleotit c th din ra (Hnh 16.19). Kt qul sau mi ln sao chp, phn t ADN si kp ngy cng ngnli v c cc u khng bng nhau (cn gi l u "ch chi").

Hin tng phn t ADN c xu hng ngn li sau mi lnsao chp thng khng xy ra cc sinh vt nhn s, bi v ccphn t ADN ca chng c dng vng (tc l khng c cc utn cng). Vy, c ch no bo v cc gen ca sinh vt nhntht khng mt i sau cc chu k sao chp ADN ni tip nhau ? l do cc phn t ADN nhim sc th sinh vt nhn thtc cc trnh t nucleotit c bit ti cc u tn cng ca chngv c gi l u mt nhim sc th (Hnh 16.20). Vng umt nhim sc th khng cha cc gen; thay vo , n thngcha cc trnh t nucleotit ngn lp li nhiu ln. Ti cc umt nhim sc th ngi, mt trnh t ngn gm 6 nucleotit lTTAGGG thng lp li t 100 n 1000 ln. Trnh t ADN tiu mt bo v cc gen ca c th. Ngoi ra, cc protein c

Hnh 16.18 Sa cha ADN kiu ct b nucleotit.Mt nhm cc enzym v protein c vai tr pht hin v sa

cha ADN sai hng. Hnh trn cho thy ADN mang mt phckp thymine, y l mt kiu sai hng ADN thng b gy ra dochiu x UV. Mt enzym nuclease ct b vng ADN sai hng,ri mt enzym ADN polymerase (ADN pol I vi khun) s thayth on b ct bng cc nucleotit ph hp trn c s dngmch ADN khng b sai hng lm khun. Enzym ADN ligase shon thnh qu trnh sa cha bng cch lp khe h (lin ktphosphodiester) cui cng trn khung ng - phosphate.

Phc kp thyminelm bin dng ADN

Mt enzym nucleasect on mang nucleotitsai hng trn mt mch

ADN ti hai im, riloi b on sai hng

ADN polymerase tnghp b cc nucleotit voon trng, s dng mchkhng b sai hng lm khun

ADN ligase hon thnhvic sa cha ADN bng viclp lin kt phosphodiestercui cng trn khung ng -phosphate

Nuclease

ADNpolymerase

ADNligase


15/20


hiu lin kt vi ADN ti u mt c vai tr ngn cn cc u"ch chi" ca phn t ADN con khng hot ha cc h thngtheo di cc sai hng ADN ca t bo. (Cc u "ch chi" caphn t ADN, nu hnh thnh do s t gy si xon kp,thng l tn hiu thc y s dng li ca chu k t bo hocdn n con ng cht theo chng trnh ca t bo).

Cu trc u mt nhim sc th khng th gip phn t

ADN mch thng trnh khi vic ngn li sao mi chu k saochp, m chng ch lm chm s "n mn" cc gen gn u tncng ca cc phn t ADN. Nh minh ha trn Hnh 16.19, umt nhim sc th ngn li sau mi chu k sao chp. Kt qul, ADN c xu hng ngy cng ngn hn trong cc t bosoma ang phn chia ngi gi hoc trong cc t bo nuicy tri qua nhiu ln phn bo. Ngi ta cho rng s ngndn ca u mt cc nhim sc th bng cch no c linquan trc tip vi qu trnh gi ha nhng m nht nh,thm tr vi s gi ha ca ton b c th.

Nhng iu g xy ra vi cc t bo m h gen ca chngvn cn c duy tr nguyn vn qua nhiu th h sinh sn?Nu cc nhim sc th cc t bo mm sinh dc (cc t bo

sinh giao t) tr nn ngn hn sau mi chu k t bo, th nhnggen thit yu cui cng s mt i trong cc t bo giao t mchng sinh ra. Tuy vy, trong thc t, iu ny khng xy ra.C mt enzym, c gi l telomerase, xc tc vic ko diu mt trong cc t bo mm sinh dc sinh vt nhn tht;

qua , b p cc nucleotit b mt sau mi chu k sao chp vphc hi li chiu di ban u ca cc phn t ADN. Enzymtelomerase khng hot ng hu ht cc t bo soma ngi,nhng hot tnh ca n trong cc t bo mm sinh dc gip umt cc nhim sc th hp t thng t c di ti a.

Vic u mt nhim sc th cc t bo soma thng ngni sau mi ln phn bo cng c th gip bo v c th khi spht sinh ung th, bi v qua c ch ny s ln phn bo cacc t bo soma b hn ch. Cc t bo ca cc khi u lnthng c cc u mt nhim sc th ngn bt thng, c thdo chng tri qua nhiu ln phn bo. Nu u mt nhimsc th tip tc ngn i th cc t bo khi u c th cht t pht.Nhng iu ngc nhin l cc nh khoa hc tm thy enzym

telomerase hot ng mnh nhiu t bo ung th; iu nycho thy kh nng ca enzym ny trong vic gip duy tr s nnh u mt nhim sc th cc t bo ung th. Nhiu t boung th dng nh c kh nng phn bo khng hn ch, chnghn nh cc dng t bo "bt t" khi c a vo nui cyinvitro (xem Chng 12). Nu enzym telomerase l mt nhnt quan trng trong pht sinh nhiu bnh ung th khc nhau, thenzym ny c th l mt "mc tiu" hiu qu trong chn onv iu tr cc bnh ung th tng ng.

Ti y, chng ta cp v cu trc v s sao chp ADNsi kp. Trong phn tip theo, chng ta s xem ADN c nggi nh th no trong cc nhim sc th, cu trc ca t bovn c coi c vai tr mang thng tin di truyn.

Hnh 16.19 u mt cc phn t ADN mch thngngn li sau mi chu k sao chp. Hnh trn ch minhha mt u ca mt mch phn t ADN si kp qua hai chu k

sao chp. Sau chu k th nht, mch ra chm mi ngn hn sovi mch lm khun. Sau chu k th hai, c hai mch dn uv mch ra chm u ngn hn so vi mch ADN m banu. Mc d khng c v y, nhng hin tng ngn dncng xy ra vi u mt th hai cn li ca phn t ADN.

Mch dn uMch ra chm

u mt cacc mchADN m

Mch dn u

Mch khun

Mi ARN

on cui on pha trc

on mi ARN cloi b v thay th bngADN nh c u 3 caon pha sau

Chu k saochp th hai

Mch dn u mi

Mch ra chm mi

Cc chu k saochp tip theo

Cc phn t conngy cng ngn

Mi c loi b nhngkhng c thay th

bng ADN do ADN polkhng th hot ng khi

thiu u 3 tdo

Hnh 16.20 u mt nhim sc th. Cc sinh vt nhntht c cc trnh t lp li, khng m ha phn tn cng cacc phn t ADN mch thng v c gi l u mt. Hnh trnl nhim sc th chut c phn u mt nhum mu vng.

16.21. Nguyn tc kt cp b sung ca cc baz nit c vai

tr th no trong sao chp ADN ?

2. Nu hai chc nng chnh ca ADN pol III trong saochp ADN?

3. Nu ADN pol I b mt chc nng,th s sao chp mch dn u s b nh hng nhth no? Trong Bng sao chp tng qut Hnh16.17, xc nh v tr hot ng ca ADN pol I trnmch dn u .Xem gi tr li Ph lc A.

Kim tra khi nim

iu g nu


16/20


Thnh phn chnh trong h gen hu ht vi khun l mt phnt ADN si kp, mch trn lin kt vi mt lng nh protein.

Mc d chng ta thng coi cu trc ny l nhim sc th vikhun, nhng thc t cu trc ny rt khc so vi mt nhim

sc th in hnh sinh vt nhn tht vn thng bao gm mtphn t ADN si kp mch thng lin kt vi mt lng ln

protein. E. coli, ADN nhim sc th bao gm khong 4,6triu cp nucleotit, tng ng vi khong 4400 gen. LngADN ny gp khong 100 ln so vi mt h gen virut inhnh, nhng ch bng khong mt phn nghn so vi lngADN c trong mt t bo soma ngi. D vy, lng ADN vi khun cng l rt ln so vi kch thc ca t bo.

Nu dui thng, phn t ADN trong mt t bo E. coli c

th o bng n v milimet v di hn khong 500 ln so vikch thc ca t bo. Tuy vy, trong t bo nh tng tc vi

16.3Khi nimMi nhim sc th gm mtphn t ADN c ng gi vicc phn t protein

Hnh 16.21

Khm ph ng gi cht nhim sc trong nhim sc th sinh vt nhn tht Chui s v nh hin vi in t truyn qua di y m t m hnh biu din cc cp gp xon v siu xon ca nhim sc th. Cc hnh minh ha c phng i t cp cutrc ca mt phn t ADN n l ti nhim sc th k gia nguyn phn l lc nhim scth co xon cc i v c th quan st c di knh hin vi quang hc thng thng.

Chuixon kp ADN(ng knh 2nm)

Cc proteinhistone

Nucleosome(ng knh 10nm)

ui histone

H1

1Chui xon kp ADN y minh ha m hnh di ruy bngADN; trong , mi di ruy bng biu dinmt khung ng - phosphate. T Hnh16.7, chng ta nh rng gc phosphatephn b dc khung phn t ny v lmcho phn t ADN c c tnh tch inm sut dc chiu di phn t. nh hinvi in t truyn qua (TEM) trn chothy mt phn t ADN trn (khng linkt protein); thit din chiu ngang(ng knh) ca ring chui xon kpny l 2nm.

2Cc histoneCc protein histone c vai tr ng gi ADNvo cht nhim sc cp u tin. Tuy miphn t histone ch c kch thc nh (khong100 axit amin), nhng tng khi lng cchistone trong cht nhim sc gn tng ngvi lng ADN. Cc axit amin tch in dng(lysine hoc arginine) chim hn 1/5 tng scc axit amin c trong histone.

Bn loi histone ph bin nht trong cht

nhim sc l H2A, H2B, H3 v H4. Cchistone ny rt ging nhau mi sinh vt nhntht. V d, histone H4 b ch khc histoneH4 cy u ng 2 axit amin, cn li lging ht nhau. S bo th ca protein histonetrong sut qu trnh tin ha cho thy vai trquan trng sng cn ca protein ny trong tchc ADN ca cc t bo sng.

Bn loi histone chnh c vai tr quyt nhcp ng gi tip theo ca ADN. (Mt loihistone th 5 l H1 lin quan n mt bcng gi tip theo ca cht nhim sc).

3Nucleosome, hay chuiht (si 10-nm)Trn nh hin vi in t, si nhim sc cp tchc ny nu khng gp xon c ng knh 10nm(si 10nm). Si nhim sc c dng ging nh mtchui ht vi cc ht xp cch nhau tng iu n. Mi ht l mt nucleosome; y chnhl n v ng gi ADN c bn; si ni gia ccht c gi l cc onADN ni.

Mt nucleosome lun gm ADN cun quanh liprotein 1,65 vng; li protein c cu to t 8phn t ca 4 loi histone chnh (mi loi ng gp2 phn t). u amino (u N) tn cng ca mihistone (ui histone) thng th ra ngoi nucleosome.

Trong chu k t bo, khi ADN sao chp, cchistone ri khi ADN trong thi gian ngn. Nhnchung, hin tng tng t xy ra khi gen cphin m v b my ca t bo phi tip cn cADN. Chng 18 s cp n nhng pht hingn y v vai tr ca nucleosome v ui histonetrong iu ha biu hin gen sinh vt nhn tht.


17/20


mt s loi protein nht nh, nhim sc th thng dng gpxon thm ch "siu xon" c th ng gi cht v ch chimmt khong khng gian hn ch trong t bo. Khng gingnhn t bo sinh vt nhn tht, vng cha mt cao caADN vi khun khng c lp mng bao bc v ch c gi lvng nhn (xem Hnh 6.6).

Mi nhim sc th sinh vt nhn tht u cha mt chuixon kp ADN mch thng duy nht; ngi, kch thc trungbnh vo khong 1,5 x 108 cp nucleotit. Lng ADN nh vy

l ln hn nhiu so vi chiu di nhim sc th khi co xon cci. Nu dui thng, mi phn t ADN h gen nhn ngi c

chiu di trung bnh trn 4 cm, tc l di hn hng nghn ln sovi ng knh ca nhn t bo - l cha k n 45 nhimsc th khc cn li ng thi c mt trong nhn t bo!

Trong t bo, ADN ca sinh vt nhn tht kt hp chnh xcvi mt lng ln cc protein. S t hp ca ADN vi ccprotein cu trc nhim sc th c gi l cht nhim sc.Nhn t bo c th cha va cht nhim sc l do s ng giADN nhiu cp khc nhau bi cc protein cu trc nhimsc th. Hiu bit hin nay ca chng ta v cc cp ng gi

ADN c minh ha trn Hnh 16.18. Hy quan st k hnhny trc khi c cc phn tip theo.

Si 30-nm

4Si 30-nmCp ng gi ADN tip theo l do tng tcgia cc ui histone ca mt nucleosome viphn ADN ni v vi cc nucleosome lin k hai bn. cp ng gi ny, c s tham giaca mt histone th nm l H1. Cc mi tngtc ny lm si nhim sc 10nm tip tc cungp v to nn si c chiu dy khong 30nm(si 30nm). Mc d si 30nm rt ph bin trongk trung gian ca chu k t bo, nhng cnnhiu quan im khc nhau v s sp xp ccnucleosome bc cu trc ny.

5Min thng lng(si 300-nm)Cc si 30nm tip tc cun vng hnhthnh nn dng cu trc ging thnglng, gi l min thng lng,nh vokhung nhim sc th c cu to nnt cc protein. Cu trc ny gi l si300nm. Khung nhim sc th thngc thnh phn giu v mt loitopoisomerase, ng thi c mthistone H1.

6Nhim sc th k giaTrong mt nhim sc th ang nguyn phn,cc min thng lng tip tc cun gp bngcch no cho n nay cha bit y to nn mt dng nhim sc th co xon cci nh c minh ha bng nh hin vi in t trn.Chiu rng ca nhim sc t khong700nm. Cc gen nht nh lun c tm thy v tr c th ca chng trn nhim sc th kgia; iu ny cho thy: cc cp ng gicao hn ca nhim sc th cng c tnh chiu v chnh xc rt cao.

Si thng lng Khung nhim sc th

Si 300-nm

Nhim sc th nhn i(1400 nm)

Nhim sc t(700 nm)


18/20


Trong chu k t bo, cht nhim sc phi tri qua nhngthay i v cp ng gi ca n (xem Hnh 12.6). Khinhum nhim sc th k trung gian v quan st di knhhin vi, cht nhim sc thng c thy dng khuch tntng i u khp nhn t bo; lc ny, nhim sc th dnggin xon. Khi t bo chun b nguyn phn, cht nhim scgp xon (c c), cui cng k gia hnh thnh nn mt slng c trng cc nhim sc th c kch thc ngn v dy,c th phn bit c bng knh hin vi quang hc.

Mc d cc nhim sc th k trung gian thng c mc c c thp hn nhiu so vi nhim sc th trong nguynphn, nhng n cng c cc cp ng gi bc cao. Mt scht nhim sc bao gm cc vng c cu trc si 10nm, bncnh nhng vng khc ng gi thnh si 30nm; nhng vngny c th tip tc ng gi thnh cc min "thng lng". Mcd mt nhim sc th k trung gian thng khng c mtkhung nhim sc th r rt, nhng cc min thng lng ca nthng c biu hin nh kt vo mt lp mng bn trong mngnhn, v c l cng c th nh kt vo cc si cu trc nnmng li nhn. Bng cch nh kt nh vy, nhim sc thc t chc dng ph hp vi s biu hin ca cc gen. Cht

nhim sc ca mi nhim sc th k trung gian chim mtkhng gian c th trong nhn t bo, v cc si nhim scthuc cc nhim sc th khc nhau khng vng mc vo nhau.

Thm ch ngay trong k trung gian, cc cu trc ca nhimsc th bao gm tm ng v cc u mt cng nh mt svng nhim sc th khc trong t bo tn ti trng thi coxon cao ging nh khi nhim sc th i vo k gia nguynphn. Kiu cht nhim sc k trung gian quan st thy diknh hin vi dng kt c khng u n c c gi l dnhim sc, phn bit vi kiu cht nhim sc c mc ktc thp hn l nguyn nhim sc ("nhim sc tht"). Do cmc ng xon cao, cc b my biu hin thng tin di

truyn ca t bo (nh b my phin m) kh c th tip cnADN trong vng d nhim sc. Ngc li, s ng gi "lnglo" ca vng nguyn nhim sc cho php cc b my phinm c th tip cn c ADN; v vy, cc gen trong vngnguyn nhim sc c th c biu hin.

Nhim sc th l mt cu trc nng ng; n c th c c,gin xon, bin i, thm ch thay i cu hnh theo yu cuca cc qu trnh khc nhau trong t bo nh nguyn phn,gim phn v qu trnh biu hin ca cc gen. Hin nay, cccon ng iu ha s bin i ca cht nhim sc vn angtip tc c cc nh khoa hc tp trung nghin cu. Tuy vy,mt vn tr nn r rng l cc histone khng ch n

thun l cc ng ch c tnh tr ADN c th qun quanhtrong cu trc cht nhim sc. Thay vo , s bin i ha hcca cc protein histone c vai tr trc tip lm thay i mc t chc ca cht nhim sc v tham gia iu ha s biu hinca cc gen. Terry Orr-Weaver, nh khoa hc c phng vn phn u ca khi kin thc Di truyn hc ny (cc trang 246- 247), cng cc cng s ca mnh nghin cu v c chphn t lin quan n ng hc nhim sc th trong nguynphn v gim phn. Vi cc nghin cu Drosophila, cc nhkhoa hc ch ra rng s phosphoryl ha mt s axit amin cth trong vng ui histone c vai tr quyt nh s ng thica nhim sc th trong k u ca gim phn I (Hnh 16.22).

S phosphoryl ha histone c vai tr g tronghot ng ca nhim sc th gim phn?

Terry Orr-Weaver v cc cng s ti Vin Cngngh Massachusetts tin hnh gy t bin thc nghim rui gim nhm tm ra cc th t bin bt th vi suy ngh chorng nhng th t bin c th lin quan n cc gen mha cho cc protein gi vai tr quan trng trong nguyn phn.T , h tm ra mt t bin gen nhk-1 gy bt th ruici. H bit rng sn phm ca gen ny l histone kinase-1(NHK-1), mt enzym c vai tr phosphoryl ha mt axit aminc th vng ui ca histone H2A. H gi thit rngnguyn nhn gy nn tnh bt th l do enzym ny khng hotng chc nng ng, dn n ng thi bt thng ca nhimsc th trong gim phn v qu trnh nguyn phn gp ri lon.

kim tra gi thit, h quan st v so snh s vnng ca nhim sc th trong gim phn cc t bo sinhtrng ca rui t bin v rui kiu di (bnh thng). Trong mtth nghim, h dng mt thuc nhum hunh quang nh du ni nh v ca ADN v mt thuc nhum hunhquang xanh lc xc nh ni nh v ca protein condensin,l protein thng bao bc cc nhim sc th vo cui K u I

v gip nhim sc th c c.Vo cui K u I, trong cc t bo sinh trng rui

kiu di, ADN v codensin nh v tp trung mt vng nhtrong nhn (hnh di bn tri; mu vng l do thuc nhum v xanh c mt ng thi). Tuy vy, rui t bin, codensinkhuch tn khp nhn; trong khi ADN ch tp trung vng binquanh nhn (hnh di bn phi; mu ca ADN tng im). Kt qu ny cho thy: codensin khng bao bc ccnhim sc th trong t bo ca cc rui t bin. Hu qu lcc nhim sc th khng c c c.

Nhn t bo bnh thng Nhn t bo t bin

Do qu trnh gim phn khng th din ra bnhthng khi enzym histone kinase NHK-1 khng biu hin ng

chc nng ca n, nn cc nh khoa hc kt lun rng sphosphoryl ha c th ui N ca histone H2A l thit yu hot ng ca cc nhim sc th trong gim phn c thdin ra chnh xc.

I. Ivanovska, T. Khandan, T. Ito and T.L.Orr-Weaver, Ahistone code in meiosis: the histone kinase, NHK-1, is required forproper chromosomal architecture in Drosophila oocytes,Gene andDevelopment19: 2571 - 2582 (2005).

Gi s mt nh nghin cu tm ra rui gim mtth t bin mt axit amin vn bnh thng c phosphoryl ha bihistone kinase NHK-1. t bin ny s nh hng th no n hotng ca nhim sc th trong gim phn cc t bo sinh trng?

Hnh 16.22Nghin cu pht hin

Th nghim

Kt qu

Kt lun

Ngun

iu g nu

Codensin vADN (mu vng)

Vin baongoi nhn

Codensin(mu xanh lc)

ADN (mu vng quanh nhn)


19/20


S phosphoryl ha v cc bin i ha hc khc ca cchistone c nhiu tc ng n s hot ng ca cc gen, sc cp Chng 18.

chng ny, chng ta tm hiu cc phn t ADN ct chc nh th no trong cc nhim sc th v bng cch nos sao chp ADN c th cung cp cc bn sao ca gen m b,m c th chuyn cho con con ci. Tuy vy, trong qu trnh ditruyn, vic cc gen c sao chp v chuyn gia cc th h lcn thit nhng cha ; iu quan trng hn l cc thng tin

di truyn phi c cc t bo s dng. Ni cch khc, ccgen phi dng "c biu hin". Trong chng tip theo,chng ta s xem cc t bo c th dch m thng tin di truync m ha trong cc phn t ADN nh th no.

16.3

1. y m t cu trc ca nucleosome, n v ng giDN c bn t bo sinh vt nhn tht.

2. ai thuc tnh gip phn bit d nhim sc v nguynhim sc l g?

3. Mc d cc protein gip nhim sch E. coli ng xon khng phi l histone, nhng

heo bn thuc tnh no ging vi histone m ccrotein ny cn phi c, xt v kh nng lin kt ADN?

Xem gi tr li Ph lc A.

Kim tra khi nim

iu g nu

Tng kt ChngHy tham kho c s hc liu gm cc hnh nh ng

ba chiu, cc bi hng dn dng file MP3, video, cc bi kim tra thc hnh,eBook v nhiu hc liu khc ti a ch Web www.masteringbio.com

ADN l vt cht di truyn (cc trang 305 310)

Tm kim vt cht di truyn: Qu trnh iu tra khoa hcCc th nghim c tin hnh vi vi khun v phag cungcp cc bng chng thuyt phc u tin chng minh ADN l vtcht mang thng tin di truyn.

Xy dng m hnh cu trc ADN: Qu trnh iu tra khoa hcWatson v Crick tm ra ADN c cu trc xon kp. Hai chuing - phosphate i song song cun quanh pha ngoi phn t;

cc baz nit hng vo pha trong, qua cc lin kt hydrochng kt cp c hiu vi nhau, gia A vi T v gia G vi C.

Hot ng Th nghim Hershey - ChaseHot ng Cu trc ca ADN v ARN

Hot ng Chui xon kp ADN

Nhiu protein phi hp trong sao chp v sa chaADN (cc trang 311 319)

Nguyn l c bn: Kt cp baz vi mch lm khunTh nghim Meselson - Stahl cho thy ADN sao chp theo c chbn bo ton: phn t m gin xon v mi mch ca n sau c dng lm khun tng hp nn cc mch mi trn c snguyn tc kt cp b sung gia cc baz nit.

Tm tt cc khi nim chnh

a phng tin

Khi nim 16.1

Khung ng - phosphate

Cc baz nit

Lin kt hydro

a phng tin

Khi nim 16.2

Sao chp ADN: Quan s t gn hn

c sa v sa cha ADN Cc enzym ADN polymerase c khnng c sa mch ADN mi, thay th cc nucleotit sai hng.Trong c ch sa cha kt cp sai, cc enzym c th sa chacc li tn ti sn. C ch sa cha bng ct b nucleotit lmt qu trnh c bn trong cc enzym c th ct b v thay

th mt on di ADN mang cc nucleotit sai hng. Sao chp u tn cng ca phn t ADN u tn cng ca

phn t ADN thuc nhim sc th sinh vt nhn tht thng ngnli sau mi chu k sao chp. S c mt ca u mt l trnh tlp li cc u tn cng ca cc phn t ADN mch thng lcch bo v cc gen gn u mt khi s "n mn". Enzymtelomerase xc tc phn ng ko di u mt nhim sc th cc t bo mm sinh dc.

Hot ng Sao chp ADN: Tng quaniu tra M hnh sao chp ADN ng nh th no ?Hot ng Sao chp ADN: Quan st gn hnHot ng Sao chp ADN: Tng kt

Mi nhim sc th gm mt phn t ADN c nggi vi cc phn t protein (cc trang 320 323)

Nhim sc th vi khun thng l mt phn t ADN mch vnglin kt vi mt s protein. Cht nhim sc sinh vt nhn thtt hnh thnh nn nhim sc th gm ADN, cc histone vcc protein khc. Cc histone lin kt vi nhau v vi ADN hnh thnh nn nucleosome, l n v ng gi ADN c bnnht. Cc ui histone chc ra ngoi phn li nucleosome. Sgp xon tip theo dn n s c c cc i ca cht nhim sc

a phng tin

Khi nim 16.3

ADN pol III tng hp mchdn u mt cch lin tc

ADNm ADN pol III bt u tng hp

ADN ti u 3 ca mi, riko di theo chiu 5 3

Mch ra chm c tng hpthnh nhng on Okazaki ngnri c ni li bng ADN ligase

Primase tng hp mton mi ARN ngn


20/20


K gia. Trong cc t bo K trung gian, phn ln chtnhim sc trng thi c c thp hn (nguyn nhim sc),nhng mt s vng vn trng thi c c cao (d nhim sc).S bin i ca cc histone nh hng n s c c ca chtnhim sc.

Hot ng S ng gi ADN

Cc cu hi t nh gi

1. Trong nghin cu ca mnh tin hnh chut v vi khungy bnh lao phi, Griffith pht hin ra rnga. v protein t cc t bo gy bnh c kh nng chuyn ha

cc t bo khng gy bnh thnh cc t bo gy bnh.b. cc t bo gy bnh sau khi un vn c kh nng gy

bnh lao phi.c. mt s cht t cc t bo gy bnh c truyn sang cc t

bo khng gy bnh v lm chng tr thnh dng gy bnh.d. v polysaccharide ca vi khun gy bnh lao phi.e. cc bacteriophag tim ADN vo vi khun.

2. Cc t bo E. coli c nui trong mi trng 15N, richuyn sang mi trng 14N v cho sinh trng qua hai thh (hai chu k sao chp ADN). Sau , ADN c tchchit t nhng t bo ny ri em li tm. Hy d on sphn b t trng ca ADN trong th nghim ny.a. Mt bng t trng cao v mt bng t trng thpb. Mt bng t trng trung bnhc. Mt bng t trng cao v mt bng t trng trung bnhd. Mt bng t trng thp v mt bng t trng trung bnhe. Mt bng t trng thp

3. Mt nh ha sinh hc phn lp v tinh sch c ccphn t cn thit cho qu trnh sao chp ADN. Khi c ta b

sung thm ADN, s sao chp din ra, nhng mi phn tADN bao gm mt mch bnh thng kt cp vi nhiuphn on ADN c chiu di gm vi trm nucleotit. Nhiukh nng l c ta qun b sung vo hn hp thnh phn g?a. ADN polymerase d. Cc on Okazakib. ADN ligase e. Primasec. Cc nucleotit

4. C s no dn n hin tng mch dn u v mch rachm c tng hp khc nhau trong qu trnh sao chpADN?a. im khi u sao chp ch c pha u 5.b. Enzym helicase v cc protein lin kt mch n ch

hot ng u 5.c. ADN polymerase ch c th ni cc nucleotit mi vo

pha u 3 ca mch ang ko di.d. ADN ligase ch hot ng theo chiu 5 3.e. Vo mi thi im, polymerase ch hot ng trn mt

mch.

5. Khi phn tch thnh phn cc baz khc nhau trong mtmu ADN, kt qu no l ph hp vi nguyn tc b sung?a. A = G d. A = Cb. A + G = C + T e. G = Tc. A + T = G + T

6. S ko di mch dn u trong qu trnh sao chp ADNa. ngy cng ri xa chc sao chp.b. din ra theo chiu 3 5.c. to thnh cc on Okazakid. ph thuc vo hot ng ca ADN polymerasee. khng cn mch lm khun

7. Khi t pht mt nhm amino, Adenine chuyn ha thnhHypoxanthine, l mt baz him thng kt cp viThymine trong phn t ADN. S phi hp ca nhng phn

t no c th sa cha c sai hng ny?a. nuclease, ADN polymerase, ADN ligaseb. telomerase, primase, ADN polymerasec. telomerase, helicase, protein lin kt mch nd. ADN ligase, cc protein chc sao chp, adenylyl cyclasee. Nuclease, telomerase, primase

8. mi nucleosome, ADN c qun quanh bia. cc polymerase d. mt phc kp thymineb. cc ribosome e. ADN vi v tinhc. cc histone

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Thc hin bi Kim tra thc hnh ti trangweb www.masteringbio.com

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9. Mt s vi khun c th p ng c vi cc tc nhn stresst mi trng bng vic tng tn s t bin trong qu trnhphn bo. Hin tng ny xy ra nh th no? N c u thg trong tin ha? Gii thch.

iu tra khoa hc

10. Xy dng cc m hnh l mt phng php quantrng trong cc nghin cu khoa hc. Hnh trn minh hamt m hnh phc h sao chp ADN c m phng bimy tnh. Mch ADN gc v mch mi tng hp c phnbit bng cc mu khc nhau; tng t nh vy l ba loiprotein: ADN pol III, protein cp trt v protein lin ktmch n. Trn c s kin thc hc c t chng ny,bn hy b sung cc thng tin lm r m hnh trn bngvic ch thch vo hnh tn cc mch ADN v mi loiprotein, ng thi v thm mi tn ch r chiu m qu trnhsao chp ang din ra.

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