chapter 15_DNA mobile_Dịch từ sách sinh học phân tử Molecular biology

8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology

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Bn cnh cc nhim sc th trong cc t bo sng , nh ton b cc yu t di

truyn vn ng rt a dng c tm thy trong t nhin. S phc tp

trong cu trc ca cc loi virus nh l cc loi plasmid v cc viroids m

chng ch n thun l cc phn t acid nucleic n ((see Ch. 16 and

17).Cc yu t l cc k sinh trng sng da vo vt ch , chng cn c

cc vt ch cung cp nng lng v cc nguyn liu th khc .Trong mt s

trng hp nh l rt nhiu cc virus , chng c th lm cho cc t bo ch

b ph hu .Trong cc trng hp khc nh l a s cc plasmid cc t bo

ch l v hi v trn thc t chng cn mang n nhiu li ch cho cc t

bo ch bng chnh cc gen mang trong cc plasmid .

K t khi tt c cc yu t mang thng tin di truyn chng c th c xemnh l cc dng sng .Chng ta i khi cng xem xt chng nh l cc

gene creatures k t khi chng thiu cc t bo cho ring chng nhng li

vn mang thng tin di truyn . V i vi cc gene creatures ny bn thn

cc t bo m n c tr cng chnh l mi trng sng ca chng .Quan

im ny c v hi l nhng n thng gip chng ta c th hiu c c

ch di truyn nu chng ta xem bn thn chng chnh l cc yu t di truynhn l coi chng nh l cc sinh vt sng hoc l cc t bo sng .

Mt trong s cc yu t di truyn thm ch khng tn ti c lp vi h

genome nhng chng chnh l cc on ADN c kh nng chn vo cc

phn t AND khc v ngi ta gi chng l cc on AND c kh nng di


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chuyn . Mt s on AND c kh nng di chuyn t vng ny n v cc

vng khc trong phm vi t bo ch v chng c bit n nh l cc yu

t di truyn vn ng transposable elements

hay l transposons.Cc cu trc khc c lu gi bn vng trong cu trcca cc ADN di chuyn chng b phn hu v tr thnh cc on hay l cc

mnh ADN .Rt nhiu cc genome ln nh l genome ngi bao gm cc

on ADN khng cn hot tnh na .

Cc phn t DNA c th c phn chia cng vi s phn chia t bo hoc

chng c th chuyn t t bo ny sang t bo khc . Nhng i vi cc nh

di truyn hc , khi nim mobile DNA c dng vi ngha c

bit .Cc Mobile DNA l cc on chui kp DNA m di chuyn nh l cc

n v ring r c kh nng di chuyn t ni ny sang ni khc ( Chc chn

l cc yu t AND c th tn ti mt kh nng chuyn t v tr ny sang

v tr khc nh l cc phn t c lp , nhng chng khng th c cc chc

nng c lp , v d nh l cc gene cassettes of integrons m chng ta s

tho lun di y )

Cc on DNA di ng c th di chuyn t v tr ny sang v tr khc trongcng mt phn t ADN ln hoc l di chuyn t mt phn t AND ch n

cc phn t ADN khc .Mt s v tr chn t nhiu l ngu nhin trong khi

mt s on khc c th chn vo cc trnh t xc nh trong DNA ca vt

ch . Mc d mt s cc on DNA ca cc loi virus bn thn n c th


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chn vo cc NST ca cc t bo ch , nhng phn ln a s cc phn t

ADN thuc yu t DNA c kh nng di chuyn c bit n nh l cc

transposons or transposable elements .i khi chng cng c gi l cc

gen nhy jumping genes bi v chng c th di chuyn t ni ny sangni khc trong phm vi cc NST .Qa trnh nhy ca cc gen t v tr ny

n v tr khc c gi l s di chuyn transposition .Bn thn cc

transposons khng n thun l cc yu t c lp trong t bo vt ch

ging nh l plasmids v cc viruses , chng l cc yu t ph thuc vo

DNA ca vt ch .Cc transposons lun lun c chn vo nhng phn t

DNA khc v vy chng khng bao gi tn ti t do nh l cc phn t ADN

ring r (Fig. 15.01).

tng kt li cc khi nim :


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Fig. 15.01 : Cc yu t di truyn vn ng khng bao gi tn ti t do .

- cc yu t di truyn vn ng l cc phn t c kh nng di chuyn t

v tr ny n cc v tr khc nhng chng lun lun c tm thy ccphn t nh l cc plasmid vi khun hoc l cc NST ca sinh vt

eukaryote .Cc transposons khng cha trnh t ca chui khi u

sao chp , nhng chng li ph thuc vo cc phn t AND ch c

th cung cp cc c im .

Nh l cc ch chng 5 , bt c phn t DNA m c cha cc v tr

khi u sao chp ca chng .Cc genome ca NST , cc plasmid v cc

virus l cc n v sao chp v chng c th c xem nh l cc phn t c

kh nng t sao chp . Ngc li , cc transsposon thiu cc im khi u

sao chp ca ring mnh v chng khng c gi l cc replicons .Chng

c th c sao chp bng cch di nhp vo cc h gen khc nh l cc

AND nhim sc th , cc plasmid cc virus ,min l cc phn t DNA ca

cc gen nhy c sao chp th gen nhy cng s c sao chp .Nu

nh transposon bn thn n chn vo v sau khng sy ra qu trnh sao

chp th cc phn t transposon cht vi n .

Cc yu t DNA c kh nng di chuyn c phn loi da trn c ch di

chuyn ca chng .S phn chia v c bn l cn c vo cc phn t

RNA trung gian v cn c cc enzyme phin m ngc c th to ra cc

phn t AND v nhng phn t di ng theo c ch da trn DNA

Cc phn t DNA thuc cc lp ph c chia thnh hai nhm chnh datheo cc bn sao mi c to thnh trong sut cc qu trnh di chuyn v tr

hoc l nguyn bn chnh ca bn sao di chuyn , li mt khong

trng trong phn t DNA trong cc v tr trc ( cc trnh t bo th

hoc l cc v tr ct v dn ) .Bng 15.01 tm tt s phn chia cc yu t


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thnh cc nhm chnh sau y ,nhng yu t s c tho lun chng sau

mt cch chi tit hn di y .

Tt c cc transposons c cha hai c im qua trng . Th nht , chng c

cc on lp li cng chiu inverted repeats hai u . Hai l , cc

transposons phi cha t nht mt gene c cha cc on m ho cho cc

enzyme transposase cn cho s di chuyn ca chng (Fig. 15.02).Cc

transposase c t chc thnh cc on lp ngc chiu .Tn s ca s thay

i v tr ca cc yu t di chuyn ht sc a dng t mt transposon

thnh cc on khc . Mt cch in hnh , n cng sp xp t 1/1000 n

1/10 000 trn mt t bo . Trn thc t , cc transposase c tp hp

thnh hai trnh t khc nhau . Trc ht l chng t chc thnh cc cc on

lp ngc chiu hai u ca transposon v cc v tr quyt nh s di

chuyn ca cc on ADN . Thm vo cc enzyme transposase c kh

nng nhn ra mt trnh t c bit trn phn t ADN m chng s chuyn

n trong tng lai . Trnh t c gi l cc trnh t ch v chngthng l cc trnh t base di t 3 n 9 cp base .Chng thng l mt s

nucleotide c thm vo , ch yu l trnh t ny di khong 9 cp base

.Sau , cc transposases thng c th nhn ra cc trnh t m c th gn

nh bt cp vi trnh t ch .Trong mt s trng hp , s t chc ca cc

trnh t ch kh lng lo n ni m n rt kh c th hnh thnh mt

trnh t lin ng .


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Ch : khng c yu t no c cc im khi u sao chp ca ring

chng , ngoi tr cc transposon lin ng - loi transposon c im khi u

di chuyn . Chiu di ca cc transposon c lin quan n loi transposon .

Bi v rt nhiu cc yu t c kh nng di chuyn c th nhn ra cc trnh t

bn trong ca phn t AND bng nhiu cch khc nhau , hn na cc yu t

mang cc trnh t nhn bit a dng c tm thy . Khng nhng vy

bn thn cc trnh t nhn bit ngn c tm thy trong s rt nhiu

cc lp khc nhau ca gen nhy.Chiu di ca cc on ny cng ph hp

vi chc nng ca chng .Chiu di ca Mu DNA bn trong ca mt virus l

39kb v trong 37 kb ca DNA Mu v xp x khong 2000 bases ca AND

nhim sc th nh vo tn cng on gen ca Mu .Cn ch rng khi m

ln u tin Mu nh vo h gen ca vt ch chng s dng hnh thc cut

and paste transposition ch khng phi l replicative transposition.Tt c cc hnh thc trung gian gia cc retroviruses c y chc nng

v cc trnh t coi nh l c ngun gc t cc virus retro c ly nhim ,

chng c tm thy trong cc nhim sc th ca sinh vt nhn chun ,

cng nh l cc yu t LTR ( long terminal repeart ) . Kt qu l,s phn


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loi v nh tn ca cc yu t retro rt a dng theo cc tc gi khc nhau .

LINES v cc yu t c lin quan v i khi cn c gi l cc non-LTR-

retrotransposons phn bit chng vi cc retrotransposons thc s -

nhng on lp li lin ng vi cc LTRs ca cc retroviruses.Retrointronsl cc on intron nhm I v nhm II , nhng on m ho cho cc enzyme

reverse transcriptase.

Cc c im cn thit ca mt transposon c cha hai on lp li ngc

chiu hai u v mt gen m ho cho enzyme transposase.Cc yu t khc

khng cn thit c th c tm thy gia cc on lp li ngc chiu .

Trong sut qu trnh di chuyn , cc transposon nhn i trong trnh t

ch ca DNA vt ch v v vy mt bn copy mi bn sn ca

transposon .

C rt nhiu cc transposon ln mang mt s lng ln cc gen khng h

lin quan n s di chuyn ca bn thn n.Cc gen khng khng sinh , cc

gen lin quan n virus , cc gen lin quan n cc qu trnh trao i cht v


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cc gen khc c th xc nh v tr trong phm vi cc transposons v chng

s tr thnh cc AND c kh nng di chuyn trong phm vi ca genome .

Mt s transposon u tin c th l cc gen mang kh nng khng khng

sinh - kh nng ny bo v cc vi khun ch khi tc dng ca acacs loithuc do con ngi s dng . Kh nng di chuyn cc vt liu di truyn t

cc phn t AND ny n cc phn t DNA khc iu c ngha ht sc

quan trng i vi s tin ho ca cc plasmid , virus v cc NST ca cc vi

khun v ca c nhng sinh vt bc cao .

Loi transposon ngn nht v n gin nht , c gi l cc on chn IS

insertion sequences ) , chng c tm thy vi khun ..chng c chia

thnh cc IS1 , IS2 . Cc trnh t ca cc on chn in hnh thng di

khong 750 n 1500 bp vi cc on lp li ngc chiu di khong 10 n

40 bp ( xem trong bng 15.02)

Thng thng cc on lp ngc chiu hai u on chn thng lpli khng chnh xc hon ton .V d nh l cc on lp ngc chiu ca

IS1 bt cp ti 20 n 23 v tr khc nhau .Cc on chn ch m ho cho c

enzyme n l , l enzyme transposase - mt loi enzyme cn cho s di

chuyn ca chng . Gia cc on lp li ngc chiu l mt vng cha

hai khung c m l orfA and orfB. Bn thn transposase c m ho t

c hai khung c m bng mt hin tng dich khung sy ra trong sut qu

trnh dch m (Fig. 15.03) . S gim tn s ca cc kt qu dch khung nh

vy kt qu s hnh thnh cc transpose ch c biu hin mt mc rt

thp . Cc on khng dch m s dn n s ph hu ln nht nhim sc th

ca vt ch .


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V kt qu l s tng hp cc transposase cn c iu khin mt cch cn

trng . Khi khng c s dch khung sy ra , khung c u tin l orfA

c biu hin .Sn phm ca gen l mt nhn t iu khin qu trnh

sao m iu khin s tng hp ca transposase v ca chnh bn thn nCc on chn IS c tm thy trong cc NST ca vi khun cng nh l

cc on DNA ca cc plasmid cng nh l cc virus ca chng .V d nh

l , mt vi bn copy ca mi on chn IS1 , IS2 , IS3 c tm thy

trong cc nhim sc th ca E coli .Cc plasmid F khng h c bn copy no

ca IS1 , c 1 bn copy ca IS2 , hai bn copy ca IS3 .Khi m cc plasmid

F v mt nhim sc th cha cc trnh t tng ng IS , plasmid c th

chn cc on chn ca bn thn n vo nhim sc th ca vt ch .N cho

php chuyn cc gen ca nhim sc th bng cc plasmid F ging nh qu

trnh s c gii thch trong chng 18 .

Cc trnh t on chn khng cha gen no cung cp mt kiu hnh thun

li .Tuy nhin s c mt ca n cng vn c cng nhn bi v s di

chuyn ca cc trnh t on chn b bt hot vi mt kiu hnh c th

ch n hay c th nhn bit c .Nhng on chn b t bin nh vy

lun lun lm mt hon ton chc nng ca cc gen v thuwowfngphaan

thnh cc downstream gene ca mt operon .Thm vo , chng cng o

ngc vi mt tn s rt thp ( xem trong chng 13)


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Cc loi transposon n gin nht bao gm cc on chn IS di chuyn bi

mt c ch ht sc n gin c gi l cc conservative or cut-and-paste

transposition (Fig. 15.04). hay l cc gen nhy theo kiu ct gin .Loi

transposon ny yu cu phi c 3 yu t : transposase,cc trnh t lp li ochiu tn cng hai u ca n v mt trnh t ch ph hp vi cc on

DNA khc mt vi v tr no trong t bo . Cc transposon c th di

chuyn t ni ny n ni khc c th trong cng mt phn t AND hoc l

cc on AND khc .Cc phn t AND m cc gen nhy c th di chuyn

n c th l mt plasmid , mt virus hoc l mt nhim sc th hay l bt

c phn t AND no min l n c cc trnh t ch ph hp .Cc enzyme

transposase ban u s lin kt vi cc on lp o chiu hai y ca cc

gen nhy , sau n s ct cc transposon cc trnh t khi u (Fig.

15.05A).Bn cnh cc transposase ngi ta tm thy mt trnh t gen ch

ph hp vi cc phn t AND m s c th l v tr nm mi ca cc gen


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nhy , n lm mt im ct m m cc trnh t gen ch c th cho cc u

tha nh ra (Fig. 15.05B). Cui cng , n chn vo cc transposone thnh

cc khong trng , s di mt v tr vi hai vng ngn ca cc on AND

mch n , chng c xc nh bi cc NST ca vt ch v sau tnghp tip tc chui th hai .V sau kt qu l transposon c

chuyn v tr v cc trnh t ch c sao chp trong qu trnh

.Dng ca cc qu trnh di chuyn nh vy cng c gi l conservative

transposition bi v cc on DNA ca cc transposon khng h bin i

trong sut qu trnh di chuyn ca chng . V tr m cc transposon ct ra

khi v tr ca n , chui AND xon kp s b ph v v c th lm cho phn

t AND c th s khng th sa cha c.R rang , tn s cao ca cc

transposon c th s gy ra mt s bin i trong nhim sc th ca vt

ch.Thm ch nh s ph v khng th c gn li , v nu nh cc

trnh t m ho nm trn cng mt nhim sc th th n s c th gy nn

t bin dch khung . Kt qu nh l ch ra trn , s di chuyn ca

chng cn c iu khin mt cch ht sc nghim ngt .


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S di chuyn lun lun li phn t DNA b ph v cc chui

xon kp .Mt vi transposons l cc dng ca replicative transposition ,

chnh l loi gen nhy c th to ra cc bn sao th 2 ca chnh n (Fig.

15.06).Kt qu l , c v tr ban u v v tr mi ca cc v tr ch u c

mt bn sao ca transposon v nh vy th phn t AND ban u khng h

b tn thng hay l b ph hu .Transposons s dng c ch c

gi l mt phc hp transposons (complex transposons)bi v qu trnh

hon ton phc tp hn c ch ct dn ht sc n gin c miu t


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trn .Complex transposons c mt enzyme transposase c kh nng nhn

bit cc trnh t lp li o chiu v cc trnh t ch ch hon ton ging

nh l cc dng transpose khc . Thm vo , cc replicative transposons

cn c thm mt enzyme na , l enzyme resolvase v cn c thm mttrnh t DNA ph thm na , l mt trnh t (IRS) , chng c nhn bit

bi cc enzyme resolvase (Fig. 15.07).

Mc d complex transposon c sao chp trong khi chng di chuyn ,

tuy nhin chng khng phi l cc are replicons, hay c th ni l chng

khng c cc v tr khi u sao chp . Loi transposon khng h to

thnh mt bn copy mi ca bn thn n


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M n c th c gii phng t do khi m dng t do trung gian tm

thy mt ngi nh mi .Thay v vy , cc complex s xm nhp vo cc t

bo vt ch bng cch sao chp DNA ca chng transposons trong sut qu

trnh di chuyn.Replicative transposition tip tc nh sau :Cc transposase bt u bng vic to ra mt im nick trn chui n tn

cng ca transposon v cc trnh t ch .Sau , n s chn vo cc u t

do c th to ra mt cointegrate - mt s ng xm nhp trong c hai

phn t DNA c th lin kt vi chui n ca transposon DNA (Fig.

15.08). S c mt ca cc chui n DNA bin i trong t bo vt ch

lm v hiu ho h thng sa cha ca cc t bo vt ch , sau s sn

sinh v tng hp nn cc chui b xung vi chui n v v vy m sao

chp transposon .Qa trnh li mt s ng xm nhp ca hai chui

xon kp c lin kt vi nhau bi hai transposon . Chng ta cng cn phi

ch l mi bn sao ca transposon bao gm mt chui DNA c v mt

chui DNA mi .Chc nng ca cc enzyme esolvase l phn chia chng

thnh hai phn t DNA. Chng c th nhn dng bng hai trnh t IRS

v gia ca hai bn copy ca transposon v thc hin s t hp li hay cn

gi l ti t hp gia chng (Fig. 15.08).N s hnh thnh nn hai phn t


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DNA t do , mi on mang mt bn copy n l ca


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transposon. Cn phi ch l s hnh thnh hai banr copy c ch r c

th trn hnh 15.08 .Phn ln cc complex transposons c bit n u

mang cc gen khc cng vi cc gen c vai tr trong s di chuyn v tiu

c.V d , Tn1 v Tn3 l cc complex transposons , thnh phn khngkhng sinh ca h penicillin v c tm thy c cc plasmid v nhim

sc th ca rt nhiu vi khun . S di chuyn ca cc transposons nguyn

nhn l do s biu hin kiu hnh ca cc gen .

Mc d , cc loi gen nhy nhm replicative and conservative transposition

dng nh l kh khc nhau , v cc c ch thc s ca cc bc c s tham

gia ca transposase c mi lin h hon ton khc nhau . Nhng trong c hai

trng hp cc trnh t ch c m ra bi cc trnh t ct so le (Fig.

15.09). Trong c hai trng hp , cc enzyme transposase u ct cc

im ni gia hai u ca transposon v DNA ch . Tuy nhin, trong s di

chuyn bo th , c hai chui c ct bt c v tr no cn trong qu

trnh di chuyn sao chp s ch c duy nht mt chui c ct .Trong c hai

trng hp, u t do pha u 3 ca chui c lin kt vi u 5

trong trnh t ch c m . Trnh t chuyn mt conservative

transposon n v tr mi ca n nhng trong khi n c th to ra mt dng

cointegrate trong trng hp ca complex transposon.Cc bc tip theo

ca qu trnh li rt ging nhau c hai dng transposon .Cc enzyme

ca t bo vt ch s tip tc lp y cc chui n bng vic s dng trnht t do u 3 ca trnh t ch c m sn nh l mt primer ,

i vi s di chuyn bo th , mt phn t ARN mi gn nh l ch c mt

s nucleotide v bc hon ton n gin ch l nhn i v sau sao

chp trnh t ch Cn trong trng hp ca s di chuyn bng hnh thc


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nhn i replicative transposition cc vng chui n di hn v bc l

nhn i transposon ca chnh bn thn n . Tng t nh vy , qu trnh

c minh ho bng mt loi transposon l Tn7 , loi m bnh thng

chng c thc hin bng cc c ch bo th.Tn7 thng thng c mtenzyme transposase bao gm hai loi protein. TnsA lm mt im nick trn

chui n u tn cng 5 ca Tn7 v TnsB thc hin ti im nick v sau

chng c th lin kt vi u tn cng 3 ca Tn7 , v v vy , c TNsA

v TnsB thc hin bng mt chui i b ct ra khi m c hai gen m ha

cho hai phn t protein , khi m c hai u c biu hin . Khi c t

bin ca Tn7 tn ti dng c mt protein TnsA ly nhim v n khng cn

ct c chui 5 .Tuy nhin , TnsB tip tc ct v sau ni li v tr ca

chui 3, hnh thnh cc cointegrates nh di chuyn theo kiu sao chp .V

v vy , TnsB ging nh l mt loi transposase ca complex transposons.

Protein TnsA phn t ct u t do Tn7 bn thn vng khi u ca n ,

chng c cu trc tng t nh l cc enzyme ct gii hn type II - type II

restriction endonuclease (see Ch. 22).

Thnh phn ca mt transposon bao gm 2 trnh t lp li o chiu t hai

loi transpson ring r , chng s di chuyn cng vi nhau nh l mt n v

v mang cc on AND gia chng (Fig. 15.10).V d nh,xem xt mt

on ca DNA cnh sn ca c hai u bng hai on chn nhn dng .Bn

thn transposase s chuyn bt c on DNA no ca DNA xung quanh

bng mt cp on lp li ngc chiu m n c th nhn ra .Kt qu l , khi

s di chuyn sy ra mt vi kh nng c th c (Fig. 15.10). Trc kia ,

mi mt trnh t ca on chn c th di chuyn hon ton c lp nhau .Hai

l , ton b cu trc gia hai on lp li ngc chiu c th di chuyn


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nh l mt n v , n cng ging nh l mt composite transposon.Rt

nhiu transposon vi khun c bit c th mang cc gen khng khng

sinh hoc l c cc c tnh khc hp li to thnh cc gen nhy , trong 3

v d in hnh nht l Tn5 (khng kanamycin ), Tn9 (khngchloramphenicol ) and Tn10 (khng tetracycline). Thng thng th cc cp

ca cc on chn hai u ca gen nhy c o chiu c lin quan

n nhau ging nh l Tn5 v Tn 10 . Cc yu t IS ca Tn5 v Tn 10 khng

c tm thy mt cch n l m chng c tn l cc IS50 and IS10 .


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C t hn hai yu t IS c hin tng lp li cng chiu nh l trong trng

hp ca Tn9 , bn sn ca chng c hai on lp cng chiu IS1.Mt trong

nhng thnh phn hu ch ca cc transposon l chng hon ton ging nhau

bi s chn lc t nhin - chusg s c th hot ng gi cc phn ca

chng vi nhau .Qa trnh t bin tch lu cc dng bt hot vi cc

on lp ngc chiu , nhng on c th ngn cc trnh t ca cc on

chn t cc on gen nhy c lp .Thng thng , mt trong hai gen m

ho cho cc transposase mt i . Kt qu l hai u v on gia s c

th lin kt hon ton v c th lun lun di chuyn cng vi nhau nh l

mt n v (Fig. 15.11). Trong thc t , tt c cc bc u c tmthy cc vi khun v v vy cc gen nhy c th s dng cho cc thao tc

di truyn trong cc phng th nghim nghin cu


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Cc hin tng thm , mt v o sy ra trong DNA ca t bo ch c th l

do cc hin tng d thng hoc l do s di chuyn v hot ng ca cc

thnh phn trong h transposon .Nh c ch n trn cc

transposase s di chuyn bt c on DNA no xung quanh bng mt cpca cc on lp li o chiu .Cc transposon lin hp c ti 4 on lp

hai u nh vy , 2 on c gi l inside ends c lin quan n cc

transposon cn hai on c gi l outside ends (Fig. 15.10 above).

Bt c cp no c s i din nhau mt cch trc tip u c th s dng

cng vi nhau . S di chuyn ca cc yu t IS u c lin quan n cc

inside end and one outside end. S di chuyn bnh thng ca ton b

cc transposon lin hp c s dng vi hai outside ends.Nhng cn nh

rng cc inside ends cng c s dng trong qu trnh di chuyn .


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Cch d dng nht l xem xt mt phn t DNA vng c kch thc b , nh

l cc plasmid (Fig. 15.12), chng c mang mt transposon lin hp , chng

s dng cc outside ends di chuyn phn transposon v s dng inside

ends c th di chuyn c phn cn li ca cc phn t DNA .Cng

cn ch l cc inside ends v cc outside ends hon ton khc nhau

.V kt qu l trong cc trng hp c minh ho , c s chn mt on

chn ca on plasmid vo cc nhim sc th vt ch .Cng cn ch rng


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s di chuyn s khng th t c nu nh cc gen m ho cho cc

transsposase cn li pha sau trong sut qu trnh thao tc .

V nu nh c hai u inside ends u c s dng trong qu trnh nhy

ca cc gen nhy vo cng mt phn t DNA ca cc vt ch th cc phn tDNA cng s l on chn vo bn thn chnh DNA ch ca n (Fig.

15.13).Cn ph thuc vo cp no tn cng c kt ni li sau khi m

chng b ph v v DNA ch sau s c th sy ra hin tng mt on

hay l o on .Cc vng bn trong ca cc transposon lin hp s b mt

i trong sut qu trnh , v v vy i khi qu trnh cng c bit

n nh l qu trnh abortive transposition.


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Transposon c phn b tha tht qua cc phn t DNA ca mi dng

sng .Mc d chng ta c nhng v d t cc vi khun minh ho lm th

no m chng c th hot ng lm vic Barbara McClintock sau khi xem

xt v nghin cu cc gen nhy u tin trong sut qu trnh trao i cho cy ng .B tin hnh nghin cu ca mnh vo nhng nm 1940 trc

cart khi pht hin ra cu trc xon kp ca cc phn t DNA c cng b ,

tuy nhin b ko nhn ra rng cc on trong nguyn liu di truyn ca

thc vt c th di chuyn t v tr ny n v tr khc .Vo nm 1951 khi b


28/52


29/52

Ac trong bt c phn t DAN no ca n v sau cc enzyme transposase

c m bi Ac cng c th lm di chuyn bt c yu t Ds no .V vy ,

c th duy tr cc mobile hay l kh nng di chuyn cc yu t Ds ny cn

phi gi c cc on lp li ngc chiu , nu ko th transposase s khngth nhn ra chng (Fig. 15.14). cho s di chuyn c th sy ra th khng

nht thit c hai yu t Ac and Ds phi nm trn cng mt NST .

Nu nh yu t Ds c chn vo gen quy nh tnh trng sc t ca ht

ng, gen s b gin on v lm cho ht c mu trng .Nhng nu nh ko

c bt c yu t Ac no ca t bo di chuyn vo th mu trng s c

di truyn mt cch bn vng cn nu ch cn n c mt trong t bo v n s

c th lm cho Ds di chuyn v sau t bo c c hai nhn t s c mu

ht ng nh ban u v tt c con chu ca n sau ny s mang mu .Nu

nh m s di chuyn xut hin khi m ht ng ch mi bt u pht trin th

m phn ht ng rt ln s xut hin cc vt m ln cn nu nh s di

chuyn sy ra khi m pht trin ca cc ht ng ny y th m

ny s rt nh (Fig. 15.15).

Cc ng vt v thc vt cng thng cha cc h transposon mang c

ngha tch cc ln tiu cc , chng ko ch ht sc a dng v chiu di

Khng ch tm thy nhng active-hay l cc loi gen nhy c kh nng hot

ng m chng ta cng thy cc transposon b bt hot hon ton do c

nhng t bin sy ra cc u terminal ca chng v lm cho cc

transposase khng th nhn ra chng v v vy chng s ko c kh nng di

ngphn ln cc transposon trong cc sinh vt bc cao l cc gen nhy thuc h


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Tc1/mariner.Thnh vin u tin c tm thy l

Tc1 t Caenorhabditis v gen nhy mariner transposon ca rui gim

Drosophila. Cc thnh vin ca h ny cng c tm thy nm ,d ng

vt v thc vt bao gm c con ngi v c cc ng vt nguyn sinh .Chiu di ca chng t 1300 to 2500 bp cha mt gen n m ho cho

transposase v c bao hai u bi cc on lp ngc chiu( inverted

repeats ). Chng hot ng theo cc c ch ct v dn , s ph hu ca


31/52

chng li thng c hn gn nh vo cc h thng sa cha.(see Ch.

14).

Cc transposons m chng ta tho lun t trc di chuyn trong khi hnh

thnh cc DNA ca chng .Thm vo , chng ta cn c cc yu t di

chuyn qua trung gian l ARN , chng l cc retroviruses (see Ch. 17),c th

a DNA ca chng vo nhim sc th ca vt ch v n l mt yu t

ging nh l transsposon , chng c th c phn loi thnh cc yu t

retro v tt c chng u s dng cc enzyme reverse transcriptase c th

chuyn t RNA ngc tr li thnh DNA .Bt c khi no cc yu t retrochn vo DNA ch trnh t ch ca n s c nhn i .Cc yu t retro

kh ph bin cc sinh vt bc cao cn cc loi DNA-based transposons li

chim u th cc loi vi khun .

Retrotransposons hay c th gi ngn gn l cc retroposons l cc yu t

c kh nng di chuyn ph thuc vo cc enzyme cn cho qu trnh di

chuyn .Chng thng kh ph bin cc sinh vt nhn chun c bit l

ng vt . cc loi Ty1 (Transposon ca nm men 1) l mt loi

retrotransposon ca nm men c chiu di 6,000 bp


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Chng c cha mt gen m ho cho reverse transcriptase v bn cnh cc

gen bng cc on lp li cng chiu di 334 bp c gi l cc long

terminal repeats (LTRs) ca mt retrovirus (Fig. 5.16).

Khi di chuyn , bc u tin l to ra mt chui n DNA (Fig. 15.17).


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Vt ch s cung cp mt RNA polymerase II chng c sao m t cc

yu t Ty-1 bt u t pha bn tri ca cc LTR . Sau , cc reverse

transcriptase s to thnh cc chui xon kp DNA .

Trc tin chng s to thnh cc chui n v sau to thnh cc phn tlai RNA/DNA v sau s hnh thnh chui xon kp DNA .Cui cng

DNA c chn vo mt v tr mi vi DNA ch v chng s c th to

thnh c 30 n 40 bn copy ca Ty1 .

Human Genetic Defects due to Retroposon Insertion

V cc on DNA khng m ho c s lng ln hn rt nhiu so vi ccphn t DNA c kh nng m ho v vy s ko c g l ngc nhin khi m

phn ln cc on chn ca cc yu t c kh nng di chuyn vo h gen ca

cc vt bc cao thng sy ra vo cc vng khng m ho . Tuy nhin cng

c rt nhiu v d ca cc on chn ca mt cc retrotransposon lm bt

hot gen v gy ra cc sai hng di truyn .Trng hp u tin c nhn

dng ngi l dng lon dng xng hay cn gi l (FCMD), cnbnh ny kh ph bin Nht mc d li rt him cc vng khc . FCM l

mt dng t bin soma ln kh ph bin Nht chng xy ra vi tn s kh

cao xp x 0.71.2 trong s 10,000 tr s sinh v tn s mang gen gy bnh

c tnh khong 1/80 . Nguyn nhn ca cn bnh ny l do s chn mt

retrotransposon di xp x 3000 base vo u 3 khng dch m ca gen

FCMD, v theo l thuyt chng c th dn n s bin i cu trc bc hai

ca cc phn t FCMD mRNA, chng c th lm cho cc phn t RNA ny

khng bn vng , v trong rt nhiu trng hp rt t hoc l ko c nh

hng no n cc m ARN c nhn thy.Cc gen FCMD m cho mt

chui peptide 461 aa m thng thng chng thng biu hin ower no ,


34/52

cc t bo c v tim , chng cng c lin quan n chc nng ca cc c

. Sau s phn tch chc nng ca gen ch ra rng cc

retrotransposon chn vo cc gene FCMD xut hin t cc t tin sng

cch y 2000 n 2500 nm , l vo khong thi gian m nhng ngiYayoi di c n Nht bn v hn quc , tuy nhin n nay iu ny vn cha

c chng minh rng nhng ngi dn di c ny mang cc gen n

cc qun th ngi dn Nht Bn

Ngi ta cho rng nhng ngi di c t i lc chu n Nht bn , v

theo hai chiu hng gii thch .u tin l do nhng ngi n t nn vn

ho Jomon qua hng 10000 nm trc . Hai l chng do nhng ngi Hn

quc sng cch y khong 2300 nm mang n . v sau nhng nm 300

trc cng nguyn , nhng ngi Yayoi truyn cho phn ln nhng

ngi Nht Bn . Cc retroposon ch c mt gen hoc hai gen v chng

thiu kh nng xm nhim nh l cc virus c th ly nhim nhanh chong

sang cc t bo khc .Tuy nhin cc yu t retro-elements ny chnh l cc

yu t trung gian gia cc retroposons v retroviruses tn ti.Mt vi yu

t transposon mang ng gi cc phn t ARN ca chng thnh cc phn

ging nh l cc virus ly nhim .Tuy nhin chng ko c kh nng gii

phng ra khi cc t bo v v vy chng s khng th ly nhim vo cc t

bo khc .Nhng yu t ny i khi cn gi l cc retrovirus ni bo.S ly

nhim ca cc loi transposon c gi l cc retroviruslike kh ph bin

cc t bo ng vt v chng thng tn ti nhiu bn copy .

Rt nhiu cc phn t DNA trong genome ca c thc vt v cc t bo ng

vt u bao gm cc trnh t lp c tho lun chng 4 . Rt nhiu

phn t trong s xut hin l do s di chuyn ca cc retrovirus . c


35/52

ng vt c v c 2 lp chnh c lp li DNA rt cao , l cc yu t

c gi l SINEs, v cc on lp trnh t di hay cn gi l cc long

interspersed elements, or LINEs. Mc d l chng c th phn chia v nh

ngha da vo di ca chng , cc on SINEs th bt ngun t DNA ch- xem di y trong khi cc LINEs bt ngun t cc retrotransposons.

Yu t LINE-1 (or L1) cc ng vt c v l cc retroposon c kh nng

di chuyn nh l cc yu t Ty1 element ca nm men .Tuy nhin, n thiu

cc trnh t LTR u 3 ca n c cc base AT bt ngun t cc ui poly

A in hnh t cc phn t mARN ca cc sinh vt nhn s (Fig. 15.18).Mt

phn t LINE-1 hon thin di 6500 bp v cha 1 gen m cho cc reverse

transcriptase. Tuy nhin , ch c khong 3000 nu ca cc trnh t LINE-1

sequences y chiu di nhng cng c cc trng hp c cc t bin

im .Rt t cc LINE-1 to thnh mt bn copy mi v c th chn vo mt

ni no trong phn t DNA no .S di chuyn c th gy ra bi mt

loi hemophilia ,v mt iu kin di truyn l do cc nhn t nh hng n

qu trnh ng mu . Trong mt s trng hp rt him l do cc y t

ny tham gia vo qu trnh ng mu do VIII trn nhim sc th X .Cc yu

t LINE-1 cn nguyn vn m nhy vo nhim sc th s 22 .N vn hot

ho bn copy ca LINE-1 c tm thy trn cng mt nhim sc th ca

gorilla,iu cng ng l n tn ti cng mt ni hng triu nm

trong qu trnh tin ho


36/52

Mt gen gi l mt bn sao , nhng ko h c chc nng , m n ch l bn

copy ca mt gen (see Ch. 4). Pseudogenes-gi gen c th c to thnh

do qu trnh nhn i DNA v v vy c th nm rt gn vi cc gen khi

u .Cc gi gen c hnh thnh theo cch s c cha c intron v exon

ca gen ban u . i khi cc reverse transcriptase ca mt retro-elemement

c th lm vic bng cc bin c sy ra trn phn t mARN hnh thnh t

gen ca vt ch .V cc on intron ca mARN b ct b trong qu trnh

splicing,reverse transcriptase hnh thnh mt DNA sao chp hay l bn copy

ca n m li thiu cc intron v d nh l cc cDNA (complementaryDNA). c bit , cc cDNA copy c th xm nhp li vo genome vt

ch (Fig. 15.19). iu cng lm cho mt gen gi thiu cc intron v v

vy chng cn c gi l cc pseudogene hoc l retro-pseudogene.

Y t SINEs l mt lp ht sc c bit ca cc gi gen l do chng bt

ngun t trnh t ca cc DNA ca vt ch .Cc yu t thng gp nht l

SINE chng l cc yu t Alu , bt ngun t 7SL RNA gene (Fig. 15.20).cc

RNA ko m ho l mt phn ca qu trnh xut cc protein mng , n

cng thng c sao m t mt promoter pha bn trong nh enzyme RNA

polymerase III (see Ch. 6). Cc gen gi pseudogenes chun thiu mt

promoter .Kt qu l , chng khng th c dch m v ko c thm bn

copy mi no c to thnh , ngc li cc yu t alu li mng cc internal

promoter. Min l promoter cn li nguyn vn , mi yu t alu c th

c gi li lm ngun cho cc qu trnh sao chp , cc qu trnh sao chp

ngc v cc qu trnh ti xm nhp.Kt qu l s lng ca cc Alu tng

ln nh nm cho n khi chng to thnh khong 1 triu bn copy trong

genome ngi .


37/52

Retrons c tm thy trong cc vi khun ngn hn v ko c quan h h

hng vi cc transposon cc sinh vt bc cao .Chng ch c mt gen c

phin m thnh mt ARN c cha cc vng ko dch m theo bi cc vng

m ho cho cc enzyme reverse transcriptase (Fig. 15.21).


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Cc enzyme reverse transcriptase c s dng cc vng khng m ho

non-coding ca cc phn t RNA nh l c khun v mi c th to ra rt

nhiu bn copy ca cc phn t va l RNA va l DNA (Fig. 15.22).in

hnh l cc vng RNA ca cc sn phm c chiu di t 100 base v ccDNA .Cc DNA c lin kt vi RNA bng cc lin kt v tr 3 OH ca

mt nhn t c bit l G residue. Trong mt s cc retrons , cc chui

n DNA c tch ra thnh cc ARN.

V trong cc trng hp khc cc retron khng c xem nh l cc phn t

c kh nng xm nhp vo cc NST . Tuy nhin , cc NST ca mt vi

chng myxobacteria c cc trnh t lp li c lin quan n cc DNA

retron .Ngi ta cho rng cc retrons c kh nng di chuyn t ni ny n

ni khc nhng vn cha bit cch thc m chng di chuyn nh th no ,

c l l cc bn copy ca n c xem xt t lu .Cc retron c tm

thy trong mt vi vi khun v ch mt bn sao n cu cc retron l lun

lun c tm thy cc nhim sc th ca vi khun . Retrons thng chn

vo DNA ca cc virus ca vi khun v sau chng s c th chn vo

NST ca vi khun .


39/52

Chng ta tho lun mt s lng rt ln cc yu t c kh nng di chuyn

v cc c ch a dng ca chng , chng c th c hoc l khng c pha

RNA phase. Cc transposons c cc c ch kh chi tit cho cc hot

ng ca n trn tuy nhin c mt s yu t di chuyn c kch thc ln

c th s dng mt lc nhiu c ch khc nhau rt a dng trong cc iu

kin khc nhau m chng ta cp n trn .Trong s rt nhiu trng

hp c cc on chn nht nh .V d nh Tn7 thng ch chn vo mt

trnh t n c bit trong nhim sc th ca E.coli . Tuy nhin , cc t

bin tn ti lm bt hot cc protein nht nh . Nhng on chn t nhiu

l ngu nhin hn l cc transposon in hnh .Cc yu t di truyn khc

cha cc c im ca c cc yu t di truyn vn ng v mt vi dng

sng ca n , nh l mt plasmid hoc l virus .V d nh cc transposon

lin hp c c enzyme transpose v cc promoter lin hp ging nh l cc

plasmid F . Mt khc , cc th thc khun Mu va l mt virus v l

transposon . Plasmids and viruses s c tho lun chng 16 v 17 .Tuy nhin , cc kha cnh ca mt transposon ca cc yu t lai s c tho

lun di y .

Cc sinh vt lai tn ti l do chng cha cc c tnh ca c hai yu t di

truyn vn ng v mt vi yu t di truyn khc .V d nh l cc th thc

khun Mu va l gen nhy va l virus , cn ch l chng ra ang ni v

mt virus mang mt transposon chn vo trong DNA - mt hin tng

thng xuyn xut hin nhng mt gen hot ng nh l mt virus v mt


40/52

yy t transposon cng mt lc .Khi m DNA Mu vo E.coli


41/52

- vi khun ch ca n , n s di nhp mt cch ht sc ngu nhin vo

nhim sc th ca vt ch bng cc phng thc di chuyn (Fig. 15.23).Ni

cch khc , ton b genome ca Mu l mt transposon. Nu nh l cc Mu

chn vo gia cc gen ca vt ch , chng s lm cho cc gen ny b bt hot.Trc y cc nh nghin cu ch thy rng s ly nhim vi cc virus

gy nn t bin v v vy chng c tn l cc Mu c ngha l cc mutor

phage .Cng ging nh l rt nhiu cc virus ca vi khun , Mu nhn i ,

chng c th chim u th khi m phage tn ti giai on prophage hoc l


42/52

bc vo qu trnh tiu th (xem chng 17) . Khi m chng nhn i s di

chuyn bng sao chp ca chng khng c iu khin , khng theo cch

sao chp nh l cc virus . Kt qu l trong cc bn copy ca Mu c chn

vo cc DNA vt ch v sau ph hu rt nhiu cc gen cu vt ch vdng chc chn l dng gy cht . Khng ging nh l cc virus khc ,

DNA ca Mu khng bao h c tm thy chng sao chp nh l mt phn t

dna c lp vi DNA ca nhim sc th.Cc on DNA c ng gi vo

cc virus c cha ton b genome Mu v c mang mt phn genome ca vt

ch hai u ca DNA vt ch .V vy,thm ch l bn trong ca cc virus

, cc DNA ca Mu vn chn vo DNA ch.Khi m cc virus DNA ly nhim

vo mt t bo mi , cc genome ca Mu c mang mt on nh ca DNA

ch cng vi n .V vy , Mu l mt transposon thc th v DNA ca n

khng bao gi c th di chuyn t do .


43/52


44/52

Cn ch l rt nhiu virus c kh nng di nhp vo cc NST ca vt ch ,

n bao gm cc virus nh l lamda v cc virus ng vt nh l cc

retrovirus (xem chng 17). Tuy nhin,nhng virus khng sao chp trong

qu trnh di chuyn ca chng v khng c bao quanh bi cc DNA chkhi chng ng gi vo cc mnh virus ca chng .V vy c lamda v cc

transposon lin hp u c tm thy trong cc vi khun v cc yu t lai

m c th di chuyn t t bo ny sang cc t bo khc ging nh l cc

plasmid c kh nng di chuyn . Trc tin l trong qu trnh phc hi ca

Tn9 c lin quan n s khng tetracycline v chng c tm thy trongcc vi khunEnterococcus faecalis. Tn916 cng mang mt vi gen cn thit

cho qu trnh di chuyn v v vy chng s ln hn rt nhiu cc transposon

khc . Tn916 di chuyn bi c ch ct v paste . Tuy nhin, n kh vi cc

transposon lin hp , trc tin l cc trnh t ch khng c sao chp khi

tn916 c chn vo.V vy , n cng c th t ct b, li DNA Ca t

bo ch .Sau n chuyn bn thn n vo cc th nhn , v mt mt , n dichuyn vo DNA ca vt ch mi . (Fig. 15.24).Tn916 v cc yu t lin

quan c th vo theo rt nhiu nhm khc nhau , c hai loi vi khun gram

m v gram dng .Bi v cc vt ch c s tn ti ca transposon lin hp

c lin quan n cc khng sinh ph rng rt ph bin trong s cc loi vi

khun .

C rt nhiu vi khun c kh nng khng li nhiu loi thuc khc nhau t

khi cc loi khng sinh c s dng ngy cng ph bin hn.cc gen khng

khng sinh thng mang cc plasmid,c rt nhiu trong s c kh nng di

chuyn gia cc loi vi khun. Trong rt nhiu trng hp cc gen khng


45/52

khng sinh thc t mang cc gen trong cc transsposon m c kh nng chn

vo cc plasmid . cc gen khng khng sinh thng xut hin trc tin

cc h transposon Tn21 trong cc vi khun gram m.Nhng nhm transpson

cha mt trnh t bn trong c bit n nh l mt integron chng hotng nh l cc gen khc trong h thng . Mt integron bao gm mt vng

nhn dng , l vng attI site, chng rt a dng cc gen cassettes ,

chng c th di nhp, v c thm mt gen m ho cho mt enzyme chu

trch nhim cho qu trnh chn vo ca chng ,cc enzyme integrase.Cc

vng attIsite c p cnh hai bn bi mt trnh t di 7 bp , chng hot

ng nh l cc vng nhn dng cho cc enzyme integrase (Fig. 15.25). Hai

promoters nm cc vng upstream khc nhau v mt trong s chng l cc

gen m cho cc integrase , cc vng khc cng ht sc a dng v chng bt

ngun t t qu trnh sao m ca bt c gen no c di nhp vo .Cc gene

cassettes rt ph hp i vi s di nhp bao gm mt gen cu trc thiu cc

promoter cng thm vi mt trnh t nhn bit cho enzyme integrase , l

attCsite.attCsites cng kh a dng , ngoi tr cc trnh t lp li di 7bp

nm hai u. attCsites cng bt ngun t mt on di 59 bp bi v v d

u tin khi m ngi ta pht hin ra l mt on di 59 bp . Tuy nhin, cc

v d sau ny c tm thy cc chiu di sau ht sc a dng v cc

trnh t bn trong ca chng . Cc Gene cassettes c th tn ti tm thi

khi m cc phn t t do trong qu trnh sao chp v s biu hin ca gen

trong cc qu trnh khc c th di chuyn n vng attI site ca

integron.Ngun sau cng ca cc gen trong h thng cassettes l ht sc khhiu .L do ti sao m phn ln cc gen gi l integron cassettes mang cc

gen khng khng sinh c l l bi v s xem xt cc cht khng khng sinh

l ht sc quan trng v v vy cn phi ch mt cch thng xuyn hn .


46/52

Transposons trong h Tn21 family c tuyn truyn rng ri i vi cc

loi khng sinh thng mi.Cc gen nhy Tn2501 transposon khng mang

cc gen khng khng sinh v chng xut hin nh l mt empty integron.

N cng c th l mt integron c th thu nhn mt cu trc a gen,v


47/52


48/52

bao gm cc yu t c kh nng di chuyn c tho lun trn , c cc

gen nhy hot ng da trn DNA v cc yu t retro .

Cc phn t DNA dng ny l cc selfish DNA bi v chng dng nh

nh l di chuyn theo s thch ring ca chng m ko ph thuc vo vaatjch ca n .V v vy nu nh vt ch ca n b ph hu th n s cht cng

vi vt ch ca n . V vy , tc lan truyn ca cc selfish DNA b gii

hn bi nhu cu cn phi trnh s ph hu vt ch hoc l bt hot qu nhiu

gen ca t bo vt ch . T nhng xem xt nh vy , cc selfish DNA trong

vt ch ca n ch ging nh l mt virus sao chp ging nh l cc virus

khc trong t bo hoc l mt vi khun ly nhim trong c th bnh nhn

.Trong hu ht cc sinh vt bc cao , mt phn quan trng l cc DNA nhiu

cc bn copy nh l cc selfish DNA. Ti sao cc t bo ch khng loi b

cc vt k sinh ra khi cc nhim sc th ca n ? cc t vi sinh trng

nhanh ging nh l cc vi khun hoc l cc loi nm , chng rt t khi c

cc selfish DNA. C l vi mt DNA quan cn thit ca cc DNA ngoi lai ,

chng s phn chia chm hn v chng c loi b bng s cnh tranh .

Ngc li trong cc t bo sinh trng v phn chia chm chng li c mi

lin h mt thit vi s sao chp ca vt ch v cui cng chng s tng

nhanh s lng ca chng.a s cc selfish DNA thng l cc phn cn li

ca virus hoc l cc gen nhy sau khi m chng c chn vo cc nhim

sc th ca vi khun .Qua mt thi gian di cc bn copy s c phn

ct bi c hai loi t bin n base v mt on.Cui cng l phn ln cc

bn sao tr thnh cc phn t khim khuyt v lm mt kh nng hnh thnhcc virus hoc l c kh nng di chuyn t v tr ny n v tr khc chng b

phn hu thnh cc junk DNA (Fig. 15.26).Genome ca cc t bo

eukaryote ni chung thng cha mt s lng ln cc junk DNA,thc s


49/52

chng l loi chim a s cc ng vt c v-loi c cc phn t DNA bao

gm cc phn t khng m ho ca mt hoc l mt s dng.

Cc phn t DNA c kh nng di chuyn khng ch bao gm mt yu t

transposon , cc intron l mt trong nhng dng him v cc dng h hng

ca DNA di ng .Cng nh l tn ca chng c ng , cc intron

cng l cc trnh t DNA c chn vo trong cc gen nm gia hai

exxon.Mi mt intron c xc nh v tr duy nht trong phm vi mt gan

c th ca cc t bo .ch ca n c th tn ti trong hai bn sao , c hocl khng c cc on chn ca intron .

s di chuyn ca cc intron cng rt gii hn khi m chng c th chim c

mt vng xc nh v s di chuyn ch c th xy ra khi m cc t bo c

cha 2 bn sao cu trnh t gen ch trong ch c mt trong hai bn sao

c cc homing intron. Cc homing intron ny sau s chn vo trnh t

ch ca gen no thiu mt bn sao cho cc intron . Trong cc sinh vt nhn

chun ,tnh trng c th xut hin sau khi bt cp khi m nhim sc th t

cc t bo cha m khc nhau s kt hp li to thnh hp t . Trong cc vi

khun , n c th xut hin khi DNA vo cc t bo nhn theo bt c c ch

no xem chng 18 . cc intron ny m ho cho cc endonuclease chu


50/52

trch nhim cho s di chuyn ca chng. Cc endonuclease sau s tch ra

thnh mt trnh t trong phm vi gia gen ch v cc on ngn b phn

hu cc phn nh ra tn cng . Chui xon kp ph hu mt bn sao

ca gen(xem chng 14) trong cc bn sao nguyn vn ca gen sau sc sao chp v s dng thay th cc on b ph hu (Fig. 15.27).

V vy , cc homing intron khng ch n thun m ho cho mt enzyme c

kh nng ct cc phn t DNA v sau li cc t bo vt ch v sau

sa cha cc sai hng , s chn ca cc homing intron lm gin on

cc trnh t nhn bit v v vy cc endonuclease ch c th ct cc trnh t

gen ch nu nh cc intron ca n vng mt.Cc trnh t nhn bit ca

homing introns cng c th di t 1820 bp v l trnh t di nht v c

hiu nht c bit n trong s cc loi nuclease

iu chc chn rng cc on chn intron s ch c chn vo cc trnh

t n duy nht trong genome ca mi t bo

Homing introns nhm 1 s dng cc endonucleases n gin nh c

cp n trn , chng cng c tm thy trong cc vi khun hoc l cc

sinh vt nhn chun , chng c tm thy trong cc t bo vi khun v cc

t bo ca sinh vt nhn chun bc thp.

Nhm II homing intron l cc yu t s retro-elements , chng thng s

dng cc RNA trung gian . Cc phn t protein m chng m ho c c hai

loi enzyme l endonuclease and reverse transcriptase c kh nng hot

ng . Trc cc endonucle ph v cu trc ca mt chui xon kp

ngay gia vng nhn bit . Tip sau l cc reverse transcriptase phn humt DNA l bn sao ca cc intron m s chn vo ca chng c thay th

, theo khun mu c sn chng s dng mt cch sao chp s cp t bn sao

ca gen ch c cha cc intron (Fig. 15.28).Cc u 3 t do b phn hu

bi cc enzyme endonuclease c s dng ging nh l mt primer cho


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qu trnh khi u tng hp DNA . Kt qu l , cc bn sao DNA c to

thnh va mi gn vo cc chui xon kp va b ph hu .


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chapter 15_DNA mobile_Dịch từ sách sinh học phân tử Molecular biology