chapter 15_dna mobile_dịch từ sách sinh học phân tử molecular biology
TRANSCRIPT
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
1/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
2/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
3/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
4/52
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 :
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
5/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
6/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
7/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
8/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
9/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
10/52
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)
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
11/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
12/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
13/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
14/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
15/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
16/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
17/52
DNA t do , mi on mang mt bn copy n l ca
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
18/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
19/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
20/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
21/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
22/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
23/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
24/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
25/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
26/52
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.
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
27/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
28/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
30/52
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
32/52
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).
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
33/52
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 ,
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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).
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
38/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
40/52
yy t transposon cng mt lc .Khi m DNA Mu vo E.coli
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
43/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
47/52
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
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
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
51/52
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 .
-
8/14/2019 chapter 15_DNA mobile_Dch t sch sinh hc phn t Molecular biology
52/52