chương 12_các yếu tố di truyền vận động

8/14/2019 Chng 12_Cc yu t di truyn vn ng

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Chng 12.t bin gene, ti t hp v cc yu t di truyn di ng

Mc tiu ca chngGii thiu cc kiu t bin gen, c ch pht sinh v hu qu ca chng. C ch sa

cha cc t bin lm gim t l t bin, duy tr tnh n nh ca vt liu di truyn.Ngoi t bin gene, yu t di truyn vn ng cng l nhn t lm tng tn s t bin

S tit: 3

Ni dung

I. t bin gene

t bin gene l nhng bin i xy ra bn trong cu trc gene. Mi t bin genedn n s thay i trnh t nucleotide to ra cc allele khc nhau. t bin gene c thxy ra do bin i ca trnh t nucleotide trong gene. t bin gene khng pht hinc khi quan st t bo hc.

Trong t nhin, tt c cc gene u c t bin c gi l t bin t nhin hayngu pht (spontaneous mutation). Cc t bin t nhin thng xut hin rt t.

1. Cc kiu t bin gene

t bin gene hay t bin im: l cc bin i rt nh trn mt on DNA, thnglin quan n mt cp base n ca DNA hoc mt s t cp base k nhau. t bin imlm thay i gene kiu di (wild-type gene),. Thc t t bin im hu nh lm gimhoc lm mt chc nng ca gene hn l lm tng cng chc nng ca gene.

V ngun gc, t bin im c phn ra lm t bin ngu nhin (spontaneous) vt bin cm ng (induced).

t bin cm ng: l dng t bin xut hin vi tn s t bin tng ln khi x lc mc ch bng tc nhn t bin hoc tc nhn mi trng c bit. t binngu nhin l t bin xut hin khi khng c s x l ca tc nhn t bin. t bin

ngu nhin c tnh l t l c s ca t bin v c dng c chng ngun bin ddi truyn t nhin trong qun th. Tn s t bin ngu nhin thp nm trong khong 10 -5

- 10-8, v vy t bin cm ng l ngun t bin quan trng cho phn tch di truyn.

Tc nhn t bin c s dng ph bin l ngun chiu x nng lng cao (high-energy radiation) hoc cc ha cht c bit.

Cc dng t bin im: c hai dng t bin im chnh trong phn t DNA:

+ t bin thay th cp base (base substitution)


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+ t bin thm bt cp base (base insertion - base delection)

Cc t bin ny c th pht sinh do nh hng ca mi trng nh nh hng cacc tc nhn gy t bin.

1.1. t bin thay th cp baseKiu t bin n gin nht l thay th mt base, trong mt cp nucleotide trong

gene c thay th bng mt cp nucleotide khc.

V d: A c thay th bng G trong si DNA. S thay th ny to ra s cp base G-T. ln sao chp tip theo to ra cp G-C trong mt phn t DNA con v cp A-T phn t DNA con kia.

Tng t, t bin thay th A bng T trn mt si, to ra s kt cp tm thi T-T.Kt qu sao chp to ra T-A trn mt phn t DNA con v A-T trn phn t DNA conkia. Trong trng hp hp ny, cp base T-A l t bin v cp A-T khng t bin. Nu

si gc DNA khng t bin c trnh t 5'-GAC-3', trn si t bin c trnh t 5'-GTC-3' v si kia khng t bin c trnh t 5'-GAC-3'.

t bin thay th cp base c chia lm hai loi:

+ t bin ng hon (transition mutations): Nu mt t bin m baz pyrimidinec thay th bng mt pyrimidine v mt purine thay bng mt purine.

t bin ng hon c th l:

T C hoc C T

(Pyrimidine pyrimidine)

A

G hoc G

A(purine purine)

t bin o hon (Transversion): t bin lm thay mt pyrimidine thnh mtpurine hay mt purine c thay th bng mt pyrimidine. Cc t bin o hon:

T A, T G, C A hoc C G

(Pyrimidine purine)

A T, A C. G T hoc G C

(Purine pyrimidine)

Nh vy c th c 4 thay th kiu t bin ng hon v c n 8 thay th kiu tbin o hon. Nu cc thay th ny xy ra vi ngu nhin xc sut nh nhau, s c t lt bin: 1 ng hon : 2 o hon. Tuy nhin trong thc t, t bin thay th base c xuhng nghing v t bin ng hon, cho nn trong s cc t bin thay th base t phtth t l xy ra t bin l: 2 ng hon : 1 o hon


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1.2. t bin thm hoc bt base (base-pair addition/deletion), cn gi l indel mutation(insertion-deletion). Trng hp n gin nht ca t bin ny l thm hoc mt mtcp base n. i khi t bin lm thm hoc mt ng thi nhiu cp base.

Hu qu ca t bin im n cu trc v s biu hin ca gene

t bin im xut hin trong vng m ha chui polypeptide ca gene (apolypetide-coding part of a gene), chng hn t bin thay th base n c th gy nhiuhu qu, nhng tt c u c tc ng ln m di truyn theo 2 hng: lm thoi ha m ditruyn hoc xut hin m kt thc qu trnh dch m. C cc dng:

t bin ng ngha (synonymous mutations): t bin thay i mt codon m haacid amine thnh codon mi m ha cho cng acid amin . t bin ng ngha cng cth xem l t bin im lng (silent mutations)

t bin nhm ngha (missense mutations), i khi cn gi l t bin khng ngngha (nonsynonymous mutations): codon m ha cho mt acid amin ny b thay i

thnh codon m ha cho mt acid amin khc.t bin v ngha (nonsense mutations): codon m ha cho mt acid amin b thay

i thnh codon kt thc dch m (translation termination/stop codon).

Bng 12.1 t bin im mc phn t

Kiu t bin Kt qu v v d

mc DNA

Transition

Purine c thay th bng mt purine khc,

pyrimidine c thay th bng mt pyrimidine

khc: A.T G.C, G.C A.T, C.G T.A,

T.A C.G

Transversion Purrin c thay th bng mt pyrimidine hoc

mt pyrimidine c thay th bng mt purine:

A.TC.G, A.TT.A, G.CT.A, G.CC.G

T.AG.C, T.A A.T, C.GA.T, C.GG.C

Insertion-deletion Thm vo hoc mt i mt hoc mt s cp base

ca DNA (thm/mt base c gch di)

AAGACTCCTAAGAGCTCCT

AAGACTCCTAAACTCCT

mc proteinSynonymous

mutation

Codon c bit m ho cho cng mt acid amin:AGG CGG

Arg Arg

Missense mutation

Loi bo th

Codon to thnh m ho cho amino acid khc

M ho cho acid amin c cng bn cht ho hc:

AAA AGA


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Loi khng bo th

Lys Arg

(kim) (kim)

M ho cho amino acid khc v bn cht ho

hc: UUU UCU

Phenylalanin Serinek nc Phn cc

Nonsense mutation Codon kt thc chui: CAG UAG

Gln Stop

Frameshift mutation Thm vo mt cp base:

AAG ACT CCT AAG AGC TCC T...

Mt mt cp base:

AAG ACT CCT AAA CTC CT...

Mc nh hng ca t bin nhm ngha v v ngha ln chui polypeptide khc

nhau ty trng hp.

Nu t bin nhm ngha thay th mt acid amin ny bng mt acid amin khc tngt v mt ha hc, c xem l t bin thay th bo th (conservative substitution). Sthay i ny hu nh t nh hng n cu trc v chc nng protein. Ngc li, nu thayth bng mt acid amin khc v phng din ha hc gi l nonconservative substitution,hu ht u gy ra s thay i ln cu trc v chc nng protein.

t bin v ngha s dn n s kt thc dch m sm. V vy chng gy ra hu qu

tng ng trn chc nng protein. Nu t bin v ngha xy ra cng gn u 3' cakhung c m, kt qu t nh hng n protein. Tuy nhin nhiu t bin v ngha vng ny vn to ra cc sn phm hon ton b mt hot tnh.


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Hnh 12.1 Hu qu ca t bin im trong gene. Codon 1-4nm trong vng m ha ca gene

Ging vi t bin v ngha, t bin thm bt base gy hu qu trn trnh tpolypetide k t im b t bin (Hnh 12.1). Trnh t trn mRNA c c theo tngkhung gm ba base (codon) mt lc. Mt hoc thm base s lm thay i khung ctrong qu trnh dch m t im b t bin cho n kt thc theo khung mi. V vy loit bin ny c gi l t bin dch khung (frameshift mutations). t bin ny to ratrnh t acid amin k t im b t bin cho n kt thc khc vi trnh t acid amingc. t bin dch khung gy ra s mt hon ton cu trc v chc nng ca protein bnhthng.

Trng hp t bin xy ra trnh t iu ha v cc trnh t khng m ha khc(Hnh 12.1). Nhng phn ca gene khng trc tip m ha cho protein m cha nhiuim bm DNA ch yu cho protein xen vo, l nhng trnh t khng nhy cm chos biu hin ca gene hoc cho hot tnh ca gene.

mc DNA, nhng im mt i (docking) gm nhng im m RNApolymerase v nhng nhn t gn kt ca n bm vo, cng nh nhng im m protein


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iu ha phin m c trng gn vo. mc RNA, nhng docking quan trng thmvo gm im bm ca ribosom (ribosome-binding site) trn mRNA vi khun, nhngim ni u 5' v 3' gn cc exon eukaryote v cc im c vai tr cho iu hadch m v nh v mRNA n vng c bit trong t bo. Nhn chung hu qu chc nngca bt k t bin im no vng nh th u ph thuc vo vic lm gin on (hocto ra) mt im bm. t bin lm gin on nhng im c kh nng lm thay iphn biu hin ca gene da vo s thay i s lng sn phm c biu hin mtthi im nht nh hoc mt m nht nh. hay bng s thay i phn ng vi nhngtn hiu (cue) ca mi trng nht nh. Ngc li, t bin mt vi im bm c thhon ton ph hy mt giai on cn cho s biu hin bnh thng ca gene, nh imbm ca mRNA polymerase hoc l nhn t splicing. V vy n lm bt hot sn phmca gene hoc ngn cn s hnh thnh sn phm.

Cn phn bit gia nhng thay i xy ra ca mt t bin gene l s thay itrnh t DNA ca gene vi s thay i mc kiu hnh. Nhiu t bin im trong

trinh t khng m ha lm t thay i hoc khng thay i trn kiu hnh nh t bingia im bm DNA cho protein iu ha hoc thay i nhng im khc trong gene lmthay i chc nng ca chng.

2. C ch gy t bin im

Khi kim tra dy t bin c gy to bi cc tc nhn t bin khc nhau cho thymi tc nhn t bin c c trng bi mt c tnh t bin khc nhau hay"preference" v c mt dng t bin nht nh v mt im t bin nht nh, c gil im d xy ra t bin (mutational hot spots). c tnh t bin nh th c ch ln u tin locus rII ca bacteriophage T4.

Tc nhn t bin hot ng t nht qua ba c ch khc nhau: chng c th lm thayth mt base trong DNA; lm bin i mt base gy kt cp nhm vi mt base khc;lm sai hng mt base, dn n khng th kt cp vi bt k base no trong iu kinbnh thng.

t bin thay th base: mt vi hp cht ha hc tng t nitrogen base bnh thngca DNA, i khi chng c th gn vo DNA thay cho base bnh thng. Nhng chtnh th c gi l cc cht tng ng vi base (base analogs). Cc cht tng ngny kt cp khng nh s kt cp ca cc base bnh thng. V vy chng c th gy rat bin do gn vo mt nucleotide khng ng trong qu trnh sao chp.

hiu hot ng ca cc cht tng ng base, trc ht cn phi xem xtkhuynh hng t nhin ca cc base i vi s hnh thnh cc dng khc nhau. Mi basetrong phn t DNA c th xut hin mt trong s nhiu dng c gi l tautomers,chng l cc ng phn khc nhau v tr v tr nguyn t v nhng lin kt gia ccnguyn t. Dng keto ca mi base thng c trong DNA (Hnh 12.2), trong khi dngimino v enol ca base l him. Tautomer imino hoc enol c th kt cp sai vi base tomt kt cp nhm (mispair). Kh nng kt cp nhm nh th gy ra t bin trong qu


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trnh sao chp c ch u tin bi Watson v Crick khi cc tc gi ny nghin cucng thc v m hnh cu trc DNA.

S kt cp nhm c th sinh ra ngu nhin, nhng cng c th sinh ra khi base b ionha. Tc nhn gy t bin 5-Bromouracil (5-BrU) l cht tng ng vi thymine, c

brome v tr carbon s 5 thay cho nhm -CH3 ca thymine. Hot tnh ca n da trnqu tnh inolization v ionization. dng keto, 5-BrU kt cp vi adenin nh trng hpthymine. Tuy nhin, s c mt ca nguyn t bromine lm thay i mt cch c nghas phn b electron vng base. V vy 5-BrU c th chuyn sang dng enol v dngion, v n c th kt cp vi guanine nh trng hp cytosine to ra cp 5-BrU-G. Trongln nhn i tip theo G kt cp vi C, to cp G-C thay cho cp A-T. Kt qu gy ra tbin ng hon. Tng t 5-BrU cng c th gy ra t bin ng hon A-T thay cho cpG-C.

Hnh 12.2 Chng minh mt vi kt cp nhm c th xy ra do kt qu ca s thay i

1 tautomer thnh 1 tautomer khc

Mt ha cht gy t bin khc l 2-amino-purine (2-AP), l ha cht tng ngadenine, c th kt cp vi thymine. Khi b proton ha, 2-AP c th kt cp nhm vicytosine, c th gy ra th h sau t bin ng hon G-C thay cho A-T do kt cp nhmvi cytosine trong ln sao chp tip theo.

Thay th base (base alteration)


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Hnh 12.3 S kt cp nhm chuyn bit do t bin cm ng alkyl ho

Mt vi tc nhn t bin khng gn vo DNA, m li lm bin i base gy ra skt cp sai. Tc nhn alkyl c s dng ph bin nh l tc nhn t bin, chng hnnh ethylmethanesulfonate (EMS) v nitrosoguanidine (NG) gy t bin theo cch ny.

Nhng tc nhn nh th s thm nhm alkyl (nhm ethyl trong trng hp EMS vnhm methyl trong trng hp NG) nhiu v tr trn c 4 base. Tuy nhin, t bin hunh ch xy ra khi nhm alkyl c thm vo oxy s 6 ca guanine to ra O-6-

alkylguanine. S alkyl ha ny dn n s kt cp nhm vi thymine (Hnh 12.3). Ktqu sinh ra t bin ng hon G-CA-T trong ln sao chp tip theo.

Tc nhn xen vo gia (intercalating agents) l nhm tc nhn quan trong khc gybin i DNA. Nhm ca cc hp cht ny bao gm proflavin, acridin cam v mt nhmcc hp cht ha hc khc. Cc tc nhn ny l nhm cc phn t bt chc cc cp basev c th xen vo gia cc nitrogen base li chui xon kp DNA. v tr xen vo nychng gy s thm vo hoc mt i mt cp nucleotide.

Sai hng base: Mt s ln tc nhn t bin gy sai hng mt hoc nhiu base. Vvy khng th kt cp vi base c trng. Kt qu lm cn tr s sao chp v DNA

polymerase khng th tip tc qu trnh tng hp DNA qua nhng base sai hng. E.coli qu trnh ny xy ra i hi hot tnh ca h thng SOS. H thng ny c kchthch nh l mt phn ng khn cp ngn cn s cht t bo khi DNA b sai hng nng.

* C ch ca t bin ngu nhin

t bin ngu nhin xy ra do nhiu nguyn nhn: gm sai hng trong qu trnh saochp DNA, cc tn thng ngu nhin, s chen vo ca yu t di ng. t bin ngunhin him nn kh xc nh c ch c bn. Tuy nhin, mt vi h thng chn lc cho


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php thu c t bin ngu nhin v phn tch mc phn t. T bn cht canhng thay i trnh t c th suy ra qu trnh dn n t bin ngu nhin.

Nhng sai hng ngu nhin (spontaneous lesions) n DNA c th sinh ra t bin.Hai tn thng ngu nhin thng xut hin nht: depurination v deamination, trong

depurination ph bin hn.Depurination do tc dng ca aflatoxin, lm mt mt base purine. Ngoi ra, qu trnh

mt purine cng xy ra mt cch t nhin. Mt t bo ng vt mt ngu nhin khong10.000 purine ca DNA trong mt th h t bo khong 20 gi 37 oC. Nu tn thngny c gi li, dn n sai hng di truyn ng k v trong qu trnh sao chp, v trmt purine khng th nh r c loi base no. Trong nhng iu kin nht nh mtbase c th chn vo to ra t bin.

Deamination ca cytosine to ra uracil. Uracil s kt cp vi adenin trong qu trnhsao chp, kt qu to ra t bin ng hon G-CA-T. Deamination 5-methylcytosineto ra thymine (Hnh 12.4). Qu trnh sao chp to ra t bin ng hon chuyn C thnhT.

Ngoi 2 qu trnh gy sai hng nh trn, s oxy ha to ra cc base b sai hng ldng tn thng th ba.. Dng oxygen hot ng nh gc superoxid (O2.-), hydrogenperoxide (H2O2) v gc hydroxyl (.OH) c to ra do sn phm ca qu trnh chuynha (aerobic metabolism). Cc dng ny c th gy tn thng oxy ha n DNA, ktqu to ra t bin.


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Hnh 12.4 Deamination ca Cytosine (a) v 5-methylcytosine

Cc sai hng trong sao chp DNA cng l ngun t bin khc.

Thay th base: sai hng trong sao chp DNA c th xy ra khi c mt cp nucleotide

ghp khng chnh xc (nh A-C) to ra trong qu trnh tng hp DNA dn n s thayth mt base.

t bin thm vo v mt base: Mt loi sai hng sao chp khc dn n thm vohoc mt mt hoc mt s cp base. Trong trng hp s base thm vo hoc mt ikhng chia ht cho 3, s to ra t bin dch khung trong vng m ha protein.

II. Sa cha v bo v DNA

1. C ch sa sai sinh hc

T bo sng c hng lot h thng sai hng DNA theo nhiu cch khc nhau. T lt bin t nhin thp do nh tnh hiu qu ca h thng sa sai ny. Sai hng ca hthng sa sai ny dn n t l t bin cao.

1.1. Quang phc hot (photoreactivation) hay sa sai nh nh sng (light repair)

Sau khi x l tia t ngoi gy t bin, nu a ra nh sng th phn ln sai hngc phc hi nh enzyme photolyase. Enzyme ny gn vo photodimer ct n thnh ccmonomer di tc dng ca nh sng mt tri c bc sng 320-370 nm. Sau phchi cc base ban u. (Hnh 12.5).


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Hnh 12.5 S to thnh v s loi b dimer thymine

1.2. Sa sai bng lm mt nhm alkyl (dealkylation)

Enzyme alkyltransferasecos th sa trc tip cc sai hng. Chng ct nhm alkyl t

cht nitrosoguanine v ethylmethnesulfonate v gn vo v tr O-6 guanine.Enzyme methyltransferase caE. coli c kh nng chuyn nhm methyl t cht O-6

methylguanine sang gc cistein trn phn t protein. Tuy nhin h thng sa sai ny cth bo ha nu mc alkyl ha cao.

* Sa sai bng ct b (excision repair pathway)

Phn ln cc c ch sa sai khc thc hin theo li ct b (excistion repair) khngcn nh sng nh cc nuclease, sau thay vo cc base ng. C th xy ra theo nhiucch:

+ Ct cc base (base excision repair) S ct b cc base sai hng nh cc enzyme

DNA glycosylase. Cc enzyme ny nhn bit cc base b bin i v cc im mt purinehay mt pyrimidine v thy gii lin kt N-glycosilic ni base vi ng. Ri enzyme APendonuclease ct lin kt ng v phosphate gn base b bin i. Sau enzyme thba, deoxyribophosphodiesterase loi b tng nucleotide k tip nhau on b hng. Sau, DNA polymerase lp y khong trng vi cc nucleotide b sung vi si khun cnli. Enzyme DNA ligase s gn cc khe h gia 2 u 3'-5' (Hnh 12.6).


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Trong t bo tn ti mt s DNA glycosylase. Chng hn, enzyme uracil-DNAglycosylase ct uracil khi DNA. Uracil to thnh do t bin mt nhm amin ngu nhin cytosine, dn n t bin ng hon thay C bng T. Enzyme ny pht hin ra uraciltrn DNA nh l mt bt thng, chng s ct b v sa sai.

+ Ct cc nucleotide: S ct b vng c nhiu pyrimidine dimer c thc hin nhenzyme exinuclease (enzyme rch mch hay enzyme to khc trn DNA) nh phc hp 3enzyme c m ha bi gene uvr ABC ca E. coli. Phc hp ny ct on 12 nucleotidetrn mt mch: 8 nucleotide t mt u b sai hng v 4 nucleotide ca u cn li.Khong trng ca 12 nucleotide ny s c lp y nh enzyme DNA polymerase I davo mch n b sung kia ca trnh t DNA gc. DNA ligase s gn vo cc khe h.

Hnh 12.6 Sa sai bng ct b nucleotide

+ c sa i vi cc base bt cp sai

C ch c sa i vi cc base bt cp sai (proofreading for base-pair matching)c thc hin trong sao chp DNA. Trong qu trnh sao chp, trc khi thc hin phnng polymer ha ni cc nucleotide, cc nucleotide triphotphate mi phi bt cp b sungvi mch khun. Nu s bt cp sai xy ra, DNA polymerase s loi b nucleotide btcp sai. Ngay c trc khi nucleotide mi rp vo, enzyme d li cp base cui, nu


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chng khng bt cp th s polymer ha tip theo b dng. Cp nucleotide u cui 3'bt cp sai s b loi b nh hot tnh exonuclease3'5' ca DNA polymerase. Khi btcp ng, qu trnh polymer ha mi c tip tc.

Hot tnh c sa i vi cc base bt cp sai l c tnh ca nhiu DNA polymerase

m bo cho s ko di chnh xc ca mch ng c tng hp.+ Sa sai da vo tnh tng ng (Homology-dependent repair system)


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Hnh 12.7 M hnh b gy si i nh trao i cho

Mt h thng sa sai quan trng pht hin tnh cht b sung i song song ca 2mch n DNA phc hi on sai hng tr li trng thi bnh thng ban u. Trong

h thng ny, on DNA sai hng b ct b v thay bng mt on nucleotide mi ctng hp b sung vi si khun i din. S sa sai xy ra qua si khun v nguyn tcca sao chp DNA bo m s sa sai hon thnh vi chnh xc cao - l s giiphng sai hng (error-free). C 2 h thng ch yu loi b sai hng: H thng sacha sai hng pht hin ra trc khi sao chp v h thng sa cha sai hng pht hintrong qu trnh din bin sao chp (sa sai sau sao chp).

+ Sa sai t mch kp (repair of double-strand break)

Khi c 2 si ca chi xon kp b t cng mt v tr, c gi l t bin tmch i, c th gy ra sai hnh nhim sc th, lm cht t bo hoc to ra trng thi tin

ung th. T bo s dng nhiu protein v con ng sa sai t gy mch i l thchin ti t hp trong gim phn. Qu trnh sa cha do trao i cho trong gim phn xyra nh sau (Hnh 12.7):

Trn mt nhim sc th xy ra s t mch i v kt qu n mn cc u mt on ngn ca DNA si n. u 3' ca mt trong nhng si ny "xm ln" vo mtchromatid.

on xm ln lm mi cho tng hp cc base b mt ca n nh s dng si isong song ca chromatid nh l si khun. S tng hp mi ny s to ra mt vng sin lai vi mt si n khng xm ln. V vy to ra mt vng d hp t nh "Aa" v sdng nh mch khun khi phc cc base b mt trn si . DNA polymerase s lpy ch trng v enzyme ligase s ni cc u mt xy ra trong cu trc c bit gingvi trao i cho 2 si n. Cu trc ny cng cha cc on bt cp khng tng ngn gin.

Trao i cho si n c gi l cu trc Holliday (Holliday structure) do Hollidaypht hin vo nhng nm 1960.

+ H thng SOS

t bo vi khun hoc t bo eukaryote b sai hng nng do chiu tia uv, tia X hoc

do tc dng ca cc ha cht gy t bin, h thng sa sai khn cp c khi ng. E. coli, h thng ny c lin quan vi hai protein c m ha bi gene lexA v recA.Protein lexA l mt cht c ch, n gn vo hp SOS, chng lp cc promotor ca ccgene SOS, ngn cn s phin m nhm cc gene ca h thng SOS. Mt vi sn phmca DNA b tn thng s lm hot ha enzyme protease recA. Protein recA b hot has ct b protein lexA, cho php cc gene ca h thng SOS phin m. Phn ng ca hthng SOS xy ra trong thi gian ngn nhng phc tp. N bao gm cc qu trnh lm


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tng hot tnh ti t hp, thay i trong khi s sao chp, c ch nuclease v kch thchphc hi sao chp v chuyn sai hng thnh sa sai p sp (error-prone replication). Tbo by gi s xy ra s sao chp DNA nhanh hn bnh thng. Nu sa sai khng kp,t bo phi chp nhn hoc b t bin hoc b cht.

III. Cc yu t di truyn vn ng (Transposable genetic elements)

1. Cc yu t di truyn vn ng prokaryote

Trnh t on xen (insertion sequence) ca vi khun

Trnh t xen on l mt on DNA ca vi khun di chuyn t mt v tr trn nhimsc th n v tr mi trn cng nhim sc th hoc trn nhim sc th khc. Khi xen vogia gene, yu t IS lm gin on trnh t m ha v lm bt hot s biu hin ca gene.Mt s trng hp, c tn hiu kt thc phin m v dch m, yu t IS lm cn tr sbiu hin sau promotor trong cng operon..

Yu t IS c tm thy u tin operongalcaE. coli, chia lm bn nhm: IS1,IS2, IS3 v IS4. Chng c th phn b ri rc trn nhim sc th chnh ca vi khun vtrn cc plasmid. V d yu t IS1 c khong 5-8 bn sao trn nhim sc th vi chiudi 768 bp.

Tt c cc yu t IS u cha on DNA m ha cho protein, c gi ltransposase, l enzyme cn thit cho s di chuyn ca yu t IS t mt v tr trn nhimsc th n v tr khc. on gene ny nm gia 2 on lp li o ngc (inverted repeat- IR) ngn. V d yu t IS1 c khong 5-8 bn sao trn nhim sc th vi chiu di 768bp, on lp li o ngc c kch thc 18-23 bp, yu t IS2 c 5 bn sao v cc yu t

IS khc. Yu t IS l nhng vng ca cc trnh t xc nh, chng l nhng v tr xy ratrao i cho. V d: S ti t hp ca plasmid v nhim sc th ca E. coli to ra nhngchng Hfr xy ra qua trao i cho n gia yu t IS trn plasmid v yu t IS trnnhim sc th.

Bng 12.2. Mt vi trnh t xen vo v kch thc ca chng

Trnh t

xen vo

S bn sao trong E. coli Chiu

di (bp)

on lp li o

ngc (bp)

IS1 5-8 bn sao trn nhim sc th 768 18-23

IS2 5 bn sao trn nhim sc th,

1 bn sao trn plasmid

1327 32-41

IS3 5 bn sao trn nhim sc th,

2 bn sao trn plasmid

1400 32-38

IS4 1-2 bn sao trn nhim sc th, 1400 16-18


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IS5 Cha bit 1250 Ngn

1.1. Gene nhy ca prokaryote

Cc yu t IS ring l khng ch c kh nng t di chuyn m khi hai yu t ny nm gn nhau th chng c th vn ng nh mt n v hon chnh v mang theo cc genenm gia chng. Cu trc phc tp ny c gi l transposon. C hai kiu transposon vi khun:

Hnh 12.8 c trng v cu trc ca transposon hn hp (composite transposon) vtranssposon n gin (simple transposon)

Transposon hn hp (composite transposon) cha nhiu gene nm gia 2 trnh t ISgn nhau, c hng ngc nhau to ra tnh t lp li o ngc (inverted repeat - IR).Mt trong 2 yu t IS m ha cho transposase xc tc cho s chuyn v ca ctransposon. Chng hn Tn10 l transposon hn hp mang gene m ha cho tnh khngkhng sinh tetracyline. Gene ny nm gia hai yu t IS10 c hng ngc nhau.

Transposon n gin (simple transposon) gia cc trnh t IR, nhng nhng trnh

t ny ngn (


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transposon. Transposon di hn yu t IS, thng cha vi kb), chng cha cc gene mha cho protein thm vo.

1.2. C ch ca s chuyn v

u tin, transposase ct vt hnh ch chi qua 5 cp base (khc vi s ct caenzyme restriction endonuclease) vi tr DNA mc tiu (target site DNA) (Hnh 12.9).Tip theo l s hi nhp ca transposon qua trung gian ca transposase, transposon xenvo gia cc u mt ca ch chi. u li ra ca si n c s dng nh l khun tng hp si b sung th hai. S gn vo to s sao chp 5 cp base, c gi l s saochp im mc tiu (target site duplication).

Hu ht cc yu t di ng ca prokaryote u s dng mt trong 2 c ch chuynv: l sao chp (replicative) v bo th (conservative) hay khng sao chp. Trong conng sao chp (nh Tn3), mt bn sao mi ca yu t di ng to ra khi chuyn v,

kt qu l mt bn sao v tr mi v bn sao cn li v tr c. Trong con ng bo th(nh trng hp Tn10) khng c s sao chp. Thay vo , yu t c ct ra t nhimsc th hoc plasmid v c gn vo v tr mi. Con ng ny cn c gi l conng "ct v dn" (cut and paste)


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Hnh 12.9. S nhn i on trnh t DNA ngn im xen vo (insertion site)

2. Cc yu t di truyn vn ng eukaryote

Cc yu t di truyn vn ng eukaryote chia lm 2 nhm: nhm l ccretrotransposon v nhm 2 l cc DNA transposon.

2.1. Cc retrotransposon

Gery Fink v cs. l nhng ngi u tin s dng nm men nghin cu iu hohot tnh gene eukaryote. Cc tc gi ny phn lp c hng ngn t bin geneHIS4 m ha enzyme tham gia tng hp histidine. Trong s hn 1.500 t bin ngunhin HIS4 c tm thy c 2 t bin c kiu hnh khng bn vng. Cc t binkhng bn vng ny c tn s phc hi li dng kiu di cao 1.000 ln hn cc t binHIS4 khc. Nhng t bin ny cho on DNA ln xen vo gene HIS4, s xen vo ny


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c thc hin do mt trong cc yu t l Ty ca nm men. C 35 bn sao ca yu t xenon gi l Ty1 genome ca nm men.

Vic to dng nhng yu t ny t cc allele t bin cho thy xen on ny khngging vi yu t IS hoc transposon ca vi khun. Thay vo chng c c tnh ca

retrovirus (virus ca ng vt). C s ging nhau trong cu trc v thnh phn gene caretrovirus v yu t Ty1 c phn lp t t bin HIS4. Ging vi retrovirus,transposon ca nm men c lp on cui di (long terminal repeat sequence) LTRs,cha hng trm cp base, c gi l trnh t nm 2 pha on m ha, c hai ucha genegagv genepol. Retrovirus c t nht 3 gene m ha cho 3 protein trong qutrnh sao chp: genegagm ha cho mt protein c vai tr lm bin tnh RNA genome.Genepolm ha enzyme reverse transcriptase. Gene env m ha cho protein v. Yu tTy ch cha genegagv genepol, khng cha gene env. (Hnh 12.10)

Hnh 12.10 S chuyn v nh retrotransposition

M hnh v s chuyn v nh retrotransposon. Mt bn phin m RNA tretrotransposon di tc dng ca enzyme phin m ngc to thnh DNA nh enzyme


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reverse transcriptase c m ha bi retrotransposon. Bn sao DNA c chn vo vtr mI trn b gene.

Vo nm 1985, J.Bocke v G. Fink chng minh, yu t Ty1, ging vi retrovirus,thc hin vic di chuyn qua trung gian RNA. Chng bt u bng bin i yu t Ty1

ca nm men c to dng trn plasmid. Trc tin mt u mt ca yu t, c sxen vo mt promotor c hot ha nh thm galactose vo mi trng. Th hai, mtintron t mt gene khc ca nm men c a vo vng m ha ca transposon Ty. Sthm vo galactose lm tng tn s chuyn v ca yu t Ty bi bin i. iu ny lmtng s lng RNA, v galactose kch thch phin m RNA Ty bt u t promotor nhycm galactose.

Nhiu t bin ngu nhin c phn lp rui dm cho thy cng charetrotransposon. Yu t copia ca rui dm cu trc tng t vi yu t Ty ca nm men.Chng xut hin t 10-100 v tr trn b gene ca rui dm. Nhng t bin nht nh carui dm l kt qu ca s xen vo ca yu t copia v nhng yu t khc. Chng hn,

t bin white-apricot (wa) v mu mt ca rui dm to ra do s xen vo yu t ca hcopia locus white.

S xen retrotransposon LTR vo cc gene thc vt nh ng, cng gp phn tora cc t bin ngu nhin.

2.2. DNA transposon

Nhiu yu t di ng c tm thy eukaryote chuyn v bng c ch ging vi vikhun. Bn thn yu t IS v transposon hoc l bn sao ca chng c th xen vo v trmi trn genome. Cc yu t di chuyn theo cch ny c gi l yu t nhm 2 hocDNA transposon. Yu t di ng c McClintok pht hin u tin ng l cc DNAtransposon. Tuy nhin tnh c trng phn t ca DNA transposon u tin li l yu t p rui dm.

Yu t p c pht hin khi nghin cu th lai mt kh nng sinh sn (dysgenesis),hin tng xy ra khi lai rui ci thuc chng phng th nghim vi rui c qun tht nhin. Trong php lai nh th, chng phng th nghim c xem l c kiu t bo M(M cytotype) v ging gc t nhin c xem l c kiu t bo P (P cytotype). Php laica M (ci) x P (c) cho th h sau kiu hnh dng mm vi gm dng bt th vi tl t bin, tn s sai hnh nhim sc th v khng chia ly nhim sc th cao. Php lai

nghch khng xy ra hin tng thoi ha ging (dysgenics). t bin dysgenics khngbn, chng c th phc hi dng kiu di hoc bin i thnh allele t bin khc vi tns cao.

S ging nhau v t bin khng bn vng rui dm v ng a n gi thuytt bin dysgenic c gy ra do s xen yu t di ng vo nhng gene c bit, lmchng b bt hot. Chng hn hu ht cc t bin trn rui dm do xen yu t di ngvo gene white. Nhng yu t nh vy gi l yu t p, c t 30-50 bn sao trn genome


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chng P v hon ton vng mt chng M. Yu t P c chiu di t 0,5-2,9 kb. Kchthc khc nhau ny do do nhiu yu t P khng y m b mt on gia. Yu t Pvi kch thc y tng ng vi transposon n gin vi khun, c u mt lon lp o ngc ngn (31 bp) v n m ha cho transposase. Gene m ha chotransposase eukaryote cha 3 intron v 4 exon.

ng, yu t Ac v Ds c th di chuyn trn nhim sc th , tc ng ln cc genekim tra mu sc ca cc ht aleuron. Cc yu t ny c phn lp cho thy c lin quanvi DNA transposon ca vi khun v ca cc eukaryote khc. Ging vi yu t P ca ruidm, yu t Ac c on cui lp li o ngc m ha cho transposase, yu t Ds likhng m ha cho transposase. Khi Ac trn genome, transposase ca n c th bm vou mt ca c yu t Ac v Ds v khi ng s chuyn v ca chng..

Yu t Ac v Ds l thnh vin ca h transposon n gin. Ngoi ra, cn c cc hyu t di ng khc ng. Mi h cha yu t t ng m ha cho transposase c thchuyn c yu t trong cng mt h, nhng khng th chuyn n cc h khc v

transposase ch c th bm vo u mt ca cc thnh vin trong h.

ngi, yu t di ng chim mt na genome ngi. a s yu t di ng thuc 2dng retrotransposon l yu t nhn ri rc kch thc di (long interspersed nuclearelement) LINEs v yu t nhn ri rc kch thc ngn (short interspersed nuclearelement) SINEs. LINE di chuyn nh retrotransposition s dng yu t m hareverse transcriptase, nhng li thiu mt vi tnh cht v cu trc ca yu t gingretrovirus bao gm LTR.

SINE c m t nh l LINE khng t ng, v chng c tnh cht cu trc ctrng ca LINE nhng khng m ha cho reverse transcriptase ring. Ngi ta cho rng

chng di chuyn c nh enzyme reverse transcriptase c m ha bi LINE tronggenome.

SINE ngi c gi l trnh t Alu v n cha trnh t im ct ca enzyme cthn ch Alu. Genome ca ngi cha hn 1 triu trnh t Alu ch mt phn hoc ton b,chim hn 10% genome ca ngi. Trnh t Alu y c kch thc 200 nucleotide.DNA genome mang yu t di ng ln hn 20 ln DNA m ha cho tt c protein ngi.

Tn s t bin ngu nhin do xen vo yu t nhm 2 l thp cha n 0,2% trongtng cc t bin ngu nhin. Trong khi nhng t bo ng vt khc nh chut doxen retrotransposon ln n 10% t bin ngu nhin, cao hn 50 ln ngi c l lin

quan vi hot tnh ca yu t retrotransposon cao hn chut.

Cu hi n tp

1. V sao phn ln cc t bin nh hng n cc gene cu trc thng l ln sovi cc allele hoang di?

2. S hnh thnh t bin dch khung din ra nh th no?


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3. Cc ha cht gy t bin c c im g?

4. Gii thch c s t bin ca cc tc nhn gy t bin sau: 5-bromuracil, acidnitr v acridin. Xc nh xem chng to ra dng t bin no (ng hon, o hon haydch khung)?

5. Hy m t mt loi sai hng ngu nhin dn n t bin.6. Phn tch s ging nhau v khc nhau gia cc kiu transposition.

7. Cc trnh t o ngc c vai tr g trong transposition?

8. Cho mt chui trnh t nucleotid trn mRNA nh sau:

Dng hoang di: ... 5' AAUCCUUACGGA 3' ...

Dng t bin: .... 5' AAUCCUACGGA 3' ...

Hy cho bit sai hng trn xy ra do loi t bin no?

9. t bin xy ra trong trnh t nucleotid do kt cp nhm nh sau:

5' AGCTGCCTT 3'

Hy cho bit acid amin no s c tm thy codon c nucleotid b thay i nhtrn?

10. bp, tn s t bin ca locus R (mu cy) rt co: 492 t bin trn 106 giaot. Gene to mu ca ni nh Pr c tn s t bin l 11 t bin trn 106 giao t. Hy

cho bit cn phi phn tch bao nhiu cy mi tm c 1 t bin kp ca 2 gen trn?

Ti liu Tham kho

Phm Thnh H. 2000. Di truyn hc. NXB Gio Dc.

L nh Lng, Phan C Nhn (1998). C s di truyn hc. NXB Gio dc.

Hong Trng Phn. 1995. Di truyn hc phn t. Trung tm o to T xa, i hcHu

Anthony J. F. Griffiths, Susan R. Wessler, Richard C. Lewontin, William M.

Gelbart, David T. Suzuki, Jeffrey H. Miller. 2004. An introduction to genetics analysis.W.H. Freeman Publishers.

Harlt D.L., Jones E.W. 1998. Genetics - Principle and analysis. Jone and BartlettPublshers, Toronto, Canada.

Stansfield W.D. 1991. Schaums outline of theory and problems of genetics.McGraw-Hill, Companies, Inc., United States of America.


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Watson D.J, Baker T.A., Bell S.P., Gann A., Levine M., Losick R. 2004. Molecularbiology of the gene. Benjamine Cummings, San Francisco, United States of America.

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