dna repair.pptx

7/26/2019 DNA repair.pptx

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DNA REPAIR

PROVIDING CHEMICAL STABILITY FOR LIFE

M. Tofazzal Islam

GEB Department, East West University


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DNA A mystery of life

From one cell to another, from onegeneration to the net.

The genetic information that governs ho!h"man #eings are shape$ has %o!e$thro"gh o"r #o$ies for h"n$re$s oftho"san$s of years.

It is constantly s"#&ecte$ to assa"lts fromthe environment, yet it remainss"rprisingly intact.


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Day after $ay, o"r D'( is $amage$ #y U) ra$iation,free ra$icals an$ other carcinogens, #"t $espite ofthat, o"r D'( at a molec"lar level is inherent.

Tho"san$s of changes happen in o"r genome at

cell"lar level, #"t these changes a*ect o"r cells!hen a process that happens tho"san$s of times a$ay are $ivi$e$.

The reason !hy o"r genetic material is not$estroye$ #y chemical levels is $"e to the largen"m#er of molec"lar systems that control an$repair contin"o"sly o"r D'(.


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Noel Pri!e i" C#emistry $%&'

The +oyal !e$ish (ca$emy of ciences has $eci$e$ to a!ar$

Tomas -in$ahl, a"l Mo$rich an$ (ziz ancar the 'o#el rize in

/hemistry 0123 for their 4Mechanistic st"$ies of D'( repair5

Damage to the genetic material poses a threat to all organisms. To

co"nteract this threat, cells have evolve$ a series of intricate D'(repair path!ays that correct D'( lesions a*ecting #ase pairing or

str"ct"re of D'(.

To$ay !e "n$erstan$ the molec"lar mechanisms of these path!ays

in great $etail, in large part $"e to the pioneering st"$ies #y -in$ahl,

Mo$rich an$ ancar that opene$ "p the 6el$.

T#ey ()(r*e* for #(+i", m(--e* ("* e.-l(i"e* #o) t#e /ell

re-(irs its DNA ("* s(fe,0(r*s t#e ,e"eti/ i"form(tio"1


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Noel Pri!e i" C#emistry $%&'


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B(/2,ro0"*

The h"man genome enco$es the information nee$e$ tocreate a complete h"man #eing.

D"ring every cell $ivision, more than 7 #illion D'( #asepairs are replicate$ an$ copies of the genome are

transferre$ to the $a"ghter cells.The D'( replication machinery responsi#le for this tas8

still ma8es occasional mista8es.

Given the size of the h"man genome an$ the large n"m#erof cells in a h"man #o$y 9a#o"t 7.: ; 2127< mista8es !illinevita#ly acc"m"late $"ring the lifetime of an in$ivi$"al.

Most of these errors !ill remain silent, #"t they can alsoca"se serio"s $iseases.


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Despite its essential role in storing geneticinformation, the D'( molec"le has limite$chemical sta#ility an$ is s"#&ect to

spontaneo"s $ecay.rocesses s"ch as hy$rolysis an$ oi$ation

occ"r at signi6cant levels in vivo, in part $"eto reactive meta#olites contin"o"sly

generate$ in vario"s physiological processes.In a$$ition, eternal factors li8e ra$iation an$

genotoic chemicals !ill f"rther stim"late ofD'( $amage formation.

B(/2,ro0"*


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The inherent insta#ility of D'( constit"tes #oth an opport"nityan$ a threat.

D'( lesions can #loc8 important cell"lar processes s"ch asD'( replication an$ transcription, ca"se genome insta#ilityan$ impair gene epression.

-esions can also #e m"tagenic an$ change the co$ing capacityof the genome, !hich can lea$ to $evastating $iseases an$con$itions associate$ !ith genome insta#ility, incl"$ingcancer, ne"ro$egenerative $isor$ers an$ #iological ageing.

F"rthermore, m"tagenic chemicals an$ ra$iation can also have

a healing e*ect= they can for instance #e "se$ to treat cancer,#y intro$"cing D'( lesions that halt cell proliferation an$stim"late programme$ cell $eath.

B(/2,ro0"*


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The cell has $evelope$ !ays to co"nteractD'( lesions an$ to 8eep D'( m"tations at atolera#le level.

( n"m#er of $i*erent D'( repairmechanisms correct lesions an$ safeg"ar$ theintegrity of the genome.

Fo"r f"n$amental D'( repair path!ays$elineate$ #y this year>s 'o#el rize la"reates!ill #e $isc"sse$ here.


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-ife eists ? so D'( m"st #erepaira#le

3Ho) st(le is DNA4 re(lly564 Tomas-in$ahl starte$ !on$ering to!ar$s the en$ ofthe 2@A1s.

(t the time, the scienti6c comm"nity #elieve$t#(t t#e DNA mole/0le t#e fo0"*(tio"of (ll life )(s e.tremely resilie"t7anything else !as simply o"t of the "estion.

Evol"tion $oes re"ire m"tations, #"t only alimite$ n"m#er per generation.


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If genetic information !ere too "nsta#le no m"ltiCcell"larorganisms !o"l$ eist.

D"ring his post$oc at rinceton University, U(, Tomas-in$ahl !or8e$ on t#e RNA mole/0le4 ( mole/0l(r

/o0si" to DNA.It $i$ not go !ell. In his eperiment he ha$ to heat +'(,

#"t this inevita#ly le$ to the molec"les> rapi$ $egra$ation.

It !as !ell 8no!n that +'( !as more sensitive than D'(,#"t if RNA )(s *estroye* so 80i/2ly )#e" s09e/te*

to #e(t4 /o0l* DNA mole/0les re(lly e st(le for (lifetime5This "estion too8 hol$ in -in$ahl>s min$.


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e #egan to loo8 for an ans!er to that "estion, an$some straightfor!ar$ eperiments prove$ that hiss"spicions !ere correct DNA 0"*er)e"t ( slo)0t "oti/e(le *e/(y.

-in$ahl estimate$ that there !ere tho"san$s of

potentially $evastating in&"ries to the genome every$ay, a fre"ency that !as clearly incompati#le !ithh"man eistence on Earth.

is concl"sion !as that t#ere m0st e mole/0l(r

systems for re-(iri", (ll t#ese DNA *efe/ts an$,!ith this i$ea, Tomas -in$ahl opene$ the $oor on anentirely ne! 6el$ of research.


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S-e/i(l e"!ymes remo+e *(m(,e i" DNA

Bacterial D'( !hich, &"st li8e h"man D'(, consists ofn"cleoti$es !ith the #ases a$enine, g"anine, cytosine, an$thymine.

ne chemical !ea8ness in D'( is that cytosine easily loses anamino gro"p, !hich can lea$ to the alteration of geneticinformation.

In D'(>s $o"#le heli, cytosine al!ays pairs !ith g"anine, #"t!hen the amino gro"p $isappears, the $amage$ remains ten$to pair !ith a$enine.

Therefore, if this $efect is allo!e$ to persist, a m"tation !illocc"r the net time D'( is replicate$.

-in$ahl realize$ that the cell m"st have some protection againstthis, an$ !as a#le to i$entify a #acterial enzyme that removes$amage$ remains of cytosines from D'(.


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B(se e./isio" re-(ir

Bit #y #it, -in$ahl piece$ together a molec"lar imageof ho! #ase ecision repair f"nctions.

Base ecision repair also occ"rs in h"man #eings an$

-in$ahl manage$ to recreate the h"man repairprocess in vitro.

The $ecisive factor for Tomas -in$ahl !as the

realisation that D'( inevita#ly "n$ergoes change,even !hen the molec"le is locate$ in the cell>sprotective environment.

o!ever, it ha$ long #een 8no!n that D'( can #e$amage$ #y environmental assa"lts s"ch as U)

ra$iation.


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B(se E./isio" Re-(ir

a. D'( glycosylase

recognizes $amage$#ase

#. +emoves #ase leaving$eoyri#ose s"gar

c.There#y pro$"cing an4a#asic5 or (9ap"rinicapyrimi$inic< site #y

#ase 4%ipping o"t5

$. ( en$on"clease c"tsphospho$iester#ac8#one

e. D'( polymerase


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A!i! S("/(r : i"+esti,(ti", #o) /ellsre-(ir ;V *(m(,e

Bacteria have t!o systems for repairing U) $amage

lightC$epen$ent photolyase,

a secon$ system that f"nctions in the $ar8

That ha$ #een $iscovere$ "sing three U)Csensitive strains of #acteria!hich carrie$ 7 $i*erent genetic m"tations "vr(, "vrB an$ "vr/.

(s in his previo"s st"$ies of photolyase, ancar #egan investigatingthe molec"lar machinery of the $ar8 system.

e ha$ i$enti6e$, isolate$ an$ characterize$ the enzymes co$e$ #y

the genes "vr(, "vrB an$ "vr/.In gro"n$C#rea8ing in vitro eperiments he sho!e$ that these enzymes

can i$entify a U)C$amage, then ma8ing t!o incisions in the D'(stran$, one on each si$e of the $amage$ part. ( fragment of 20C27

n"cleoti$es, incl"$ing the in&"ry, is then remove$.


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Simil(r me/#("isms for ;V *(m(,ere-(ir i" #0m("s ("* (/teri(

(ziz ancar mappe$ the net stages of n"cleoti$e ecisionrepair.

In parallel !ith other researchers, incl"$ing Tomas -in$ahl,ancar investigate$ n"cleoti$e ecision repair in h"mans.

The molec"lar machinery that ecises U) $amage fromh"man D'( is more comple than its #acterial co"nterpart#"t, in chemical terms, n"cleoti$e ecision repair f"nctionssimilarly in all organisms.

ancar event"ally ret"rne$ to the enzyme, photolyase

"ncovering the mechanism responsi#le for reviving the#acteria. In a$$ition, he helpe$ to $emonstrate that ah"man e"ivalent to photolyase helps "s set the circa$iancloc8.


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N0/leoti*e e./isio" re-(ir

+ecognizes #"l8y lesions that #loc8 D'(replication 9i. e. lesions pro$"ce$ #y carcinogens


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Nucleotide Excision Repair

(in E. coliof a T-T dimer)

Endonuclease cuts on either side ofdamage (~20 nt altogether).

Strands unwound by helicase.

!r" has the endonuclease acti!ity#

!r$ and % bind to the damaged site.

&E' can also reair other tyes of &$damage.

Essential in humans# eole with

*eroderma igmentosum ha!e reduced

&E' + e,treme sensiti!ity. $lso a

&E' deficiency in humans romotes

cancer. /robably a minor athway fordimer reair in organisms with

hotolyases.

. UvrA,B

2. UvrC

1. UvrD


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It -(ys o< to le(r" (o0t 3DNAst0


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I"ter)e(+i", t)o str("*s ofrese(r/#

Dam methylase co"ples methyl gro"ps to D'(.These methyl gro"ps co"l$ f"nction assignposts, helping a partic"lar restriction

enzyme to c"t the D'( stran$ at the correctlocation.

o!ever, there !as a $i*erent signallingf"nction for the methyl gro"ps on D'(.

( #acterial vir"s !ith several occ"rrences ofmismatching #ases in the D'( !as constracte$.For instance, A co"l$ #e place$ opposite C,instea$ of T.


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When these vir"ses infect #acteria, the #acteriacorrecte$ the mismatches.

(mong other things, that it co"l$ #e a repairmechanism that correcte$ the fa"lty matchesthat sometimes occ"r !hen D'( is replicate$.

erhaps the methyl gro"ps on the D'( helpe$the #acteria i$entify !hich stran$ to "se astemplate $"ring correction.

(s the ne! D'( stran$, the fa"lty replica, !asstill "nmethylate$, may#e that !as ho! it co"l$#e i$enti6e$ an$ correcte$H


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T#e met#yl(tio" of DNA Mo$rich>s an$ Meselson>s

!or8e$ together, they create$ a vir"s !ith a n"m#er ofmismatches in its D'(.

This time, Mo$rich>s *(m met#yl(se !as also "se$ toa$$ met#yl ,ro0-s to one of the D'( stran$s.

When these vir"ses infecte$ #acteria, the #acteriaconsistently correcte$ the D'( stran$ that lac8e$methyl gro"ps.

Mo$rich an$ Meselson>s concl"sion !as that D'(mismatch repair is a nat"ral process that corrects

mismatches that occ"r !hen D'( is copie$, recognisingthe $efect stran$ #y its "nmethylate$ state.


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The D'( replication machinery is not errorCfree.There is al!ays the possi#ility that an incorrectn"cleoti$e is intro$"ce$ $"ring synthesis of a

ne! D'( stran$.These types of errors are 8no!n as mismatches

an$ they have the capacity to change these"ence of D'(, i.e. to intro$"ce m"tations.

(s a 6rst line of $efence against mismatches,replicating D'( polymerases contain a 7 to 3eon"clease activity that allo!s them toproofrea$ the ne!ly synthesize$ D'( stran$.

DNA mism(t/# re-(ir


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The eon"clease activity can correct mista8es$"ring D'( replication #y reversing the$irection of the polymerase an$ ecising

incorrectly intro$"ce$ n"cleoti$es.

Even if proofrea$ing eJciently corrects mostmista8es ma$e $"ring D'( synthesis, somenonCWatsonC/ric8 #ase pairs still remain. Tocorrect these errors, cells "se mismatch repair.

DNA mism(t/# re-(ir


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DNA mism(t/# re-(ir

To$ay !e 8no! that all #"t one o"t of atho"san$ errors that occ"r !hen the h"mangenome is copie$, are correcte$ #y mismatchrepair.

o!ever, in h"man mismatch repair, )e still*o "ot 2"o) for s0re #o) t#e ori,i"(lstr("* is i*e"ti=e*.

D'( methylation has other f"nctions in o"rgenome to that of #acteria, so something elsem"st govern !hich stran$ gets correcte$ ?an$ eactly !hat remains to #e clari6e$.


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Mism(t/# re-(ir >MMR?

Despite etraor$inary 6$elity of D'( synthesis, errors $opersist"ch errors can #e $etecte$ an$ repaire$ #y the postC

replication mismatch repair systemro8aryotes an$ e"8aryotes "se a similar mechanism

!ith common str"ct"ral feat"resDefects in MM+ elevate spontaneo"s m"tation rates 21C

2111Defects in MM+ "n$erlie h"man pre$isposition to colon

an$ other cancers 94'//5

dna repair.pptx

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