Day 2 Note--DNA Replication & Repair

8/8/2019 Day 2 Note--DNA Replication & Repair

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DNA ReplicationDNA ReplicationDuring mitosisDuring mitosis


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DNA by Watson & Crick & FranklinDNA by Watson & Crick & Franklin

When discovered, the structure suggested howWhen discovered, the structure suggested howDNA was able to replicate:DNA was able to replicate:

The HThe H--bonds between complementary bases breakbonds between complementary bases break This allows the DNA to unzipThis allows the DNA to unzip

Each DNA strand then acts as a template to buildEach DNA strand then acts as a template to build

the complimentary strandthe complimentary strand

This results in two identical DNA molecules; one for eachThis results in two identical DNA molecules; one for each

daughter celldaughter cell


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That process is calledThat process is called

Semiconservative ReplicationSemiconservative Replication

Text page 217, figure 1 a & b


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DNA Strand SeparationDNA Strand Separation

Replication begins when proteins bind at aReplication begins when proteins bind at a

specific site on the DNA known as the origin ofspecific site on the DNA known as the origin of

replication (replication (oriori).).

Eukaryotic replication is similar to prokaryoticEukaryotic replication is similar to prokaryoticreplication but more complexreplication but more complex

The closed circular DNA ofprokaryotes usuallyThe closed circular DNA ofprokaryotes usuallyonlyhas one origin of replication (onlyhas one origin of replication (oriori))

Linear eukaryotic DNA has multipleLinear eukaryotic DNA has multipleorioriss


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DNA Strand SeparationDNA Strand Separation The strands cannot simply bepulled apartThe strands cannot simply bepulled apart

because they areheld together byhydrogenbecause they areheld together byhydrogen

bonds and twisted around each other in thebonds and twisted around each other in the

doublehelix.doublehelix.

Specific enzymesSpecific enzymes

work together towork together toexpose the DNAexpose the DNA

template strandstemplate strands


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DNA StrandDNA Strand

SeparationSeparationText page 219, figure 3.Text page 219, figure 3.

----The d.s. DNA isThe d.s. DNA isunwound byhelicase.unwound byhelicase.

---- The single strandedThe single stranded

bindingproteins (SSBs)bindingproteins (SSBs)

bind to theexposed basesbind to theexposed basesto prevent them fromto prevent them from

annealing.annealing.


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DNA Strand SeparationDNA Strand Separation What is involved?What is involved?

DNA helicase:DNA helicase:unwinds the doublehelix by breakingunwinds the doublehelix by breaking

the Hthe H--bonds at the replication fork.bonds at the replication fork.

ReplicationReplication

fork:fork:regionregion

wherewhere

enzymesenzymesreplicatingreplicating

DNA bindDNA bind

to anto an

untwisted,untwisted,s.s. DNAs.s. DNA


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SingleSingle--strandedstranded

BindingProteinsBindingProteins

(SSBs):(SSBs):bind thebind the

exposed DNAexposed DNA

template strands totemplate strands to

block new Hblock new H--

bonds that wouldbonds that wouldrere--join the strandsjoin the strands


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DNA gyrase:DNA gyrase:relieves tension from the unwindingrelieves tension from the unwinding

of the DNA strands during bacterial replication.of the DNA strands during bacterial replication.

It cuts nicks in both strands of DNA, allowingIt cuts nicks in both strands of DNA, allowing

them to swivel around one another and thenthem to swivel around one another and thenresealing the cut strands.resealing the cut strands.

Similar enzymes perform the task in eukaryotes.Similar enzymes perform the task in eukaryotes.


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ReplicationReplication

Replication begins in 2 directions from theReplication begins in 2 directions from theorioriss

as a region of the DNA is unwound.as a region of the DNA is unwound.

DNA replication proceeds toward the directionDNA replication proceeds toward the direction

of the replication fork (leading strand) on oneof the replication fork (leading strand) on onestrand and away from the fork (lagging strand)strand and away from the fork (lagging strand)

on the other strand.on the other strand.

In eukaryotes, when 2 replication forks are too near,In eukaryotes, when 2 replication forks are too near,

a replication bubble formsa replication bubble forms


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Text page 220, figure 4Text page 220, figure 4


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Building Complimentary StrandsBuilding Complimentary Strands

In prokaryotes, there are 3 enzymes known toIn prokaryotes, there are 3 enzymes known to

function in replication & repairfunction in replication & repair

DNA polymerase I, II & IIIDNA polymerase I, II & III

In eukaryotes,In eukaryotes,

there are 5 enzymesthere are 5 enzymes

known to functionknown to function

in replication & repairin replication & repair


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Building Complimentary Strands inBuilding Complimentary Strands in

prokaryotesprokaryotes DNA polymerase III builds the complimentaryDNA polymerase III builds the complimentary

strand of DNAstrand of DNA

It only functions under certain conditionsIt only functions under certain conditions

DNA polymerase III adds complimentaryDNA polymerase III adds complimentary

nucleotides (nucleotides (deoxyribonucleosidedeoxyribonucleosidetriphosphatestriphosphates) in the) in the

5 to 3 direction, usingRNA primers as starting5 to 3 direction, usingRNA primers as starting

pointspoints

The segments are called Okazaki fragmentsThe segments are called Okazaki fragments


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Text page 221, figure 7 aText page 221, figure 7 a


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prokaryotesprokaryotes RNA primers are synthesized byRNA primers are synthesized byprimaseprimaseand areand are

temporarytemporary

TheThe leading strandleading strand (uses 3

(uses 3--5

template) is5

template) is

synthesizedsynthesized continuouslycontinuously

TheThe lagging strandlagging strand (uses 5(uses 5--3 template) is3 template) is

synthesizedsynthesized discontinuouslydiscontinuously in short fragmentsin short fragments

Text page 220, figure 5


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prokaryotesprokaryotes DNA polymerase IDNA polymerase Iremoves theRNA primersremoves theRNA primers

from the leading strand and fragments from thefrom the leading strand and fragments from the

lagging strand and replaces them with thelagging strand and replaces them with the

appropriate deoxyribonucleotides.appropriate deoxyribonucleotides.

Text page 221, figure 7b


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prokaryotesprokaryotes DNA ligaseDNA ligasejoins the Okazaki fragments into onejoins the Okazaki fragments into one

strand on the lagging strand of DNA through thestrand on the lagging strand of DNA through the

formation of a phosphodiester bond.formation of a phosphodiester bond.

Text page 221, figure 7 c


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prokaryotesprokaryotes

As the 2 new strands of DNA are synthesized, 2As the 2 new strands of DNA are synthesized, 2

d.s. DNA molecules areproduced thatd.s. DNA molecules areproduced that

automatically twist into a helix.automatically twist into a helix.

TAKE A MOMENT AND VISUALIZETHISTAKE A MOMENT AND VISUALIZETHIS


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Guided PracticeGuided Practice

ActivityActivity


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DNA RepairDNA Repair

DNA polymerase III and DNA polymerase IDNA polymerase III and DNA polymerase I

proofread the newly synthesized DNA strands.proofread the newly synthesized DNA strands.

When mistakes occur, either enzyme canWhen mistakes occur, either enzyme canfunction as an exonuclease.function as an exonuclease.

Theenzyme backtracks and excises the incorrectlyTheenzyme backtracks and excises the incorrectly

paired nucleotidepaired nucleotide

Then it continues forward adding nucleotides to theThen it continues forward adding nucleotides to the

complimentary strandcomplimentary strand


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DNA RepairDNA Repair

Repairs must be made immediately to avoidRepairs must be made immediately to avoid

errors being copied in subsequent replications.errors being copied in subsequent replications.

Errors missed byproofreading can be correctedErrors missed byproofreading can be correctedby one of several repair mechanisms thatby one of several repair mechanisms that

operate after the completion of DNAoperate after the completion of DNA

replication.replication.


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DNA Replication & RepairDNA Replication & RepairSummarypage 222Summarypage 222

Homework:

-page 223, q # 1, 2 (due: tomorrow)

-something, in the format of your choice,

describing:

a)structure of DNA

b)replication

c)methods of repair following an error.

Documents

Day 2 Note--DNA Replication & Repair