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C H A P T E R 1 2 . 3
DNA & RNA
REPLICATION
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GETTING STARTED
The replication of a DNA molecule begins at specialsites called origins of replication.
These are short stretches of DNA having a specific
sequence of nucleotides.
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BUBBLES
Proteins that initiate DNA replication recognize thissequence and attach to the DNA, separating thetwo strands and opening up a replication bubble.
Replication proceeds in both directions.
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STEP 1 EUKARYOTIC DNA
At the end of a replication bubble is a replicationfork. A Y-shaped region where the parental strandsof DNA are being unwound.
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STEP 2
Several kinds of proteins participate in theunwinding.
Helicases are enzymes that untwist the double helix
at the replication forks.
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STEP 2 CONTINUED
After parental strand separation, single-strandbinding proteinsbind to the unpaired DNA strands,stabilizing them.
The untwisting of the double helix causes tightertwisting and strain ahead of the replication fork.
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CONTINUED
Topoisomerasehelps relieve this strain by breaking, swiveling,and rejoining DNA strands
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STEP 2
The unwound sections of parental DNA strands arenow available to serve as templates for thesynthesis of new complementary DNA.
The initial nucleotide chain that is produced duringDNA synthesis is a short stretch of RNA. It is calledthe primerand is synthesized by the enzymeprimase.
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SYNTHESIZING A NEW DNA STRAND
Enzymes called DNA polymerases begin thesynthesizing by adding nucleotides to a preexistingchain.
Two major DNA polymerases play a major roll in DNAreplication:
DNA polymerase I
DNA polymerase III
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INCORPORATION OF A NUCLEOTIDE
INTO A DNA STRAND
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SYNTHESIS OF THE LAGGING STRAND
1. Primase joins RNA nucleotides into a primer
2. DNA pol III adds DNA nucleotides to the primer,forming Okazaki fragment.
3. After reaching the next RNA primer to the right,DNA pol III detaches
4. After fragment 2 is primed DNA pol III adds DNAnucleotides until it reaches the fragment 1 primer
and detaches. 5. DNA pol I replaces the RNA with DNA, adding to
the 3 end of fragment 2.
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SYNTHESIS OF THE LAGGING STRAND
6. DNA Ligase forms a bond between the newestDNA and the DNA of fragment 1.
7. The lagging strand is now complete.
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1. Primase joins RNAnucleotides into aprimer.
DNA pol III addsDNA nucleotidesto the primer,
forming Okazakifragment.
After
reaching thenext RNAprimer to theright, DNA polIII detaches
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. After fragment 2is primed DNA polIII adds DNAnucleotides until it
reaches thefragment 1 primerand detaches.
. DNA pol I
replaces the RNAwith DNA, addingto the 3 end offragment 2.
DNA Ligaseforms a bondbetween thenewest DNAand the DNA offragment 1.The lagging strand is
now complete.