dna & rna replication2

8/13/2019 DNA & RNA Replication2

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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

2

3

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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4

5

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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.

dna & rna replication2

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