1. DNA REPLICATION
AHL Topic 7.2
IB Biology
Miss Werba

2. AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS
7.1 DNA STRUCTURE
7.2
DNA REPLICATION
7.3
TRANSCRIPTION
7.4
TRANSLATION
7.5 PROTEINS
7.6 ENZYMES
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3. THINGS TO COVER Direction of DNA replication
Prokaryotic DNA replication:
Enzymes
Okazaki fragments
deoxynucleoside triphosphates
Eukaryotic DNA replication
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4. Ref: IB Biology, OSC

5. DIRECTION OF REPLICATION Command term = STATE
7.2.1
DIRECTION OF REPLICATION Command term = STATE
DNA replication occurs in a 5’→ 3’ direction.
This means the 5’ end of the new strand
The 5’ end of the free DNA nucleotide is added to the 3’ end of the chain of nucleotides that is already synthesized.
Addition of nucleotides is catalysed by DNA polymerase.
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6. 7.2
DNA REPLICATION
The free floating nucleotides in the nucleus are called deoxyribonucleoside triphosphates
These molecules are composed of a deoxyribose sugar, a nitrogen base and three phosphate groups.
eg. dATP, dCTP, dTTP, dGTP C1 C2 C3 C4 C5 P
BASE
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7. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
Review of SL information:
Several processes are required for DNA replication to occur:
DNA double helix “unwinds”
Two strands of DNA “unzip”
Free nucleotides pair up with exposed bases
A new complementary strand forms
Now we have to add the HL details to this very simplified version...!
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8. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
1) Unwinding & unzipping the DNA
Enzyme involved:
DNA helicase
Process:
DNA helicase unwinds the DNA double helix
Hydrogen bonds are broken between the base pairs, allowing the strands to separate
One of the old strands will be 5’3’ and the other will be 3’5’
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9. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
Becomes complicated because DNA replication only occurs in a 5’ 3’ direction
problems arise due to the fact that the two DNA strands run in opposite directions
The two strands are referred to as:
the leading strand
ends in 3’ (the right way)
Replication occurs continuously
the lagging strand
ends in 5’ (the wrong way)
Replication needs to ‘leap frog’
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10. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
2) Replicating the leading strand (5’ 3’)
Enzyme involved:
DNA polymerase III
Process:
DNA polymerase III binds new nucleotides to the new growing strand
Uses condensation reactions to add nucleotides
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11. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) Replicating the lagging strand (3’ 5’)
Enzymes involved:
RNA primerase
DNA polymerase III
DNA polymerase I
DNA ligase
Process:
RNA primer is added
Okazaki fragments are formed
RNA primer is removed
Okazaki fragments are connected
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12. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) Replicating the lagging strand (3’ 5’)
Adding the RNA primer
Enzyme involved:
RNA primerase
Process:
A primer is needed to create a starting point for replication of the lagging strand
A few RNA nucleotides (an RNA primer) are bound to the template strand
RNA primase binds the RNA nucleotides together
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13. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) Replicating the lagging strand (3’ 5’)
Forming the Okazaki fragments
Enzyme involved:
DNA polymerase III
Process:
Deoxyribonucleoside triphosphates form hydrogen bonds with the exposed lagging strand
DNA polymerase III binds the nucleotides together
forms an Okazaki fragment ( nucleotides)
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14. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) Replicating the lagging strand (3’ 5’)
Removing the RNA primer
Enzyme involved:
DNA polymerase I
Process:
DNA polymerase I removes the RNA primer from the Okazaki fragment
It replaces the RNA nucleotides with corresponding DNA nucleotides
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15. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
3) Replicating the lagging strand (3’ 5’)
Connecting the Okazaki fragments
Enzyme involved:
DNA ligase
Process:
DNA ligase catalyses the formation of bonds between the Okazaki fragments
It acts as the “glue” to “stick” the new DNA strand together
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16. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
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17. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
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18. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
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19. PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
7.2.2
PROKARYOTIC DNA REPLICATION Command term = EXPLAIN
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Ref: IB Biology, Oxford Study Courses

20. EUKARYOTIC DNA REPLICATION Command term = STATE
7.2.3
EUKARYOTIC DNA REPLICATION Command term = STATE
DNA replication is initiated at many points in eukaryotic chromosomes.
In order to speed up the replication process, it must start at many points along the same DNA helix at the same time.
These points are known as replication bubbles.
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21. EUKARYOTIC DNA REPLICATION Command term = STATE
7.2.3
EUKARYOTIC DNA REPLICATION Command term = STATE
DNA replication is initiated at many points in eukaryotic chromosomes.
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22. Sample questions Q1. What are Okazaki fragments?
Short lengths of RNA primase attached to the DNA during replication
Short sections of DNA formed during DNA replication
Nucleotides added by DNA polymerase I in the same direction as the replication fork
Sections of RNA removed by DNA polymerase III and replaced with DNA
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23. Sample questions
Q2. Which enzyme removes the RNA primer during replication?
RNA primase
DNA polymerase I
DNA ligase
Helicase
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24. Sample questions Q3. Explain the process of DNA replication. [8]
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25. Sample questions
A1. & A2. B
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26. Sample questions DNA replication is semi-conservative;
unwinding of double helix / separation of strands by helicase;
hydrogen bonds between two strands are broken;
each strand of parent DNA used as template for synthesis;
synthesis continuous on leading strand but not continuous on lagging strand;
leading to formation of Okazaki fragments (on lagging strand);
synthesis occurs in 5´ → 3´ direction;
RNA primer synthesized on parent DNA using RNA primase;
DNA polymerase III adds the nucleotides (to the 3´ end) according to complementary base pairing;
adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.)
DNA polymerase I removes the RNA primers and replaces them with DNA;
DNA ligase joins Okazaki fragments;
as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond;
Accept any of the points above shown on an annotated diagram. 8 max
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