1. WHY DNA Replication? DNA replicates to make duplicate copies for cell division DNA replication occurs during S (synthesis) of Interphase of cell cycle 1. What are the differences between DNA replication in eukaryotes and prokaryotes?

2. DNA Replication Origins of replication
1. Breaking of hydrogen bonds between two anti-parallel strands
2. Replication Forks: hundreds of Y-shaped regions of where the splitting starts is called the “origin of replication”.
Replication
Fork
Parental DNA Molecule 3’ 5’

3. DNA Replication Strand Separation:
1. Helicase: enzyme which catalyze the unwinding and separation (breaking H- Bonds) of the parental double helix.
The separation happens in chains rich in A-T since it is easier to break two bonds than the three bonds in C-G
Single-Strand Binding Proteins: proteins which attach and help keep the separated strands apart.

4. Reflection How does replication start?
Who prevents the unwound DNA from twisting back?

5. DNA Replication Origins of replication 2. Replication Bubbles:
a. Hundreds of replicating bubbles (Eukaryotes).
b. Single replication fork (bacteria).
Bubbles

6. DNA Replication Strand Separation:
3. Topoisomerase: enzyme which relieves stress on the DNA molecule by allowing free rotation around a single strand.
Enzyme
DNA
Enzyme

7. DNA Replication Priming: (Initation)
1. RNA primers: before new DNA strands can form, there must be small pre-existing primers (RNA) present to start the addition of new nucleotides (DNA Polymerase).
2. Primase: enzyme that polymerizes (synthesizes) the RNA Primer in the initiation point of the 3’-5’ chain.
3. RNA primase can attract RNA nucleotides which bind to the DNA nucleotides of 3’-5’ strand with hydrogen bonds

8. DNA Replication (Elongation) Synthesis of the new DNA Strands:
1. DNA Polymerase: with a RNA primer in place, DNA Polymerase (enzyme) catalyze the synthesis of a new DNA strand in the 5’ to 3’ direction.
RNA
Primer
DNA Polymerase
Nucleotide 5’ 3’

9. DNA Replication (Elongation)
2. Leading Strand: synthesized as a single polymer in the 5’ to 3’ direction.
RNA
Primer
DNA Polymerase
Nucleotides 3’ 5’

10. DNA Replication
3. Lagging Strand: also synthesized in the 5’ to 3’ direction, but discontinuously against overall direction of replication.
RNA Primer
Leading Strand
DNA Polymerase 5’ 3’
Lagging Strand 5’ 3’

11. DNA Replication
4. Okazaki Fragments: series of short segments on the lagging strand.
Lagging Strand
RNA
Primer
DNA
Polymerase 3’ 5’
Okazaki Fragment

12. Reflection
Why the two strands of the helix have to be elongated by two slightly different mechanisms?
How are Okazaki fragments on the lagging strand joined into one continuous strand?

13. DNA Replication
5. DNA ligase: a linking enzyme that catalyzes the formation of a covalent bond from the 3’ to 5’ end of joining stands.
Example: joining two Okazaki fragments together.
Lagging Strand
Okazaki Fragment 2
DNA ligase
Okazaki Fragment 1 5’ 3’

14. Reflection
List the proteins/enzymes involved in the process of replication

15. DNA Replication DNA Pol I-exonuclease reads the fragments and
removes the RNA primers.
Gaps are closed with
the action of DNA polymerase by
adding nucleotides
DNA ligase adds phosphate in the backbone to close the gaps
Each new double helix is one old and one new strand. This is called Semi-Conservative replication

16. DNA Replication (Termination) The primer is removed
from the last section of
Lagging strand
DNA polymerase cannot
seal the gap
The end of the parental strand is not replicated
These non coding DNA sequences called telomeres
As a result part of telomere is removed in every subsequent replication
Enzymes like nucleases fix the possible errors and remove the wrong nucleotides and DNA fills the gaps.

17. Inquiry..
How is DNA packaged in eukaryotes? Describe the different size fibers and the proteins they use.
What are telomeres and what is the relation between telomeres and aging?