1. DNA Replication the big event during S phase

2. Review Required prior to cell division (both mitosis and meiosis)
Occurs in both prokaryotes & eukaryotes
In eukaryotic cells it occurs during S (synthesis) stage of interphase in cell cycle. The synthesis that is occurring is of the DNA
Takes place in nuclei of eukaryotes

3. Creates replication bubbles
Replication starts at the ORIGIN OF REPLICATION (a specific nucleotide sequence)
Creates replication bubbles

4. In prokaryotic cells there is one origin
In eukaryotic cells there are 100’s to 1000’s of origins

5. DNA is semi-conservative
This means that one half of the DNA strand is being used to make the other half.
This also means that one strand of the DNA is an original and the other half is brand new.

6. DNA Replication is semi-conservative:
each replicated DNA molecule consists of one “old” and on “new” strand.

7. Step One The DNA molecule unwinds
Helicase breaks the hydrogen bonds between the two strands.
Is an enzyme made during G1
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8. Step Two
Single stranded binding proteins (SSBs) hold the two strands of DNA apart
This adds stability.

9. Step Three
RNA Primase makes short sections of RNA nucleotides that bind with complementary base sequences
These RNA nucleotides act as a primer for DNA nucleotides.
Primers are short segments of RNA, about 10 nucleotides long
Must have a primer because DNA polymerase can only add nucleotides to another nucleotide
Primase

10. Step Four
DNA polymerase III adds DNA nucleotides to the RNA primer sequence in a 5’ to 3’ direction. New nucleotides can only be added to the 3’ end of a growing DNA chain So we say DNA elongates in a 5’ to 3’direction

11. Step Four Leading Strand:
DNA polymerase III can synthesize a complementary strand on one side of the template in the 5’ to 3’ direction with no problem.

12. What about the other strand??5’ 3’
DNA Polymerase III
Can only add to this side …
AWAY from the
replication fork

13. Lagging Strand
DNA polymerase III must work away from the replication fork.
Makes a short strand of DNA, called an Okazaki fragment.
As the bubble widens, it can make another short strand, and so on.

14. Step Four Lagging Strand
DNA polymerase III must work away from the replication fork.
Makes a short strand of DNA, called an Okazaki fragment.
As the bubble widens, it can make another short strand, and so on.

16. Step Five
RNA primers are removed and replaced with DNA nucleotides by DNA Polymerase I.

17. Step Six
Along the lagging strand the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.

18. Step Seven
Throughout the process proofreading is done by DNA Polymerase III and I and again replication is complete.

19. Know them well

21. Accurate Only 1:1,000,000,000 nucleotides are incorrectly paired
DNA replication is ...
Accurate
Only 1:1,000,000,000 nucleotides are incorrectly paired

22. DNA replication is ... Extremely rapid
In prokaryotes, up to 500 nucleotides are added per second
50 per second in eukaryotes

23. Other Good Animations http://www.ncc.gmu.edu/dna/repanim.htm