1. Mr. Briner Unit 7.1b DNA Replication HL
American School of Milan
DP Biology 11
Unit 7.1b
DNA Replication HL

2. DNA Replication

3. 7.1.U3
DNA polymerases can only add nucleotides to the 3’ end of a primer.

4. DNA Replication Process of DNA Replication
DNA replication occurs in a 5’  3’ direction
Each nucleotide contains a deoxyribose molecule with 5 carbons
Nitrogenous base attached to carbon 1
Phosphate attached to carbon 5

5. DNA Replication Process of DNA Replication
DNA replication occurs in a 5’  3’ direction
DNA is built with a repeating backbone
sugar - phosphate - sugar – phosphate
Thus, DNA has a free 5’ phosphate and a free 3’ carbon
Replication only adds nucleotides at the 3’ end

6. DNA Replication
Process of DNA Replication

7. DNA Replication Process of DNA Replication
5' end of the free nucleotide is added to the 3' end of the exsisting nucleotide chain
Bonded covalently to the 3' end on the already formed polynucleotide chain

8. DNA Replication (a) A free nucleotides in isolation
5 ' is orientated to the 3' so that DNA polymerase enzyme can form the phosphodiester bond between carbon 5 and carbon 3

9. DNA Replication (b) There is a polynucleotide chain already in place
Note the position of the free 3' on the left end of this chain

10. DNA Replication Process of DNA Replication
DNA Polymerase III can only add nucleotides in the 5’ to 3’ direction
DNA is antiparallel

11. DNA Replication Process of DNA Replication
DNA polymerase III works in two directions:
LEADING STRAND
New strand copied in one piece moving towards replication fork
LAGGING STRAND
New strand copied in many pieces moving away from replication fork

12. DNA Replication Process of DNA Replication
DNA polymerase III works in two directions:

13. 7.1.U5
DNA replication is carried out by a complex system of enzymes.

14. DNA Replication Enzymes in DNA Replication Helicase:
Controls unwinding of coiled DNA
Separates complementary strands of DNA, producing a replication fork
DNA Gyrase (= Topoisomerase)
Reduces the twisting strain created by the unwinding of DNA by helicase

15. DNA Replication

16. DNA Replication
Enzymes in DNA Replication
Helicase:

17. DNA Replication Enzymes in DNA Replication DNA polymerase III:
Can only add nucleotides to a free 3’ end of an existing nucleotide strand
Thus, it can only synthesize continuously on one of the two strands of DNA
In the direction toward the replication fork
This is known as the leading strand

18. DNA Replication Enzymes in DNA Replication DNA polymerase III:
Joins the 5' to 3' of the NEW STRAND
Prokaryotic DNA polymerases can work at around 1000 bases per second
Whole circular (loop) of DNA can replicated between 20 and 40 minutes

19. DNA Replication
Enzymes in DNA Replication
DNA polymerase III:

20. DNA Replication Enzymes in DNA Replication
Hydrolysis of phosphate group provides the energy to form phosphodiester covalent bonds between nucleotides

21. DNA Replication

22. DNA Replication
Enzymes in DNA Replication

23. DNA Replication Enzymes in DNA Replication Primase:
DNA polymerase III can only add DNA nucleotides to a free 3’ end
Primase uses the lagging strand to synthesize a short RNA nucleotide sequence known as an RNA primer
DNA polymerase III attaches to the primer to replicate the lagging strand

24. 7.1.U4
DNA replication is continuous on the leading strand and discontinuous on the lagging strand.

25. DNA Replication Okazaki fragments
DNA polymerase III is unable to work directly on the lagging strand 
It lacks a free 3’ end on an existing DNA strand
So Primase uses the lagging strand to synthesize a short 10 RNA nucleotide
RNA primer
DNA polymerase III uses the free 3’ end of the RNA primer to synthesize longer sequences of DNA
Known as Okazaki fragments

26. DNA Replication
Enzymes in DNA Replication

27. DNA Replication Enzymes in DNA Replication DNA polymerase I:
DNA polymerase I = proofreading enzyme
Removes RNA nucleotides of the primer
Replaces them with DNA nucleotides
This leaves unconnected DNA fragments on the lagging strand
Okazaki fragments

28. DNA Replication Enzymes in DNA Replication DNA ligase:
Forms covalent bonds linking together Okazaki fragments
Completes DNA synthesis on the lagging strand

30. DNA Replication
Enzymes in DNA Replication

32. DNA Replication Replication in Eukaryotic organisms
Eukaryotic genome is so large it would take days to replicate using a single initiation point
Many initiation points are found in each eukaryotic chromosome
About 100,000 nucleotides apart
With replication forks moving in opposite directions away from each initiation point
Until they meet in the middle

33. DNA Replication
Replication in Eukaryotic organisms

34. DNA Replication Replication in Eukaryotic organisms
Eukaryotic DNA polymerase synthesizes about 50 bases per second
With up to 80 million bases to replicate the job is achieved in about one hour by having many replication forks

35. MAJOR SOURCES
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Stephen Taylor (Bandung, IN)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at