DNA Replication Notes

1. DNA

2. DNA review
 DNA codes for all information necessary for
life
 The information is stored in the sequence of
nucleotide bases
 It is a quaternary code…
 The sequence is important
– Human Genome Project: Sequence of roughly
3.2 billion bases in human DNA!

3. A Perfect Copy
When a cell divides, each new cell receives a
complete set of chromosomes.
This means that each new cell has a complete
set of the DNA code.
Before a cell can divide, the DNA must be copied
so that there are two sets ready to be distributed to
the new cells.

4. DNA structure
 Recall that once Watson and Crick made the
correct model, they instantly realized a
copying mechanism.
 Due to the structure of DNA (specifically the
base-pairing arrangement), if the order of
nucleotides on one chain is known, the order
on the other chain is obvious.

5. A
A
T
G
G
C
A

6. A
A
T
G
G
C
A
T

7. A
A
T
G
G
C
A
T
T

8. A
A
T
G
G
C
A
T
T
A

9. A
A
T
G
G
C
A
T
T
A
C

10. A
A
T
G
G
C
A
T
T
A
C
C

11. A
A
T
G
G
C
A
T
T
A
C
C
G

12. A
A
T
G
G
C
A
T
T
A
C
C
G
T

13.  DNA is copied through a process called DNA
replication.
 The process, though seemingly simple, is
actually quite complex.
 Energy is required.
– Comes from which molecule?
 Many enzymes and proteins are required for
DNA replication.

14. Where does replication happen?
 Eukaryotes?
 Prokaryotes?

15. In a nutshell…
 To start, an enzyme breaks the hydrogen
bonds that hold base-pairs together.
 This process “unzips” the DNA molecule.
 Other enzymes find and correctly match free-
floating nucleotides to the two sides of the
now open DNA molecule.
 This process continues until the entire
molecule has been replicated.

17. Replication fork… what happens
 Many different enzymes are needed to open
up the DNA helix for replication
 Two strands must be separated first
 Helicases do this …
 Pry apart the hydrogen bonds of the two
DNA strands…

19. Template Strands and New
Strands

20. DNA polymerases build new strands.
Growth
Growth
Replication
fork
DNA
polymerase
New strand
Original
strand
DNA
polymerase
Nitrogenous
bases
Replication
fork
Original
strand
New strand

21. DNA Replication fork is
asymmetrical
 Leading and lagging strands are different
 The leading strand is continuous
 The lagging strand has fragments and is
synthesized discontinuously.
– Okazaki fragments

23. High fidelity requires proof
reading
 DNA polymerase synthesizes in a 5’ to 3’
direction
 Proof reading is in a 3’-5’ direction
 Can take out mismatched bases with reverse
action

24. Fig 5-9

25. Helicases…
 Many different helicases have been identified
 Single stranded binding proteins are
important in maintaining the single stranded
nature at the replication fork & so no hair
pins can form

27. Replication starts at
replication origins.

28. • Each eukaryotic chromosome contains one DNA double helix
• Average ~106
base pairs long
• With a replication rate of 2 kb/minute, replicating one human
chromosome would require ~35 days.
• Solution ---> DNA replication initiates at many different sites
simultaneously.
Rates are cell
specific!

29. Three main features of the DNA synthesis reaction:
1. DNA polymerase catalyzes formation of phosphodiester bond
between deoxyribose and the phosphate on the new strand.
• Energy for this reaction is derived from the release of two of the
three phosphates on the free nucleotide.
2. DNA polymerase “finds” the
correct complementary nucleotide
at each step in the
lengthening process.
• rate 50-1000 bases/second
• low error rate (~1/107
)
3. Direction of synthesis is 5’ to 3’ Image credit:
Protein Data Bank

30. DNA Replication Overview
Replication Animation