1. Describe the role
of DNA in relation
to gene expression
DNA Replication

2. Contents
 Why replicate?
 The basics
 The enzymes
 Okazaki fragments
 Fixing errors
 Review Questions

3. Why replicate?
 DNA replicates before it forms a
chromatid. I.e. when it still looks
like…
 Over an organisms life the DNA in
the zygote is copied trillions of
times with minimal error. Error
rates are generally 1 in 50 million
Stuff that any respectful
Bio student really
should know…
Mitosis and Meiosis
What are they and
where they happen!
Meiosis Revision
exercise
When we copy this
100,000,000,000,000 times it
turns into this

4. Why replicate?
www.cellsalive.com
 Each new cell must
have a copy of the
entire DNA genome.
Some of the
DNA released
from a single
human
chromosome

5. DNA Replication - the very basics
The basics of DNA REPLICATION
 Unwind – Unzip – Add nucleotides – wind it all
back up.
 If only it could be that simple
 Two things make it a little more fiddly
 DNA is a VERY LONG double helix chemical
molecule
 It has a anti-parallel structure

6. • DNA Synthesis
 The DNA bases on each
strand act as a template to
synthesize a complementary
strand
• Recall that Adenine (A)
pairs with thymine (T)
and guanine (G) pairs
with cytosine (C)
 The process is
semiconservative because
each new double-stranded
DNA contains one old
strand (template) and one
newly-synthesized
complementary strand
DNA Replication
A
G
C
T
G
T
C
G
A
C
A
G
C
T
G
T
C
G
A
C
A
G
C
T
G
T
C
G
A
C
A
G
C
T
G
T
C
G
A
C
T
C
G
A
C
A
G
C
T
G

7. Anti-parallel
structure of DNA
It’s a long Double
Helix

8. 8
DNA Replication
 Begins at origins of replication
 Two strands open forming Replication
Forks (Y-shaped region)
 New strands grow at the forks
Replication
Fork
Parental DNA Molecule
3’
5’
3’
5’
copyright cmassengale

9. DNA replication forks
 Because the DNA
chromosome is so
long it needs
multiple replication
forks working at
the same time.

10. Anti-parallel structure (DNA)
 This diagram shows
a lot of stuff not yet
discussed.
 What you need to
understand at this
point
 When the enzyme
travels along a
strand of DNA it
travels in the 3’ to 5’
direction of the
original strand
DNA Polymerase travels 3’ to 5’ on original/template
strand

11. 3’ end has a free deoxyribose
5’ end has a free phosphate
The enzyme:
 can only build the new strand in
the 5’ to 3’ direction
 Thus scans the template strand in
3’ to 5’ direction
DNA Replication

13. Okazaki Fragments
An Okazaki fragment is a relatively short fragment of DNA
created on the lagging strand. Each Okazaki fragment is
joined together by DNA ligase after the primers have been
removed.
http://www.youtube.com/ Replication animation
http://www.youtube.com/ crash course in DNA (summary)

14. Semi-conservative replication
A simple idea really…
 S.C.R is simply the
formation of two double helix
molecules where each
molecule contains one of the
original strands and one new
strand of nucleotides
(daughter strand).
 This helps to minimise the
errors made in replication
as each molecule contains
a copy of the original
nucleotide sequence.

15. Checking for errors
 In general, enzymes
(DNA polymerases)
are extremely
accurate. Even so,
some DNA
polymerases also have
proofreading ability;
they can remove
nucleotides from the
end of a strand in
order to correct
mismatched bases.
You don’t need to know the
detail in this box. But read it
cause it is interesting ;-)

16. Review Questions
ENZYME FUNCTION
Helicase
DNA Polymerase III
DNA Polymerase I
RNA Primase
DNA Ligase
http://www.biologycorner.com DNA Quizs

17. Review Question
2011 NCEA exam – Q1 (b)
 When DNA is replicated, each of the parent
strands acts as a template.
 Explain why there is a difference in the way in
which the parallel strands of DNA are replicated.
 You may use a labelled diagram to support your
answer.

18. Review Question
 What words are hidden under the yellow boxes?
Click to show answer
Click to show answer
Click to show answer
Click to show answer

19. Review Question
 Add the 3’ and 5’ labels to the diagram
Click for answers

20. Review Question – taken from 2008 NCEA Paper