Describe the role of
DNA in relation to
gene expression
DNA Replication
Contents
 Why replicate?
 The basics
 The enzymes
 Okazaki fragments
 Fixing errors
 Review Questions
Why replicate?
Stuff that any respectful
Bio student really
should know…
Mitosis and Meiosis
What are they and
where they happen!
When we copy this
100,000,000,000,000 times it
turns into this
 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 base pairs (bp).
Meiosis Revision
exercise
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
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
It’s a long Double Helix
Anti-parallel
structure of DNA
DNA Replication
• 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 newlysynthesized complementary
strand
AT
GC
CG
TA
GC
AT
GC
AT
CG
TA
CG
GC
GC
T A
GC
DNA Replication
 Begins at origins of replication
 Two strands open forming Replication Forks (Y-
shaped region)
 New strands grow at the forks
3’
5’ Parental DNA Molecule
3’
8
copyright cmassengale
Replication
Fork
5’
T G G T A C A G C T A G T C A T CG T A C CG T
T GG T A C A G C T A G T C A T CG T A C CG T
An enzyme “unzips” or opens up the DNA strand
DNA replication forks
 Because the DNA
chromosome is so long
it needs multiple
replication forks
working at the same
time.
Anti-parallel structure (DNA)
 When an
enzyme travels
along a strand
of DNA it
travels in the 3’
to 5’ direction
of the original
strand
DNA Replication
3’ end has a free deoxyribose
5’ end has a free phosphate
An enzyme:
 can only build the new strand in
the 5’ to 3’ direction
 Thus scans the template strand in
3’ to 5’ direction
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.
Crash course in DNA replication:
http://www.youtube.com/watch?v=teV62zrm2P0
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.
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 ;-)
DNA Replication video
 http://www.youtube.com/watch?v=teV62zrm2P0
Review Questions
http://www.biologycorner.com DNA Quizs
ENZYME
Helicase
DNA Polymerase III
DNA Polymerase I
RNA Primase
DNA Ligase
FUNCTION
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.
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
Review Question
 Add the 3’ and 5’ labels to the diagram
Click for answers
Review Question –
taken from 2008 NCEA Paper