Slides covering material from Topic 2.7 (and some AHL material from 7.1) of the updated IB Biology syllabus for 2016 exams

1. DNA Replication (2.7 & 7.1 HL)
IB Diploma Biology
Essential Idea: The structure
of DNA enables it to create
identical copies of itself

2. 2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.

3. Semi-Conservative?
2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.
DNA replication is…

4. Semi-conservative?
2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.

5. 2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.

6. Each time DNA is copied, the new
double stranded molecule consists
of one old template strand plus a
new complementary strand made
from previously free bases
2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.

7. A = T and C = G,
they are complementary
The way the molecules fit
together makes it very unlikely
that they will bond with the
wrong partner.
So the genetic code is faithfully
copied during replication
When things do go wrong, we
have a mutation!
2.7.1 The replication of DNA is semi-conservative and depends on
complimentary base-pairing.

8. 2.7.12 Analysis of Meselson and Stahl’s results to obtain support for the theory of
semi-conservative replication of DNA / 7.1.1 DNA structure suggested a mechanism
for DNA replication.
• Watson & Crick’s 1953 paper on DNA
structure ended by suggesting a
semi-conservative model for DNA
replication…
• Matthew Meselson and Franklin
Stahl designed what is often called
the “most beautiful experiment in
biology” to prove their hypothesis
about DNA replication
http://www.learnerstv.com/animation/biology/con20ani.swf

9. 2.7.12 Analysis of Meselson and Stahl’s results to obtain support for
the theory of semi-conservative replication of DNA.
http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535
::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and
%20Stahl%20Experiment
• E. coli bacteria were grown in two
different isotopes of Nitrogen:
• Bacteria grown in N-14 isotope
had ‘lighter’ DNA
• Bacteria grown in N-15 isotope
had ‘heavier’ DNA
• N-15 bacteria were transferred into N-
14 solution and samples were
collected every 20 minutes (time of E.
coli replication cycle)
• Samples were centrifuged to separate
DNA by weight (see right)

10. Helicase
• The ‘-ase’ ending indicates it
is an enzyme
• This family of proteins varies,
but are often formed from
multiple polypeptides and
doughnut in shape
2.7.2 Helicase unwinds the double helix and separates the two strands
by breaking hydrogen bonds.

11. • Unwinds the DNA Helix
• Separates the two polynucleotide strands by breaking the hydrogen
bonds between complementary base pairs
• ATP is needed by helicase to both move along the DNA molecule and to
break the hydrogen bonds
• The two separated strands become parent / template strands for the
replication process
2.7.2 Helicase unwinds the double helix and separates the two strands
by breaking hydrogen bonds.

12. DNA Polymerase
• The ‘-ase’ ending indicates it
is an enzyme
• This protein family consists of
multiple polypeptides
• The polymerization reaction is
a condensation reaction
2.7.3 DNA polymerase links nucleotides together to form a new strand,
using the pre-existing strand as a template.

13. • Free nucleotides are nucleoside triphosphates
• The extra phosphate groups carry energy which is
used for formation of covalent bonds
• DNA polymerase always moves in
a 5’ to 3’ direction
• DNA polymerase catalyzes the
covalent Phosphodiester bonds
between sugars and phosphates
• DNA Polymerase proofreads the
complementary base pairing so
mistakes are very rare, occurring
approx. once in every billion bases
2.7.3 DNA polymerase links nucleotides together to form a new strand,
using the pre-existing strand as a template.

14. 2.7.3 DNA polymerase links nucleotides together to form a new strand,
using the pre-existing strand as a template.

15. The Polymerase Chain Reaction (PCR)
Synthetic method of amplifying specific sequences of DNA.
Useful when only a small amount of DNA is available for
testing e.g. crime scene samples of blood, semen, hair, etc.
http://highered.mcgraw-
hill.com/olc/dl/120078/micro15.swf
2.7.9 Use of Taq DNA polymerase to produce multiple copies of DNA
rapidly by the polymerase chain reaction (PCR).
• Processes artificially recreates DNA replication
• Taq DNA Polymerase is used for PCR
• Comes from heat-resistant bacterium, Thermus
aquaticus, that lives in hot springs…
• Can resist denaturation at high temperatures
required to separate DNA strands in PCR
• Copies up to 1000 nucleotides / minute

16. The PCR Process:
PCR occurs in a thermal cycler and involves 3 steps:
1. Denaturation: DNA sample is heated to 95⁰C
to break hydrogen bonds and separate it into
two strands
2. Annealing: DNA sample is cooled to 54 ⁰C,
allowing primers attach to opposite ends of the
target sequence
3. Elongation: A heat-tolerant DNA polymerase
(Taq) copies the strands
• One cycle of PCR yields two identical copies of
the DNA sequence
• A standard reaction of 30 cycles would yield
1,073,741,826 copies of DNA (230)
2.7.9 Use of Taq DNA polymerase to produce multiple copies of DNA
rapidly by the polymerase chain reaction (PCR).

17. 2.7.9 Use of Taq DNA polymerase to produce multiple copies of DNA
rapidly by the polymerase chain reaction (PCR).

18. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.
DNA Polymerase can
only add nucleotides
to the 3’ end…

19. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.

20. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.

21. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.

22. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.
http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316
/120076/bio23.swf::How%20Nucleotides%20are%20Added%20in%20DNA%20Replication

23. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.

24. 7.1.3 DNA replication is continuous on the leading strand and discontinuous on the
lagging strand / 7.1.5 DNA polymerases can only add nucleotides to the 3’ end of a
primer.

25. 7.1.4 DNA replication is carried out by a complex series of enzymes.
DNA Polymerase III
DNA Gyrase (Topoisomerase)
stabilizes the helix as it is unwound

26. 7.1.4 DNA replication is carried out by a complex series of enzymes.

27. 7.1.4 DNA replication is carried out by a complex series of enzymes.

28. 7.1.4 DNA replication is carried out by a complex series of enzymes.

29. Enzyme Function
DNA Gyrase
(Topoisomerase)
Stabilizes DNA helix as it is unwound by Helicase
Single Stranded
Binding Proteins
Hold unzipped, single-stranded sections of DNA
apart during replication
Helicase
Unwinds DNA helix and unzips strands by breaking
Hydrogen bonds
DNA Polymerase III Adds new DNA nucleotides in the 5’  3’ direction
DNA Primase
Adds primers of RNA nucleotides to the lagging
strand as starting points for replication
DNA Polymerase I Replaces RNA primers with DNA nucleotides
DNA Ligase Joins Okazaki fragments on lagging stand
7.1.4 DNA replication is carried out by a complex series of enzymes.

30. 7.1.4 DNA replication is carried out by a complex series of enzymes.
http://www.stolaf.edu/people/giannini/flas
hanimat/molgenetics/dna-rna2.swf
http://sites.fas.harvard.edu/~biotext/
animations/replication1.swf

31. 7.1.9 Use of nucleotides containing dideoxyribonucleic acid to stop
DNA replication in preparation of samples for base sequencing.
DNA Sequencing:
• Unknown DNA sequence mixed with DNA
nucleotides and enzymes needed for replication
• Dideoxyribnonucleotides with different
fluorescent markers are added
• These modified nucleotides stop replication at
the point they are added to the DNA since new
nucleotides cannot be attached to their 3’ end
• Fragments are separated by size and analyzed
by fluorescence

32. 7.1.9 Use of nucleotides containing dideoxyribonucleic acid to stop
DNA replication in preparation of samples for base sequencing.

33. Bibliography / Acknowledgments
Jason de Nys
Chris Paine