NATIONAL QUALIFICATIONS CURRICULUM SUPPORT
Human Biology
and Biology
Unit 1: How Does DNA Replicate?
Teacher’s Notes
[HIGHER]
The Scottish Qualifications Authority regularly reviews
the arrangements for National Qualifications. Users of
all NQ support materials, whether published by
Learning and Teaching Scotland or others, are
reminded that it is their responsibility to check that the
support materials correspond to the requirements of the
current arrangements.
Acknowledgement
Learning and Teaching Scotland gratefully acknowledges this contribution to the National
Qualifications support programme for Human Biology and Biology.
The publishers gratefully acknowledge permission to use the following sources: figures 4.17
and 4.18 from ‘Biology’, by Martin Rowland, Nelson Thornes, reproduced with the permission
of Nelson Thornes Ltd from Bath Advanced Biology, Martin Rowland, ISBN 978-0-17438425-0; figure 4a from Proceedings of the National Academy of Sciences, Vol 44, ‘The
Replication of DNA in Escherichia Coli’ by M Meselson and FW Stahl, p 675
http://www.pnas.org/content/44/7/671.full.pdf+html, image courtesy of Matthew Meselson;
Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid (Reprinted
from Nature, April 25, 1953) Author(s): CRICK, F. H. C.; image from
http://www.achievement.org/achievers/wat0/large/wat0-001.jpg © Academy of Achievement;
image of Watson and Crick from http://www.flickr.com/photos/ludens/2414798676 © A
Barrington Brown/Science Photo Library; text ‘The Replication of DNA in Escherichia Coli by
Matthew Meselson and Franklin W Stahl, Proceedings of the National Academy of Sciences, p.
671, text courtesy of Matthew Meselson; image of DNA from
http://ocw.mit.edu/courses/biology/7-a12-freshman-seminar-structural-basis-of-geneticmaterial-nucleic-acids-fall-2005/chp_dna.jpg © National Library of Medicine
Every effort has been made to trace all the copyright holders but if any have been inadvertently
overlooked, the publishers will be pleased to make the necessary arrangements at the first
opportunity.
© Learning and Teaching Scotland 2011
This resource may be reproduced in whole or in part for educational purposes by educational
establishments in Scotland provided that no profit accrues at any stage.
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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Contents
Introduction
4
Suggested approach
5
Revision: What is the structure of DNA?
6
Hypothesising: How might DNA replicate?
8
Experimental procedure: How can we find out how DNA replicates?
10
Prediction: What would you expect to happen?
13
Analysis: Which model of DNA replication is supported?
15
Assessment: How does DNA replicate?
16
Extension: How was DNA polymerase discovered?
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
How does DNA replicate?
Introduction
The following is a case study that is designed to support students to answer
the question ‘How does DNA replicate?’ It is assumed that they already know
the structure of DNA and you now want them to fi nd out how DNA makes
copies of itself during cell division.
The case study is designed to provide students with an opportunity to find out
the answer to the question themselves, using the real experimental data from
the scientists who first successfully answered it in 1958.
It is intended that by completing this case study students should not only
learn the mechanism by which DNA replicates, but should also have had the
opportunity to develop their scientific enquiry skills.
Learning outcomes
Higher Biology
1. b. (i) DNA can direct its own replication.
Higher Human Biology
2. a. (iii) Replication of DNA.
Useful weblinks
 http://www.sumanasinc.com/webcontent/animati ons/content/meselson.html
 http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/007243
7316/120076/bio22.swf::Meselson%20and%20Stahl%20Experiment
 http://www.learnerstv.com/animation/animation.php?ani=20&cat=biology
 http://www.dnalc.org/view/15331-Proposed-models-of-DNA-replicationMatthew-Meselson-.html
 http://www.dnalc.org/view/15880-Models-of-DNA-replication.html
 http://www.dnalc.org/view/15879-Semi-conservative-replication.html
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
© Learning and Teaching Scotland 2011
HOW DOES DNA REPLICATE?
Suggested approach
The case study is intended to provide a challenging learning experience for
students and could be used in a variety of ways, depending on the students
and/or the context.
The following is one possible suggested format and assumes 60-minute
lessons:
Revision: This could be used as an introductory lesson or provided as a
pre-case study homework task.
Hypothesising to analysis: These sections may be completed in a single
lesson, but are very likely to extend into a second.
Assessment and extension: The research for the assessment is likely to take
one lesson, with another for producing the resource. Both of these could be
completed as homework/independent study.
Students are likely to encounter some difficulty in analysing the results from
Meselson and Stahl’s paper. This challenge is intentional and is included to
provide students with an opportunity to develop their scientific thinking
skills.
If students are finding the task to be overly challenging it might help to
prompt them towards the relevant areas of the results. The full figure from
Meselson and Stahl’s paper is included deliberately to provide an opportunity
to interpret real evidence. This, however, may prove to be too challenging for
some. In this case the following diagram may prove useful:
Generation
Result
1
2
Lighter
Heavier
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Revision: What is the structure of DNA?
In April 1953 Francis Crick and James D.
Watson had a paper published in the Journal
Nature entitled ‘A Structure for Deoxyribose
Nucleic Acids’.
In this paper they wrote:
‘We wish to put forward a radically different
structure for the salt of deoxyribose nucleic
acid. This structure has two helical chains
each coiled around the same axis.’
Watson and Crick (1953)
Watson & Crick
Image from Preoccupation
Use this information and your prior learning to draw an annotated
diagram of Watson and Crick’s structure of DNA.
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HOW DOES DNA REPLICATE?
So, we know the structure of DNA – but why does it replicate?
Why does DNA need to be able to copy itself?
 Cell division – mitosis/meiosis
 Maintain constant number of chromosomes in cells
 Ensure all cells contain the same genes
DNA replication occurs as part of a process that occurs in nearly every cell in
our bodies.
Name this process and summarise it as a diagram in the space below.
Mitosis
46 chromosomes (23 from each
parent)
DNA replication
96 sister chromatids generated
Nuclear envelope disassembles…
Dissolve is the wrong term
Chromosomes align at middle of
cell
Sister chromatids pulled apart
Use Cytokinesis rather than ‘cell
division’ which is misleading
Two daughter cells formed each
with 46 chromosomes
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Hypothesising: How might DNA replicate?
In the same 1953 paper, Watson and Crick went on to say:
‘If an adenine forms one member of a pair, on eit her chain, then…the other
member must be thymine; similarly for guanine and cytosine…It has not
escaped our notice that the specific pairing we have postulated immediately
suggests a possible copying mechanism for the genetic material. ’
Watson and Crick (1953)
DNA can direct its own replication, but how does this happen? Watson and
Crick hinted at a possible mechanism, but they were not able to supply any
experimental evidence to support their theory. What do you think? How does
one DNA molecule turn into two?
Perhaps an entire new DNA molecule is made from an ‘old’ DNA molecule ,
like this:
DNA
REPLICATION
‘old’ DNA
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‘old’ DNA ‘new’ DNA
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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In this hypothesis, the
two new DNA strands
occur together in one
daughter cell, the two
old in the other.
Conservative model
HOW DOES DNA REPLICATE?
There are at least two other possible hypotheses to explain how n ew DNA
strands might be made. Draw these in the space below.
DNA
REPLICATION
New and old DNA occur in
both daughter cells. The
old and new DNA form
their own strands.
Semi-conservative model
DNA
REPLICATION
New and old DNA occur in
both daughter cells. The
old DNA is distributed
amongst all strands in the
daughter cells.
Dispersive model
Once you have drawn the other models, describe them and try to find out
their names.
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Experimental procedure: How can we find out how DNA
replicates?
Matthew Meselson and Franklin W. Stahl set
out to answer this very question. In 1958
they published a paper that outlined their
experimental procedure for determining the
mechanism of DNA replication. They
introduce the paper by saying:
‘Hypotheses for the mechanism of DNA
replication differ in the predictions they
Meselson & Stahl
make concerning the distribution among
Image from achievement.org
progeny molecules of atoms derived from
parental molecules. Radioisotopic labels have been employed in experiments
bearing on the distribution of parental atoms amo ng progeny molecules in
several organisms. We anticipated that a label which imparts to the DNA
molecule an increased density might permit an analysis of this distribution by
sedimentation techniques.’
Meselson and Stahl (1958)
What does this mean? Look up any words you do not understand and
explain in your own words what Meselson and Stahl are saying in the
above quote.
The predictions made about how DNA replication happens are different
because they say that the new molecules can be arranged in different ways.
Isotopes have been used in experiments to follow what happens to the atoms
in several organisms. We will be able to trace what happens to the atoms in
the DNA molecules as the isotopes will make some DNA heavier, causing it to
sink in a centrifuge experiment.
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HOW DOES DNA REPLICATE?
Meselson and Stahl’s experiment relied on the different properties of the two
isotopes of nitrogen, 14 N and 15 N. Isotopes can be distinguished on the basis
of their mass: 14 N is lighter than 15 N; neither of these isotopes is radioactive.
Why is it important that these isotopes are not radioactive?
If the isotopes were radioactive they could damage the DNA, interfering with
replication and damaging the organism.
Meselson and Stahl grew bacteria in a medium where their nitrogen source,
used to synthesise nucleotides and eventually DNA, was the ammonium ion
(NH 4 + ) containing heavy 15 N. After several generations, all the DNA in these
bacteria would be heavy.
Where are the nitrogen atoms in DNA?
The bases in DNA (adenine, thymine, cytosine and guanine) all contain
nitrogen.
The parental bacteria were then placed in a fresh medium containing light
( 14 N) ammonium ions and allowed to divide and c opy their DNA just once to
produce generation 1. A sample removed now from this beaker would
contain generation 1 DNA.
Some of the generation 1 bacteria were then allowed to divide once more in
14
N medium to produce generation 2 bacteria.
Bacteria
cultured in
medium
containing
heavy
nitrogen
(15N)
Bacteria
transferred
to medium
containing
light
nitrogen
(14N)
DNA sample taken
after 20 minutes.
Generation 1
DNA sample taken
after 40 minutes.
Generation 2
To find out which form of replication is occurring, Meselson and Stahl
isolated and separated the DNA from generation 1 and generation 2. This was
done by density gradient centrifugation. This technique allows the separation
of molecules on the basis of their density.
After centrifugation, the tubes were irradiated with ultraviolet light. DNA
absorbs this light so the position of the DNA in the centrifuge tube shows up
as a black band.
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Light 14 N DNA is found at the top of the centrifuge tub e and heavy 15 N DNA
is located at the bottom of the tube. Any mixture of 14 N and 15 N DNA is
found in the centre of the tube.
light 14N DNA
14N
and 15N hybrid DNA
heavy 15N DNA
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Prediction: What would you expect to happen?
Before you find out which of the models described above is the correct one,
you must first work out what actual results Meselson and Stahl would have
obtained for each of the predicted models.
In the table below, draw the expected centrifuge tube for each of the
predictions for generation 1 and generation 2. The centrifuge tubes for
the conservative model have been done for you.
Prediction
Model
Conservative
Semiconservative
Dispersive
Generation 1
Generation 2
On the following page you will find the actual results from Meselson and
Stahl’s experiments.
On their diagram the heavier DNA is found on the right.
They conducted a range of experiments. You will need to identify which
of two results show generations 1 and 2.
Compare Meselson and Stahl’s results with your own predictions above
to determine which of the models of DNA replication their experiment
supports.
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Analysis: Which model of DNA replication is supported?
Which is the correct model of DNA replication?
Semi-conservative
Explain how you arrived at this conclusion.
 Generation 1 contains only one band of DNA, which is a mixture of
the two isotopes of nitrogen. This eliminates the conservative model ,
which predicted two bands, one at 14 N and one at 15 N.
 Generation 2 contains two bands of DNA: one shows a mixture of the
two isotopes of nitrogen and the other shows the presence of 14 N.
This eliminates the dispersive model, which predicted one band of
mixed DNA.
 The evidence supports the semi-conservative model of DNA
replication, which predicted two bands of DNA for generation 2, one
a mixture of the two isotopes of nitrogen and the other at 14 N, which
matches the experimental evidence.
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Assessment: How does DNA replicate?
You now need to demonstrate your understanding of how DNA replicates a nd
take your understanding further.
Your task is to prepare a resource that could be used by students learning
about DNA replication using the following as your title:
How does DNA replicate?
It is up to you to choose how to present your understanding but through your
resource you must:
 demonstrate a clear understanding of semi -conservative DNA replication
 explain why the data from Meselson and Stahl’s experiment supports semiconservative DNA replication over other hypotheses
 outline the steps involved in semi-conservative DNA replication.
You should consider the following questions when preparing your resource:






Why and when does DNA replicate?
What is the model of DNA replication?
What evidence supports this model of DNA replication?
How does the structure of DNA relate to the mechanism of replication?
Which enzymes catalyse DNA replication?
What is the sequence of steps involved in DNA replication?
Use the space below to begin to plan your resource.
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HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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HOW DOES DNA REPLICATE?
Extension: How was DNA polymerase discovered?
Now that you know how the model of DNA replication was tested, what about
the enzyme which catalyses DNA replication – DNA polymerase? What do
we know about this enzyme? Who discovered it? When did they discover it?
What methods did they use?
Find out what you can about the discovery of DNA polymerase and
summarise your findings in the space below.
Useful weblinks
 http://www.jbc.org/content/278/37/34733.full?maxtoshow=&HITS=10&hit
s=10&RESULTFORMAT=&searchid=1&FIRSTINDEX=30&resourcetype
=HWCIT
 http://www.dnalc.org/resources/nobel/kornberg_a.html
HOW DOES DNA REPLICATE? (H, HUMAN BIOLOGY AND BIOLOGY)
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