3.4 DNA Replication
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Topic 3: Chemistry of Life
3.4 DNA Replication
Orange book: pg. 54-60
Green book: pg. 41-42
3.4.1 Explain DNA replication in terms of unwinding the double helix and
separation of the strands by helicase, followed by the formation of the new
complementary strands by DNA polymerase.(pg. 54, 41)
3.4.2 Explain the significance of complementary base pairing in the
conservation of the base sequence of DNA. (pg. 57, 42)
3.4.3 State that DNA replication is semi-conservative. (pg. 60, 42)
3.4.1 DNA Replication
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3.4.1 Explain DNA replication in terms of unwinding the double helix and
separation of the strands by helicase, followed by the formation of the new
complementary strands by DNA polymerase.
Orange book: pg. 54
Green book: pg. 41
To do:
Visit the following website
 http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter14/animations.html#
In your green books answer the following questions:
Why does DNA replicate?
During which phase of the cell cycle does DNA replicate?
From the board copy down the simple diagram it illustrate DNA replication.
Why does DNA replicate?
Cells must prepare for a cell division by doubling the DNA content of the cell in
a process called DNA replication. This process doubles the quantity of DNA and
also ensures that there is an exact copy of each DNA molecule.
When does DNA replicate?
In the nucleus during interphase of the cell cycle in preparation for mitosis the
DNA will replicate. During interphase there is a nuclear membrane which
separates the fluid of the nucleus from the cytoplasm. The DNA is in the form
of chromatin.
Among the variety of molecules present in the nucleus are two types that are
particularly important for the process of DNA replication:
 enzymes needed for the replication – these include helicase and a
group of enzymes collectively called DNA polymerase.
 free nucleotides – these are nucleotides that are not yet bonded and
are formed floating free in the nucleus, some contain adenine, some
thymine, some cytosine and some guanine (free nucleotides are more
correctly called nucleoside triphospates – which is explain for HL).
The process of replication
The basic idea of DNA replication is evident from its base pairing, but the way
in which DNA is organised in a double stranded α-helix introduces some
complications. The steps of the replication process are as follows:
1. The double helix unwinds into two single strands
2. Like a zipper, an enzyme called DNA helicase opens up a short segment of
the helix, exposing its nitrogenous bases. The point where one strand of
DNA is ‘unzipped’ and separates from its complementary strand is called a
replication fork. DNA helicase breaks the hydrogen bonds which hold
together complementary base pairs.
3. Enzymes called DNA polymerase move along the open strands, reading
the exposed bases. And like matchmakers, arrange ‘marriages’ with
complementary free nucleotides. This is known as complimentary base
pairing. If the DNA polymerase finds the sequence TCG, it assembles AGC
opposite it.
4. As one DNA polymerase molecule moves away from the replication fork
replicating one strand of the open DNA, another DNA polymerase moves in
on the other strand and moves in the opposite direction, replicating the
other strand.
Thus from the old DNA molecule, two new ones are made. Each new DNA
consists of a new strand from free nucleotides and an old strand conserved
from the parent DNA, hence the name semi-conservative replication.
*Continue to scroll down – there are more illustrations of DNA replication.
Semi-Conservative DNA Replication
At the replication fork, DNA helicase (not shown) unwinds the double helix and
exposes the bases. DNA polymerases begin assembling new bases across from
the existing ones, moving away from the replication fork on one strand and away
from it on the other strand.
The result is two DNA double helices, each composed of one strand of the
original DNA and one newly synthesised strand.
Replication of DNA
The two strands of the double helix separate by breaking the hydrogen bonds
(shown as dotted lines) between nucleotides. New, complementary nucleotides
attach at the proper sites, and a new strand of DNA is synthesised along-side
each of the original stands. Arrows indicate hydrogen bonds forming again
between pairs of bases.
3.4.2 Complementary Base Pairing
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3.4.2 Explain the significance of complementary base pairing in the conservation
of the base sequence of DNA.
Orange book: pg. 57
Green book: pg. 42
To do:
In your own words explain why complementary base pairing is so important in
the replication of DNA.
Genetically Identical Cells
The pattern of DNA replication ensures that two identical copies of DNA are
produced from one. For growth, repair and replacement it is essential that the
cells being made are genetically identical to the parent cell. All somatic cells
(body cells) of an organism contains exactly the same DNA, the characteristics
of a cell depends on which genes are ‘switched on’. As a growing child is making
more bone cells, it is important that each new cell as all the information it needs
to ensure it will carry out its function properly. The new bone cells will be
identical to the parent cells and will act as bone cells because they will have the
same genes ‘switched on’ as the parent cell.
Fine Control of Replication – Semi-Conservative Replication
When a cell divides it needs to provide each new cell with an exact copy of its
DNA. Since DNA controls all cellular function, this replication process must be
very exact.
The law of complementary base pairing shows that we can predict the base
sequence of one DNA strand if we know the sequence of the other. More
importantly, it enables a cell to reproduce one strand based on information in
the other.
In the diagram above where replication has already taken place, the two strands
are absolutely identical to each other. This is because the original doublestranded molecule had complementary pairs of nucleotides and it was the
complementary nucleotides that used the unzipped single-stranded areas as
templates.
3.4.3 Semi-Conservative Replication
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3.4.3 State that DNA replication is semi-conservative.
Orange book: pg. 60
Green book: pg. 42
To do:
The following animation demonstrates experiments carried out to support the
theory of semi-conservative replication:
http://www.sumanasinc.com/webcontent/animations/content/meselson.html
Copy the diagram from the board to explain semi-conservative replication
DNA replication results in two identical DNA double helices, each consisting of
one ‘old’ and one ‘new’ strand – hence “semi-conservative”.
In 1958, Matthew Meselsohn and Kranklin Stahl carried out experiments
involving the bacterium E. coli. They used two isotopes of nitrogen and
determined what proportions of the isotopes were present in strands of DNA
after one and two replications. After one replication, each daughter DNA
molecule possessed one strand with heavy isotope of nitrogen (15N) and one
strand with the light isotope of nitrogen (14N) – a hybrid had formed. After a
second replication, the DNA molecules were either hybrid or without the heavy
isotope of nitrogen. This evidence shows that the replication process of DNA is
semiconservative.