IB Topics 3 and 7
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DNA replication is a means to produce new
molecules that have the same base
sequence
Occurs during interphase of the cell cylce
DNA replication is semi-conservative
◦ The parent DNA strand separates into two
◦ Each strand serves as a template for new
complementary strands
◦ The new double helix is half original
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1. Bacteria were grown in a medium
containing nitrogen 15 (N15) for several
generation
2. If the medium contains no other nitrogen
source, the E. coli will use N15 and
incorporate it into their DNA
3. Eventually, they will only have N15
4. Once the E. coli had only N15 they were put
into a growing medium contain only N14
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5. N15 is heavier than N14 making new
incorporation of nitrogen easy to distinguish
6. The differences were measured according
to the densities of the new strands
-Explain what is
happening in this
diagram.
-Which color is N14?
N15?
-Which model of DNA
replication is
illustrated?
-Why aren’t there
double helices with two
red strands in the 2nd
generation?
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Occurs in a 5’→3’ direction
◦ The 5’ end of the ‘new’ nucleotide attached to the
3’ end of the nucleotide before it
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DNA is unwound and unzipped by the
enzyme helicase
DNA polymerase III creates links between the
nucleotides
◦ It creates a strand that is complementary to the
original strand)
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Replication takes place at several places on
one double helix at the same time
Once the double helix is unwound and
unzipped the two parent strands become the
leading and lagging strands
This happens because the strands are
antiparallel
3’
5’
5’
3’
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The lagging strand runs from 3’ to 5’
1. Before DNA replication begins there must
be an RNA primer
2. The RNA primer is made by adding
complimentary RNA nucleotides to the
lagging DNA strand by hydrogen bonding of
the bases
◦ RNA has uracil instead of thymine
3. RNA primase (an enzyme) then bonds
the RNA nucleotides together
RNA primer
RNA primase
3’
5’
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4. After RNA primer is in place DNA
nucleotides are added by DNA polymerase III
5. Eventually, the segment of DNA will run
into another RNA primer
6. The DNA segments are called Okazaki
fragments
Okazaki fragment
RNA primer
DNA polymerase III
3’
5’
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7. Once Okazaki fragments are formed DNA
polymerase I replaces the RNA primer with
DNA nucleotides
8. DNA ligase links the fragments together
5’
3’
New strand
Parent strand
3’
5’
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In the lagging strand DNA replication is
discontinuous
DNA is replicated in segments that become
joined together
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Replication is continuous
There are no fragments
Helicase unwinds and unzips the double helix
DNA polymerase III adds nucleotides in the
direction of 5’→3’
◦ DNA polymerase only works in the direction of 5’→3’
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Lagging strand
◦ Formed in short segment of 100-200 nucleotides
(these are the Okazaki fragments)
◦ Grows in direction of 5’→3’ because DNA polymerase III
only works in the 5’→3’
◦ 1. Helicase unwinds and unzips the parent DNA
◦ 2.RNA primer is formed by RNA nucleotides that are
joined together by RNA primase
◦ 3. DNA polymerase III bonds DNA nucleotides to the
RNA primer
◦ 4. DNA polymerase I replaces the RNA primer with DNA
nucleotides
◦ 5. DNA ligase joins the Okazaki fragments together
Leading strand
◦ 1. Helicase unwinds and unzips
◦ 2. DNA polymerase III adds complimentary DNA nucleotides
The leading and lagging strand are determined by the 5’
and 3’ end of the strand.
5’
3’
Which template
will be the
leading strand?
3’
5’
5’
3’
3’
5’
McGraw Hill
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Explain DNA replication.
Use p. 284-289 to help you.
Include drawings and annotations to aid your
explanation.