Chapter 16.
DNA
The Genetic Material
Replication
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Copying DNA
 Replication of DNA

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base pairing allows
each strand to serve
as a pattern for a
new strand
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Models of DNA Replication
 Alternative models
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needed to be verified through experimentation
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Semi-conservative replication
1958
 Meselson & Stahl



labeled nucleotides of “parent” DNA strands with heavy
nitrogen = 15N
labeled new nucleotides with lighter isotope = 14N
replicated strands were found to be half 15N & half 14N
“The Most Beautiful Experiment in Biology”
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DNA Replication
 Large team of enzymes coordinates replication
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Enzymes
more than a dozen enzymes & other proteins participate in
DNA replication
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Helicase
 Opens DNA helix enabling replication
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DNA Polymerase III
 Adds nucleotides only to 3’ end
 nucleoside-P-P-P links to sugar-P backbone
 losing 2-P provides energy for bonding
 Does this sound familiar??
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A common thread in biology…
Why 3Ps?
5’
3’
3’
5’
You don’t just
get energy from
ATP → ADP
TTP → TMP
GTP → GMP
CTP → CMP
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Leading & Lagging strands
 Leading strand

continuous synthesis
 Lagging strand
Okazaki fragments
 joined by ligase

 “spot welder”
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Okazaki fragments
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Priming DNA synthesis
DNA polymerase
can only extend an
existing DNA
molecule. Cannot
start a new one.
short RNA primer
is built first on
parent DNA strand
by primase
 RNA primer later
removed by DNA
polymerase I
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Replication fork
DNA
polymerase
ligase
lagging strand
Okazaki
fragments
3’
primase
5’
3’
5’
SSB
5’
3’
helicase
DNA
polymerase
5’
3’
leading strand
direction of replication
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Replication bubble
Adds 1000 bases/second!
Which direction does DNA build?
List the enzymes & their role
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Replication enzymes
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DNA polymerases
 DNA polymerase I
20 bases/second
 editing, repair & primer removal

 DNA polymerase III
1000 bases/second
 main DNA building enzyme
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Editing & proofreading DNA
 At 1000 bases/second,
lots of typos!
 DNA polymerase I
excises mismatched
bases
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reduces error rate
from 1 in 10,000 to 1
in 100 million bases
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Fast & accurate!
 It takes E. coli <1 hour to copy
5 million base pairs in its single
chromosome & divide to form 2
identical daughter cells
 Human cell copies its 6 billion bases &
divide into daughter cells in only few
hours
remarkably accurate
 only ~1 error per 100 million bases
 ~30 errors per cell cycle

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Problems in the end
 Ends of
chromosomes
are eroded
with each
replication
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Telomeres
 expendable,
non-coding
sequences at
ends of DNA


short sequence
of bases
repeated 1000s
times
TTAGGG in
humans
 Telomerase
enzyme in certain
cells
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The “Central Dogma”
 flow of genetic information within a cell
transcription
DNA
RNA
translation
protein
replication
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