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BIOL239 Study Guide - Quiz Guide: Dna Polymerase Iii Holoenzyme, Semiconservative Replication, Dna ReplicationPremium

8 pages67 viewsWinter 2017

Department
Biology
Course Code
BIOL239
Professor
Christine Dupont
Study Guide
Quiz

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DNA Replication
The Watson-crick DNA replication model
oThis model proposed a mechanism
whereby DNA replication is
semiconservative: in daughter molecules one strand is conserved
from the parental molecule and the
other is newly synthesized
oDuring replication the strands
separate and used as templates to
make two new daughter strands
What are the three possible models of DNA replication?
oSemiconservative:
DNA molecules split and daughter molecules
form one new strand one old strand
oConservative:
DNA molecule makes two new molecules by itself
oDispersive:
Parts of a new molecule is dispersively used to
form old bits from new bits
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Who proved DNA semi-conservation? How?
oMatthew Meselson and Franklin Stahl
Experimental proof of semiconservative replication
oStudies with controlled isotopic composition
of nucleotides incorporated into daughter DNA strands
oGrew cells with N14 isotope and N15 isotope
so they are incorporated into their DNA.
oDNA was centrifuged so that heavier DNA (N15) was at
the core of tube and lighter DNA (N14) was closer
to the tip.
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o Hybrid DNA was in-between these two areas.
oBy looking at the banding
patterns could rule out the
conservative model as you
would see two clear N15 and
N14 bands
oTo rule out the semiconservative
model of DNA replication they let the
replicated strands to re-replicate
and found that they saw ¼ of the
progeny DNA hybrid and ¾ light
(N14)
The molecular mechanism of DNA
replication
oComplex process which occurs at
a precise moment is cell cycle
oTwo steps: initiation phase and
elongation (copying)
oE. coli DNA replication as a model
The reason E. coli is a very important model organism is because it is very
similar
Difference between prokaryote and eukaryote DNA?
oProkaryotes: Small circular (binary
fission)
Eukaryotes: Large linear
Describe the Initiation phase
oEukaryotes have this at s phase
oOrigin of replication = OriC
oOriC is at a site of A-T base pairs since hydrogen bonds on these bases are easier to
pull apart from each other
oInitiator proteins: Proteins that
recognize the sequence and the
origin and start the strand separation
of the two strands
oDNA Helicase: One one the first
enzymes acting on the replication
bubble and works to unwind the
DNA strand and works in both
directions of the replication bubble
oWhen this bubble gets to a certain
size as it gets bigger, DNA starts to
base pair together as this is the
energetically favorable reaction.
oSingle-stranded- DNA- binding proteins: Stabilize the single strands and stop them
from re-pairing together
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Elongation:
oDNA polymerase
(the enzyme that
builds DNA)
MUST have free
3’ OH to extend
upon and
requires a primer
to begin synthesis
oDNA primase: An enzyme that builds
RNA primer which provides a free 3’ OH
which can be extended on by DNA
polymerase
oAs helicase continues to make the
bubble bigger, DNA Polymerase
III starts to add nucleotides to the
3’OH end of the new strands in the
5’ to 3’ direction
oLeading strand: continuous
synthesis happens here as the
bubble opens up as DNA
polymerase III synthesizes DNA in
the 3’ to 5’ direction
oLagging strand: The discontinuous
synthesis of DNA occurs on the
lagging strand due to the direction
of the synthesis of DNA
polymerase III being in the 3’ to 5’
direction
oOkazaki fragments: DNA- RNA strands that make up the lagging strands,
which are small fragments of DNA due to the synthesis of the lagging strand
oDuring elongation, the RNA primers are removed by DNA polymerase I
oDNA polymerase 1 fills in the gap (still 5’ to 3’)
oDNA ligase seals the 3’ OH and 5’ PO4 nicks by catalyzing the formation of
phosphodiester bonds
oThis completes the formation of the lagging strand
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