BIOL 1010U Lecture Notes - Lecture 12: Nucleoside Triphosphate, Semiconservative Replication, Dna Replication

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21 Jun 2018
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Chapter 12 – DNA Replication
12.1 – DNA Replication
During DNA replication, the parental strands separate and new partners are
made
The two strands of the parental molecule separate and each strand serves as
a model for the synthesis of the daughter strand
As each daughter strand is synthesized, the order of the bases in the
template strand determines the order of the complementary bases added to
the daughter strand
oE.g. Sequence 5’ – ATGC – 3’ specifies the sequence 3’ – TAGC – 5’
This process is called semi-conservative replication
oAfter replication, each new DNA molecule consists of a strand that was
originally part of the parental molecule and a newly synthesized strand
New DNA strands grow by the addition of nucleotides to the 3’ end
The semi-conservative model implies that both daughter strands should grow
in length by the addition of nucleotides near the site where the parental
strands separate, a site called the replication fork
As more and more of the parental strand is unwound and the replication fork
moves forward, both new strands would also grow in the direction of the
replication fork - This scenario is impossible
DNA is anti parallel – one strand, left-to-right, is 5’ to 3’. The other strand is,
left-to-right, 3’ to 5’. Therefore the daughter strands also have opposite
orientations
Near the replication fork, the daughter strand on the bottom of the replication
fork ends in a 3’ hydroxyl and the strand on the top ends in a 5’ phosphate
The strand that terminates in the 5’ phosphate cannot grow in the direction of
the replication fork because DNA only grows in a 5’ to 3’ direction
DNA polymerization occurs when the 3’ hydroxyl at the growing end of the
polynucleotide chain attacks the triphosphate group at the 5’ end of an
incoming nucleotide
oEach of these incoming nucleotides is a nucleotide triphosphate (three
phosphate groups attached to the 5’ carbon of the deoxyribose)
As the incoming nucleotide triphosphate is added to the growing DNA strand,
one of the nucleotide’s high-energy phosphate bonds is broken to release the
outermost two phosphates (called pyrophosphate), and immediately the high-
energy phosphate bond in the pyrophosphate is cleaved to drive the
polymerization reaction forward and make it irreversible
The polymerization reaction is catalyzed by DNA polymerase
oDNA polymerase – an enzyme that is a critical component of a large
protein complex that carries out DNA replication; exists in all
organisms and is highly conserved (they vary little from one species to
another because they carry out an essential function)
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A cell typically contains several different DNA polymerase enzymes, each
specialized for a particular situation but all share the same function in that
they synthesize a new DNA strand from an existing template
In replication DNA, one daughter strand is synthesized continuously and the
other in a series of short pieces (see Fig. 12.5 if unclear)
The daughter strand with its 3’ end pointed towards the replication fork can be
synthesized continuously by adding nucleotides to the 3’ end – called the
leading strand
The strand with its 5’ end pointed toward the replication fork cannot grow
continuously – called the lagging strand
As the replication fork unwinds, the lagging strand grows away from the fork
and forms a stretch of single-stranded DNA (a few hundred to a few thousand
nucleotides, depending on the species)
As the parental DNA continues to unwind, a new daughter strand is initiated
with its 5’ end near the replication fork, and this strand is elongated at the 3’
end as usual
The result is that the daughter strand is synthesized in short, discontinuous
pieces
A new piece is initiated at intervals and each new piece is elongated at its 3’
end until it reaches the piece in front of it
These short pieces are sometimes called Okazaki fragments
A small stretch of RNA is needed to being synthesis of a new DNA strand
Each new DNA strand must being with a short stretch of RNA that serves as
a primer for DNA synthesis
The primer is needed because DNA polymerase can’t begin a new strand on
its own; it can only elongate the end of an existing piece of DNA or RNA
The primer is made by RNA primase
oSynthesizes a short piece of RNA complementary to the DNA template
and does not require a primer of its own
Once the primer has been created, the DNA polymerase adds DNA
nucleotides to the 3’ end of the growing strand
Since DNA polymerase extends off an RNA primer, all new DNA strands have
a short stretch of RNA at their 5’ end
For the lagging strand, there are many primers – one for each of the
discontinuous fragments of newly synthesized DNA
When the DNA polymerase on the lagging strand runs into the RNA primer, a
different DNA polymerase complex takes over, removing the RNA primer and
extending the fragment with DNA nucleotides to fill the space left by the RNA
primer
When the replacement is completed, the adjacent fragments are joined by
DNA ligase (see Fig. 12.6)
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Document Summary

During dna replication, the parental strands separate and new partners are made. The two strands of the parental molecule separate and each strand serves as a model for the synthesis of the daughter strand. As each daughter strand is synthesized, the order of the bases in the template strand determines the order of the complementary bases added to the daughter strand: e. g. Sequence 5" atgc 3" specifies the sequence 3" tagc 5". This process is called semi-conservative replication: after replication, each new dna molecule consists of a strand that was originally part of the parental molecule and a newly synthesized strand. New dna strands grow by the addition of nucleotides to the 3" end. The semi-conservative model implies that both daughter strands should grow in length by the addition of nucleotides near the site where the parental strands separate, a site called the replication fork.

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