Transcript DNA Replication and Recombination - HMartin

PowerPoint® Lecture Presentation for
Concepts of Genetics
Ninth Edition
Klug, Cummings, Spencer, Palladino
Chapter 11
DNA Replication and Recombination
Lectures by David Kass with contributions from
John C. Osterman.
Copyright
© 2009©Pearson
Education,
Inc.
Copyright
2009 Pearson
Education,
Inc.
Section 11.1
• DNA Is Reproduced
by Semiconservative
Replication
• The complementarity
of DNA strands
allows each strand to
serve as a template
for synthesis of the
other.
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Section 11.1
• 3 possible modes of
DNA replication are
possible:
• conservative
• semiconservative
• dispersive
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Section 11.1
• The Meselson-Stahl experiment
demonstrated that:
• DNA replication is semiconservative
• each new DNA molecule consists of one old
strand and one newly synthesized strand
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Figure 11.3
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Figure 11.4
Section 11.1
• The TaylorWoods-Hughes
experiment
demonstrated that
DNA replication is
semiconservative
in eukaryotes.
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Section 11.1
• DNA replication
begins at the origin of
replication and is
bidirectional rather
than unidirectional.
• A replicon is the
length of DNA that is
replicated following
one initiation event at
a single origin.
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Section 11.2
• DNA Synthesis in Bacteria Involves Five
Polymerases, as well as Other Enzymes
• DNA polymerase I catalyzes DNA
synthesis and requires a DNA template
and all four dNTPs.
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Section 11.2
• Chain elongation occurs in the 5' to 3'
direction by addition of one nucleotide at a
time to the 3' end.
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Section 11.2
• DNA polymerases I, II, and III can
elongate an existing DNA strand (called a
primer) but cannot initiate DNA synthesis.
• All three possess 3' to 5' exonuclease
activity.
• But only DNA polymerase I demonstrates
5' to 3' exonuclease activity.
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Section 11.2
• DNA polymerase III is the enzyme
responsible for the 5' to 3' polymerization
essential in vivo.
• Its 3' to 5' exonuclease activity allows
proofreading.
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Section 11.2
• Polymerase I is believed to be
responsible for:
• removing the primer
• the synthesis that fills gaps produced during
synthesis
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Section 11.2
• DNA polymerases I, II, IV, and V are
involved in various aspects of repair of
damaged DNA.
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Section 11.2
• DNA polymerase III has 10 subunits
whose functions are shown in Table 11.3.
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Section 11.3 Many Complex Tasks Must Be
Performed during DNA Replication
• 7 key issues that must be resolved
during DNA replication:
• unwinding of the helix
• reducing increased coiling generated during
unwinding
• synthesis of a primer for initiation
• discontinuous synthesis of the second strand
• removal of the RNA primers
• joining of the gap-filling DNA to the adjacent
strand
• proofreading
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Section 11.3 – Unwinding DNA Helix
• DnaA binds to the
origin of replication
(oriC) and is
responsible for the
initial steps in
unwinding the
helix.
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Section 11.3 - RNA Primer
• To elongate a polynucleotide chain, DNA
polymerase III requires a primer with a free 3'OH group.
• Enzyme primase synthesizes an RNA primer
that provides the free 3'-OH required by DNA
polymerase III
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Section 11.3
• As replication fork
moves, only 1 strand
can serve as
template for
continuous DNA
synthesis—the
leading strand.
• Opposite lagging
strand undergoes
discontinuous DNA
synthesis.
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Section 11.3
• Both DNA strands are synthesized
concurrently by looping the lagging strand
to invert the physical but not biological
direction of synthesis.
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Section 11.3
• Proofreading and error correction are
an integral part of DNA replication.
• All of the DNA polymerases have 3' to 5'
exonuclease activity that allows
proofreading.
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Section 11.4
• DNA synthesis at a single replication fork:
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Section 11.6
• Eukaryotic DNA Synthesis Is Similar to
Synthesis in Prokaryotes, but More Complex
• In eukaryotic cells:
• there is more DNA than prokaryotic cells
• the chromosomes are linear
• the DNA is complexed with proteins
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Section 11.6
• Eukaryotic chromosomes contain multiple
origins of replication to allow the genome to
be replicated in a few hours.
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Section 11.6
• 3 DNA polymerases are involved in
replication of nuclear DNA.
• 1 involves mitochondrial DNA replication.
• Others are involved in repair processes.
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Section 11.6
• Pol  and d
• major forms of the enzyme involved in initiation
and elongation.
• Pol 
• possesses low processivity.
• functions in synthesis of RNA primers during
initiation on the leading and lagging strands.
• Polymerase switching occurs
• Pol  is replaced by Pol d, which has high
processivity, for elongation.
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Section 11.7
• Telomeres Provide Structural Integrity at
Chromosome Ends but Are Problematic to
Replicate
• Telomeres at the ends of linear
chromosomes consist of long stretches of
short repeating sequences and preserve
the integrity and stability of chromosomes.
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T-Loop in Telomeres
http://www.ncbi.nlm.nih.gov/bookshelf/picrender.fcgi?book=eurekah&part=A74250&blobname=ch516f1.jpg
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Section 11.7
• Lagging strand
synthesis at end of
chromosome is a
problem b/c once
the RNA primer is
removed, there is
no free 3'-hydroxyl
group from which to
elongate.
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Section 11.7
• Telomerase directs
synthesis of the
telomere repeat
sequence to fill gap.
• This enzyme is a
ribonucleoprotein w/an
RNA that serves as
the template for the
synthesis of its DNA
complement.
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Section 11.8
• DNA Recombination, Like DNA Replication,
Is Directed by Specific Enzymes
• Genetic recombination involves:
•
•
•
•
•
endonuclease nicking
strand displacement
ligation
branch migration
duplex separation to generate the characteristic
Holliday structure (chi form)
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Figure 11.18
Section 11.9 Gene Conversion Is a
Consequence of DNA Recombination
• Gene conversion is characterized by
nonreciprocal genetic exchange between
two closely linked genes.
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The End
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