Nucleic acid chemistry part 3 : DNA replication

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Nucleic acid chemistrypart 3 DNA replication
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DNA replication, as predicted by Watson Crick

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Unzipping the DNA helix
During replication, the double helix is
unzipped as a result of DNA topoisomerases (afte
r action of helicases). The replication fork is
therefore able to proceed along the molecule
without the helix having to rotate.
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Drawing showing that the unwinding motion
(curved arrows) of the daughter branches of a
replicating circle, lacking positions at which
free rotation can occur, causes overwinding of
the unreplicated portion.
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Bidirectional DNA replication of (A) a circular
bacterial chromosome (B) a linear eukaryotic
chromosome
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The mode of action of Type I and Type II DNA
topoisomerases.

(A) a Type I topoisomerase makes a nick in one
strand of a DNA molecule, passes the intact
strand through the nick, and reseals the gap.
(DL 1 one superhelical winding is
changed) (B) a Type II topoisomerase makes a
double-stranded break in the double helix,
creating a gate through which a second segment
of the helix is passed. (DL 2 two
superhelical windings are changed)
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The processes of catenation and decatenation,
catalyzed by DNA gyrase.
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Three possible schemes for DNA replication. (For
the sake of clarity, the DNA molecules are
drawn as ladders rather than helices.)

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The Meselsohn-Stahl experiment (1958)

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(Dispersive replication would give a single
intermediate band with decreasing density when
the number of generations increases.)
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Variations (A) replication of the human
mitochondrial genome. (B) rolling-circle
replication as used by various
bacteriophages
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Overview of cellular DNA replication.
(Bidirectional) Replication of a linear segment
of a chromosome is shown. In circular
procaryotic genomes, the two replication forks
eventually would fuse with one another. In
linear eucaryotic chromosomes, these two
replication forks would fuse with replication
forks from adjacent origins.

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The Escherichia coli origin of replication

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A Saccharomyces cerevisiae (yeast) origin of
replication.

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Origin of replication of ColE1 plasmid

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Origin of replication of plasmid pSC101

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Three mechanisms by which RNA primers are
synthesized in single-stranded viral DNA
molecules.

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Replication of bacteriophage l at different
stages of development.

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Priming of DNA synthesis in bacteria (A) and in
eukaryotes (B)

In eukaryotes, the primase forms a tight
complex with DNA polymerase a, which adds about
20 20 deoxynucleotides to the first 8-12
nucleotides synthesized by the primase.
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Priming and synthesis of the lagging-strand copy
during DNA replication in E. coli
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A model for parallel synthesis of the leading-
and lagging-strand copies by a dimer of DNA
polymerase III enzymes

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The simultaneous synthesis of both leading and
lagging progeny strands by a dimeric complex of
DNA polymerase III holoenzyme in association
with the primosome and helicases (the collection
of which is known as a replisome)

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Replication of double-stranded circular
DNA molecules through the rolling-circle
mechanism. The active force that unwinds the
5-tail is the movement of the replicome
propelled by its helicase components.

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Some of the replication genes of E. coli

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The series of events involved injoining up
adjacent Okazakifragments during DNA replication
in E. coli.

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The flap endonuclease FENI cannot initiate
primer degradation because its activity is
blocked by the triphosphate group present at the
5 end of the primer.

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Two models for completion of lagging-strand
replication in eukaryotes.

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The role of terminator sequences during DNA
replication in E. coli.

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In eukaryotes telomeres at the end of the
linear chromosomal DNAs gt telomerase
maintaining the ends Regulation -
replication factories are not broken down, but
are preserved in the cell. - not all origins
ussed at random, but little is known how their
use is regulated. - cell cycle G1 - S -
G2 - M - replication is
coordinated with the cell cycle so that two
copies of the genome are available when the
cell divides. - genome replication
during S phase - checkpoints are
necessary the key cell cycle checkpoints are
at the periods immediately
before entry into S and M phase (the
cell cycle may become arrested if e.g. the
DNA is extensively damaged)