Download DNA Structure and Replication

DNA Part II:
The "Stuff" of Life
Its Structure, Replication,
& Unending Repair
Structure of a Nucleotide
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Sides of the Ladder
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Hydrogen Bonding and Nitrogenous Bases
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Hydrogen Bonding and Nitrogenous
Bases
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Hydrogen Bonding and Nitrogenous
Bases
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Hydrogen Bonding and Nitrogenous
Bases
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Forming the Double Helix
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DNA Forming Chromosomes
Structure in eukaryotes.
•the DNA is wrapped around
proteins called histones
forming nucleosomes.
•This forms a fiber known as
chromatin.
•This forms a coil within a
coil.
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Replication of DNA
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Possible Scenarios for DNA Replication
Experiment of Meselson and Stahl
Matt Meselson (left) and Frank Stahl (right) in 1958
demonstrated that replication was semi-conservative using
radioactive nucleotides with dividing bacteria supporting
Watson’s and Crick’s hypothesis
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Proof for DNA Replication
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Simple Model Proposed for DNA Replication
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Prokaryotic DNA Replication
The prokaryotic
chromosome attaches to
the plasma membrane.
The DNA is then
replicated in both
directions.
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Eukaryotic DNA Replication
DNA replication occurs
simultaneously in many
locations along the very
long eukaryotic
chromosomes.
Three replication bubbles are visible along the
DNA within this cultured Chinese hamster
cell. The arrows indicate the directions of
DNA replication at the two ends of the
bubble.
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Helicases
Helicases are enzymes responsible for the unwinding
of the DNA molecule. They unwind the DNA in
both directions
Releasing Stress in the DNA Molecule
Since DNA is a double helix, there will be
tension in the DNA strand that causes it to
tangle as it is unwound by the helicase.
The enzymes topoisomerase I and II are
responsible for relieving that stress by
clipping one or two strands of the DNA.
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Adding Nucleotides as Triphosphates
Nucleotides are always added on as triphosphates. When
the nucleotides are added then two phosphates are cleaved
off making a pyrophosphate.
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Synthesis ALWAYS occurs in the 5 to 3 direction!
When a DNA molecule is being synthesized , the nucleotides
are added as triphosphates, and two phosphates are removed.
Nucleotides are always added to the 3 end!
In Need of a Primer!
DNA polymerase must
always attach the complementary nucleotide to a 3
end of the deoxyribose sugar
molecule.
So, in the very beginning a
small RNA primer must be
laid down in order to start the
process of DNA replication.
Primase is the enzyme
responsible for this.
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Putting Down a RNA Primer
RNA nucleotides (red pentagons) are being
laid down by primase before DNA polymerase
begins DNA replication.
The Lagging Strand
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The Lagging Strand and Ligase
This animation, shows the leading strand being
synthesized followed by the lagging strand. The
enzyme named ligase ties them together.
Function of Telomeres
Once DNA has been replicated,
there is one problem. The usual
replication machinery provides no
way to complete the 5 ends after
the RNA primer is removed, so
repeated rounds of replication
produce shorter and shorter DNA
molecules.
To compensate for this repeated
shortening process, repetitive
sequences of DNA are added.
These are noncoding sequences and
called telomeres.
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More About Telomeres
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Maintaining Telomere Length in
Gametes and Germ Cells
In gametes, the shortening of
telomeres would cause serious
problems. If chromosomes of
gametes became shorter each time
during replication, then essential
genes would eventually be missing.
An enzyme complex called
telomerase catalyzes the lengthening
of telomeres in gametes.
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Telomerase Function
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DNA Repair
Errors in DNA replication
occur about 1 in every
10,000 base pairs. Not
bad, but with 6 billion
bases being replicated that
amounts to 60,000
mistakes every time a cell
divides.
DNA repair systems repair
about 99% of these
mistakes.
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A Recap!
Once More With Feeling!
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Experiment of Meselson and Stahl
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Topoisomerase Animation
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