DNA Part II:
The "Stuff" of Life
Its Replication &
Its Unending Repair
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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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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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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Created by:
Carol Leibl
Director of Science Programs
The National Math and Science Initiative
Dallas, TX