Molecular Biology
DNA Replication
Processes
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Replication
Transcription
Translation
protein synthesis
Is the Genetic Material DNA or Protein?
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Chromosome contains
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60% protein
40% DNA
Evidence that DNA can Transform
Bacteria
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experiment by Frederick Griffith, 1929
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S bacteria are
pathogenic
R bacteria are not
CONCLUSIONS
 R forms were transformed
into pathogenic S bacteria
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Transforming Principle
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Change in the genotype and phenotype due to
the assimilation of external DNA by a cell.
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In 1944 Oswald Avery et. al announced that the
transforming agent is DNA
Additional Evidence that DNA is
Genetic Material
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Erwin Chargaff (1947)
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noticed that the amount of Adenine is equal to the
amount of Thymine and the same between Cytosine
and Guanine
Chargaff Rules
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A-T
C-G
Viral DNA can Program Cells
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Alfred Hershey & Martha Chase (1952)
used radioactive S and P to trace the fates of proteins and
DNA of a T2 phages (virus)
they wanted to see which of
these molecules entered the
bacteria and could reprogram
the cell to make more phages.
labels proteins
labels DNA
Conclusions: DNA entered bacteria but proteins did not
therefore the DNA is what functions as genetic material
Research Conclusion
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By 1950’s DNA was accepted as the genetic
component
3-D structure of the DNA was unknown
A-T and C-G are the accepted combinations
Structural Model of DNA
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Rosalind Franklin (1950’s)
x-ray diffraction photo of DNA
James Watson & Francis Crick (April
1953)
Replication
DNA
DNA
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Semi conservative
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Prokaryotic replication
Eukaryotic replication
Replication
DNA
DNA
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DNA builds in the order 5'
5'
3'
5'
3'
5'
3'
3'
5'
3'
5'
3'
5'
3'
3'
5'
3'
5'
Prokaryotic Replication
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circular double strand DNA
Origin of replication
Bidirectional movement
Termination of replication
and form 2 circular dsDNA
DNA
E. coli
Eukaryotic Replication
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DNA replication begins at specific sites
Eukaryotic Replication (5’ to 3’)
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Chemical direction: 5’
3’
phosphate
OH
OH
phosphate
Enzymes in Eukaryotic Replication
makes RNA primer,
it will be used as a
"starting point" for the
DNA
stabilizes DNA
Helps by breaking, swiveling
and rejoining DNA strands
untwists the DNA
Eukaryotic Replication
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leading strand is made continuously along the 5' 3' order
lagging strand is made discontinuously against the 5'
3'
3'
5'
5'
3'
Okazaki fragments
Synthesizing the New Strand of DNA
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DNA polymerases
requirements
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a primer
template
Primer
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DNA cannot initiate
replication
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primer is formed
10 nucleotide stretch of
RNA
primase
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enzyme that makes
the primer
Antiparallel Elongation
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DNA polymerases
DNA ligase
 bonds the 3' of the 2nd fragment with the 5' of the 1st fragment
Okazaki fragment
 includes RNA as starter
Shortening of DNA
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DNA polymerase can only add
a nucleotide to the 3' end of
the DNA
lack of 3' puts it in
disadvantage
even for the use of the
Okazaki fragment, there is no
3' carbon for the nucleotide
addition
therefore DNA becomes smaller
every time it replicates
Telomeres
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Located at eukaryote DNA ends of
chromosomes
Prokaryote chromosome doesn't have ends, it is circular,
so no problem during replication
It is not genes, rather repetitions
protect genes from being eroded through multiple rounds
of DNA replication
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typical, TTAGGG (G rich)
about 100-1000 repetitions
believed that these repetitions shorten with increasing age
Telomerase
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Enzyme that makes telomeres
It is present in
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Quantity: the more telomerase, the longer it takes to
reach cell division limit (lots = immortal)
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germ line cells (zygotes with long telomeres)
Stem cells
(some) cancerous cells
Enzyme makes telomeres without templates
Telomere length maybe a limiting factor in the life span of
tissues and organisms
Telomerase enzyme = RNA + protein
The End