Molecular Basis of Inheritance

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Molecular Basis of Inheritance The DNA
molecule How is it replicated? How is it
transcribed? What are the different types of RNA
and what is their contribution to protein
synthesis?
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The discovery of DNA as genetic information
was a long time coming Early 1900s- chromosomes
contained genetic information what was it-
theDNA or the proteins? 1928- the Griffiths
experiment genetic informtation could be
transferred from one organism to another
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WHAT transformed those bacteria? 1944- Avery,
MacLeod and McCarty Took Streptococcus
apart proved transforming molecule was
DNA and not carbohydrate, protein or lipid
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1952- Hershey-Chase experiment Used
viruses very simple only DNA and
protein labeled DNA in one virus and protein in
another Some viruses use RNA- no virus has both
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DNA structure Chargaff same amount of A as T,
and C as G Franklin and Wilkins DNA is a double
helix (Franklin sugar-phosphate backbone)
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DNA replication and repair Watson and Crick
predicted a semiconservative model of
replication Each new molecule of DNA would
contain one new strand paired with an old
strand Confirmed by Meselson and Stahl
experiment several years later
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The process of DNA replication How does it
begin? Bacteria have circular chromosomes eukar
yotes have one or more linear chromosomes In
both, replication starts at specific origins of
replication
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One existing strand serves as the template the
other strand is newly synthesized Nucleotides
are added one by one to the growing strand by
DNA polymerases Free nucleotides are
triphosphates energy comes from the removal of
the two phosphates and their hydrolysis
to monophosphates
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Nucleotides can only be added in one orientation
(5?3)
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Strands are antiparallel. Note orientation of 3
and 5 ends. Nucleotides can be added only to
the 3 end. Therefore one new strand can be
made as a continuous strand, but the other is
made in small pieces.
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ligase
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DNA polymerases can only add nucleotides to a
growing strand they cannot start the
process Primers are needed Primases make the
primer (from ribonucleotides) Many other enzymes
involved helicases, topoisomerases, etc.
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Nucleotide addition occurs rapidly (50/sec in
prokaryotes, 500/sec in eukaryotes Errors
happen Some DNA polymerases have
proofreading capability Several repair
systems have been identified in both prokaryotes
and eukaryotes Some repair single nucleotides,
others larger segments of DNA
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What about the ends?
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Telomeres are special structures on the ends
of eukaryotic chromosomes that help
stabilize them Multiple repeat sequences
TTAGGG These are not genetic sequences, but
protect sequences from being lost Telomeres are
themselves shortened after repeated
replications Telomerase restores them but is not
usually active
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