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Lecture

BIOB11H3 Lecture Notes - Pyrimidine Dimer, Replisome, Radiography


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Aarti Ashok

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Lecture #21
Date: Tuesday, July 20th/2010
DNA Replication and Repair
- DNA is a double helix with anti-parallel strands
- Backbone is made of sugar-phosphate chains; in middle base-pairs
- The structure of DNA suggests a ^copying_ mechanism; parent strands + new strands yield
^daughter DNAs_
Models proposed for DNA replication
- Conservative model
o daughter DNA composed of 2 new strands
o parent strands join back together to form parent DNA
- Semi-conservative model
o Daughter DNA composed of 1 parent strand and 1 new strand
- Dispersive model
o Daughter DNA composed of pieces from both new strands and parent strands
o Hybrid molecules (still relatively 50% parent and 50% new; even though they are
patches from each)
The Meselson-Stahl experiment
- E.coli grown in medium with 15N Æcertain isotope of nitrogen (ammonium chloride = source of
nitrogen)
- Shifted to second medium
- DNA extracted after 1 round of replication; centrifuged to separate by density
o 20 mins
- SNA extracted after 2 rounds of replication; centrifuged
o 40 mins
- Separated using a cesium chloride gradient; less dense at top, more dense at bottom
- Found that semi-conservative model holds true for DNA replication
DNA replication is semi-conservative
- True for bacteria and eukaryotic cells
A closer look at replication
- Bidirectionality of DNA replication at each origin/bubble
- In bacterial cells:
o DNA polymerases replicate DNA; require a primer strand (=RNA strand needed to
initiate replication) and all 4 deoxyribonucleotides to replicate DNA
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