BIOB11H3 Lecture Notes - Pyrimidine Dimer, Replisome, Radiography

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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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Document Summary

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. Conservative model: daughter dna composed of 2 new strands, parent strands join back together to form parent dna. Semi-conservative model: daughter dna composed of 1 parent strand and 1 new strand. Dispersive model: daughter dna composed of pieces from both new strands and parent strands, hybrid molecules (still relatively 50% parent and 50% new; even though they are patches from each) E. coli grown in medium with 15n certain isotope of nitrogen (ammonium chloride = source of nitrogen) Dna extracted after 1 round of replication; centrifuged to separate by density: 20 mins. Sna extracted after 2 rounds of replication; centrifuged: 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.

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