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Chapter 7-2.docx

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BIOL 2000
Joseph Rasmussen

Chapter 7-2 Representations of the DNA double Helix  Each nitrogenous base is in the same plane o Equal planar  Minor and major grooves o Think of a screw-  Thread is the phosphates  Different sequences of DNA help code for proteins Chemical structure of DNA  Strand polarity o 5’ and 3’ ends  Hydrogen bonds between bases o G-C o A-T o Stability  Phosphodiester bond links nucleotides o 5’ to 3’  Strand has directionality o 3’ is the end o 5’ is the beginning o 3’ and 5’ are opposite each other  Sequence of DNA o What is the linear order of the nitrogenous bases  Starting at 5’ to 3’  5’ C A G T 3’  3’ G T C A 5’ Question:  Do the base pairing rules observed within the double helical structure also control the process of DNA replication? o OR  Is replication like photocopying process? Alternative models for DNA replication  Semi conservative o Watson and Crick o One parent o One new strand  Conservative o Photocopy o Two parentals together o Two new strands together  Dispersive replication o Each strand contains 50% new and 50% old Meselson-Stahl Experiment  Strategy: o Distinguish newly synthesized strands from parental strands using different isotope labeling  Different weights o Separate DNAs with different relative densities  Procedure 15 o Labeled parental DNA by growing E.Coli in N medium for many generations  Heavy DNA o Transferred cells to N medium  Lighter o Extracted DNA after 1 and 2 generations o Centrifuge in CsCl gradient  Separates DNA of different densities\  Looking for: o Replication once  See what weights o Replication twice  To come to conclusion  Predicitons o Conservative model  First generation  One parental
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