Lecture #21 - BioMI 290, Fall 2007, Lecture 21 1 Lecture 21...

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Unformatted text preview: BioMI 290, Fall 2007, Lecture 21 1 Lecture 21 Lecture 21 DNA Replication DNA Replication (DNA Synthesis) (DNA Synthesis) BBOM 11e Sec. 7.5-7.6 Escherichia coli Watson and Crick: It has not escaped our notice that the specific pairing we have postulated immediately suggests a plausible copying Semiconservative ds DNA mechanism for the genetic material. DNA double helix stores information redundantly, once per strand. Separation of the two strands allows new DNA synthesis to occur on each strand, using the other as a template. BioMI 290, Fall 2007, Lecture 21 2 1957 Meselson & Stahl 1. Labeled original DNA with heavy 15 N . 2. Cells grown in 14 N . 3. DNA extracted & nt if d in CsCl ft 1 1 generation Many generations 15 15 15 15 14 14 centrifuged in CsCl after 1 round or more of replication. Semi- conservative Conservative Semi- conservative Semi- conservative Conservative Semi- conservative 15 N 15 N 15 N 15 N 14 N 14 N 15 N 15 N 15 N 14 N 14 N 15 N Conservative Semi-conservative 14 N 15 N 14 N 15 N + 14 N 15 N 14 N 15 N + 14 N Generations after change from 15N to 14N 0.3 0.7 1.1 1.0 1.5 1.9 2.5 2.5 3.0 4.0 BioMI 290, Fall 2007, Lecture 21 3 Main Enzyme: DNA polymerase III holoenzyme 5 Elongation--DNA Polymerase Direction of Synthesis (5=>3) Elongation--DNA Polymerase Direction of Synthesis (5=>3) 5 3 ALL nucleic acid polymerases can ONLY go in one direction, 5 3 (because they all use nucleoside 5-triphosphates as a substrate for synthesis). Last nucleotide of nascent strand Next nucleotide to be added 3 5 3 Fidelity of DNA Replication Fidelity of DNA Replication 1. DNA polymerase III is very accurate in dNTP selection 2. DNA polymerase corrects its mistakes (proofreading, Fig. 7.20) 3. DNA repair enzymes correct mismatches Overall Fidelity ~ 1 mistake in every 10 10 bp synthesized! BioMI 290, Fall 2007, Lecture 21...
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Lecture #21 - BioMI 290, Fall 2007, Lecture 21 1 Lecture 21...

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