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DNA Replication Pt. 2 10.5 Pgs. 188 – 189 Objective: I can describe in EXCRUCIATING detail how DNA is replicated. File:/dna-replication1.swf
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1) DNA unwinds and unzips Enzyme: Helicase Single-stranded DNA stabilized Single-Strand Binding Proteins (SSB) Replication Fork – site where unzips Template strand = strand that is read (a.k.a. parent strand) Replication fork
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2) Primer added to each template strand Made of RNA (few hundred nucleotides) Provides place to start adding DNA Enzyme: Primase
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3) Build new DNA using old as template (semi-conservative replication) Enzyme: DNA Polymerase (III) Starts at primer (can’t start on single-strand) Adds matching nucleotide Can only build 5’ to 3’ (read 3’ to 5’) Polymerase adds nucleotides to 3’ end of newly made strand
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Because DNA is anti-parallel (and DNA polymerase only builds 5’ to 3’)… Leading strand: build continuously Lagging strand: build discontinuously Segments in lagging strand called Okazaki fragments http://highered.mheducation.com/sites/0072943696/student_view0/chapter3/animation__dna_replication__quiz_1_.html http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA+Replication+Fork File animations
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4) Primers (made of RNA) removed and replaced with DNA nucleotides Enzyme: RNase H (a.k.a. DNA polymerase I) 5) Gaps between newly constructed strands filled in – making last phosphodiester bond (because DNA Polymerase can only…) Enzyme: DNA ligase http://www.youtube.com/watch?v=4PKjF7OumYo http://www.youtube.com/watch?v=4jtmOZaIvS0
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Enzymes are good at proofreading and repairing mistakes Especially DNA Polymerase (III) However, proofreading not perfect This is OK, because this leads to mutation, and with mutation… Can have evolution http://highered.mheducation.com/sites/9834092339/student_view0/chapter14/proofreading_function_of_dna_polymerase.html
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