DNA Replication Pt Pgs. 188 – 189 Objective: I can describe in EXCRUCIATING detail how DNA is replicated. File:/dna-replication1.swf

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

2) Primer added to each template strand  Made of RNA (few hundred nucleotides)  Provides place to start adding DNA Enzyme: Primase

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

 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 File animations

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

 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