IB›IB Biology HL›Mind mapsD1.1 DNA replicationIB Biology HL: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsWhy and howMakes exact copies for growth, reproduction, repairSemi-conservative: one old strand, one newHelicase unwinds and breaks hydrogen bondsDNA polymerase links nucleotides to the templateComplementary base pairing keeps copying accurateDirectionStrands are antiparallelNew nucleotides join the 3' end of the strandNew DNA grows 5' to 3'Polymerase needs a primer with a free 3' endTwo strandsLeading: continuous, towards the forkLeading needs its RNA primer onceLagging: discontinuous, made away from the forkLagging forms Okazaki fragmentsEach fragment needs a new primerReplicationcopying DNAA–TC–G5'3'EnzymesPrimase makes short RNA primersPol III adds DNA nucleotides, makes most of both strandsPol I replaces RNA primers with DNALigase seals Okazaki fragments togetherPol III proofreads: cuts mismatches at the 3' endPCR and gels95 °C separates strands55 °C primers bind either side of target72 °C Taq polymerase extendsCopies double each cycle: 2ⁿDNA is negative; shorter fragments move furtherMore markers → fewer false matchesExam tipsNew strand grows 5'→3'; template is read 3'→5'Say why Taq survives PCR heatNo ligase → fragments stay unjoined