Obstacles to DNAPolymeraseDNA is antiparallel No problem on strand that runs 3' to 5Problem encountered with strand that is 5' to 3SolutionSynthesizeleading strand continuouslySynthesize lagging strand discontinuouslyTemplatestrandFormOkazakifragments(100-200ntinhumansNewstrandsSynthesisinthiswaywouldpullstrands inopposite directions
Obstacles to DNA Polymerase DNA is antiparallel No problem on strand that runs 3’ to 5’ Problem encountered with strand that is 5’ to 3’ Solution Synthesize leading strand continuously Synthesize lagging strand discontinuously • Form Okazaki fragments (100-200 nt in humans)
A model for DNA replicationNewprimerDNALagging-polymerasestrandOkazakifragmentsLeadingstrand353'53F5John wiley
A model for DNA replication
Features ofDNA Replication:DNA replication is bidirectional:Bidirectional replication involves two replication forks, which move in oppositedirections·DNAreplicationissemidiscontinuous·Theleading strand copies continuously: The lagging strand copies in segments (Okazaki fragments) which must be joined5LeadingstrandLagging strandParental strandsOkazakifragmentsMovement ofreplicationfork
Features of DNA Replication • DNA replication is bidirectional • Bidirectional replication involves two replication forks, which move in opposite directions • DNA replication is semidiscontinuous • The leading strand copies continuously • The lagging strand copies in segments (Okazaki fragments) which must be joined
DNA replication is semi-discontinuous: One parental strand is copied continuously to give a newly synthesizedcopy, called the leading strand, at each replication fork.: The other parental strand is copied in an intermittent, or discontinuous,mode to yield a set of fragments, called the Okazakifragments. Thesefragments are then joined to form an intact lagging strand.: Overall, each of the two DNA duplexes produced in DNA replicationcontains oneold""and onenew"DNA strand,and halfof thenew strandwas formed by leading strand synthesis and the other half by laggingstrand synthesis
DNA replication is semi-discontinuous • One parental strand is copied continuously to give a newly synthesized copy, called the leading strand, at each replication fork. • The other parental strand is copied in an intermittent, or discontinuous, mode to yield a set of fragments, called the Okazaki fragments. These fragments are then joined to form an intact lagging strand. • Overall, each of the two DNA duplexes produced in DNA replication contains one “old” and one “new” DNA strand, and half of the new strand was formed by leading strand synthesis and the other half by lagging strand synthesis
DNA ReplicationRelax supercoiling by nicking one strand (topoisomerase)Unwind helix (separate2 strands)(helicase)Need proteins to protect as DNA (SSB: single-strand binding protein)RNAprimers synthesizedHeDNAreplicatedRNAremovedandreplacedwithDNAGaps sealedSupercoiling addedEukaryoticSSB
DNA Replication Relax supercoiling by nicking one strand (topoisomerase) Unwind helix (separate 2 strands) (helicase) Need proteins to protect as DNA (SSB: single-strand binding protein) RNA primers synthesized DNA replicated RNA removed and replaced with DNA Gaps sealed Supercoiling added