DNAReplicationMaking an exact duplicate of the DNA involves 30 differentenzymesBegins at an origin of replicationHelicase unwinds and unzips the DNA double helixCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display(b)Replicationforks16
16 DNA Replication • Making an exact duplicate of the DNA involves 30 different enzymes • Begins at an origin of replication • Helicase unwinds and unzips the DNA double helix Replication forks (b) Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or display
DNA ReplicationAn RNA primer is synthesized at the origin of replicationbyPrimaseDNA polymerase Ill adds nucleotides in a 5'to 3directionCopyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display(b)Replicationforks17
17 DNA Replication • An RNA primer is synthesized at the origin of replication by Primase • DNA polymerase III adds nucleotides in a 5′ to 3′ direction Replication forks (b) Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or display
DNAReplicationDNA polymerase Ill adds nucleotides in a 5'to 3'direction- Leading strand - synthesized continuously in 5'to 3'direction- Lagging strand - synthesized 5' to 3' in short segments;overall direction is 3'to 5Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display(b)Replicationforks18
18 DNA Replication • DNA polymerase III adds nucleotides in a 5′to 3′direction – Leading strand – synthesized continuously in 5′ to 3′ direction – Lagging strand – synthesized 5′ to 3′ in short segments; overall direction is 3′ to 5′ Replication forks (b) Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or display
DNAReplicationDNA polymerase I removes the RNA primers andreplacesthemwithDNAWhen replication forks meet, ligases link the DNAfragments along the lagging strandSeparation of the daughter molecules is completeCopyright The McGraw-Hill Companies, Inc. Permis sion required for reproduction or display(b)Replicationforks19
19 DNA Replication • DNA polymerase I removes the RNA primers and replaces them with DNA • When replication forks meet, ligases link the DNA fragments along the lagging strand • Separation of the daughter molecules is complete Replication forks (b) Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or display
OverallBacterialDNA ReplicationCopvriaht@TheMcGraw-HillCompanies.Inc.PermissionreguiredforreproductionordisplayForks4.Beforesynthesisof thelagging strandcanstart,aprimasefirstconstructsashortRNAprimertodirecttheDNA5.AsecondpolymerasepolymeraselLSynthesiscanproceed(DNApolymerase)actsonlyinshortsectionsandproducesontheOkazakifragments6.Openspaces inthenePsegments of RNA primer and new DNAbyremovingtheprimerslagging strandarefilledcalled Okazaki fragments.in bya ligase that adds(a)thecorrectnucleotides3.Thetemplateforthelaggingstrandrunsakitheoppositedirection(3'to5)andmustagmentbereplicatedbackwardsawayfromthe5'replication forksotheDNApolymerase3canadd the nucleotides in the necessary5'to3'arrangement.NickLagging strand synthesis3'3Y5NTN42.Thetemplatefortheleading strand (bottom)iscorrectlyorientedfortheDNApolymeraselllto1:Thechromosometobeaddnucleotidesinthe5'to3'directiontowardsthereplicatedis continuouslyreplicationfork,soitcanbesynthesizedasaunwoundbyahelicase,continuous strand.Notethatdirection of synthesis2formingareplicationforkreferstotheorderofthenew strand (red).withtwotemplatestrands.Lagging strand synthesis3553136840906999103942838384983Key:佳Template strandPrimase丰NewstrandDNApolymerase IIHRNAprimerDNApolymerase1LigaseHelicaseDaughtercellDaughtercell(b)
Overall Bacterial DNA Replication Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or display. 4.Before synthesis of the lagging strand can start, a primase first constructs a short RNA primer to direct the DNA polymerase III. Synthesis can proceed only in short sections and produces segments of RNA primer and new DNA called Okazaki fragments. 3.The template for the lagging strand runs the opposite direction (3′ to 5′) and must be replicated backw ards away from the replication fork so the DNA polymerase can add the nucleotides in the necessary 5′ to 3′ arrangement. 5. A second polymerase (DNA polymerase I) acts on the Okazaki fragments by removing the primers. 6. Open spaces in the lagging strand are filled in by a ligase that adds the correct nucleotides. 1. The chromosome tobe replicated is continuously unw ound by a helicase, forming a replication fork w ith tw o template strands. 2. The template for the leading strand (bottom) is correctly oriented for the DNA polymerase III to add nucleotides in the 5′ to 3′ direction tow ards the replication fork, so it can be synthesized as a continuous strand. Note that direction of synthesis refers to the order of the new strand (red). Lagging strand synthesis Lagging strand synthesis Template strand New strand RNA primer Helicase Primase DNA polymerase III DNA polymerase I Ligase Key: 5′ 3′ 3′ 5′ 3′ 5′ 5′ 3′ 3′ 5′ 2 4 3 5 6 1 Nick Forks (b) Daughter cell Daughter cell (a)