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Dna replication1
The document describes DNA replication. It explains that DNA must be copied before cells divide so that new cells have identical DNA. Replication begins at origins of replication and forms replication bubbles or forks. Enzymes such as helicase unwind the DNA strands, primase adds RNA primers, and DNA polymerase adds nucleotides to the 3' ends of the strands. The leading strand is synthesized continuously while the lagging strand is synthesized in short segments called Okazaki fragments that are later joined by ligase. The process results in two new DNA molecules each with one original and one new strand, supporting the semiconservative model of replication.
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Dna replication1
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- 2. 2 DNA StructureDNA Structure •Rosalind Franklin took diffraction x-ray photographs of DNA crystals • In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays copyright cmassengale
- 3. 3 Discovery of DNADiscoveryof DNA StructureStructure • Erwin Chargaff showed the amounts of the four bases on DNA ( A,T,C,G) • In a body or somatic cell: A = 30.3% T = 30.3% G = 19.5% C = 19.9% copyright cmassengale
- 4. 4 Chargaff’s RuleChargaff’s Rule •AdenineAdenine must pair with ThymineThymine • GuanineGuanine must pair with CytosineCytosine • The bases form weak hydrogen bonds G CT A copyright cmassengale
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- 6. 6 DNADNA • Two strandscoiled called a double helix • Sides made of a pentose sugar Deoxyribose bonded to phosphate (PO4) groups by phosphodiester bonds • Center made of nitrogen bases bonded together by weak hydrogen bonds copyright cmassengale
- 7. 7 DNA Double HelixDNADouble Helix NitrogenousNitrogenous Base (A,T,G or C)Base (A,T,G or C) ““Rungs of ladder”Rungs of ladder” ““Legs of ladder”Legs of ladder” Phosphate &Phosphate & Sugar BackboneSugar Backbone copyright cmassengale
- 8. 8 HelixHelix • Most DNAhas aMost DNA has a right-handright-hand twist withtwist with 10 base pairs10 base pairs in ain a complete turncomplete turn • Left twisted DNA is calledLeft twisted DNA is called Z-DNAZ-DNA oror southpawsouthpaw DNADNA • Hot spotsHot spots occur where rightoccur where right and left twisted DNA meetand left twisted DNA meet producingproducing mutationsmutations copyright cmassengale
- 9. 9 DNADNA • Stands for Deoxyribonucleicacid • Made up of subunits called nucleotidesnucleotides • NucleotideNucleotide made of:made of: 1. Phosphate groupPhosphate group 2. 5-carbon sugar5-carbon sugar 3. Nitrogenous baseNitrogenous base copyright cmassengale
- 10. 10 DNA NucleotideDNA Nucleotide O=P-O O PhosphatePhosphate GroupGroup N NitrogenousbaseNitrogenous base (A, G, C, or T)(A, G, C, or T) CH2 O C1 C4 C3 C2 5 SugarSugar (deoxyribose)(deoxyribose) O copyright cmassengale
- 11. 11 Pentose SugarPentose Sugar •Carbons are numbered clockwise 1’ to 5’ CH2 O C1 C4 C3 C2 5 SugarSugar (deoxyribose)(deoxyribose) copyright cmassengale
- 12. 12 DNADNA P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C TA copyright cmassengale
- 13. 13 Antiparallel StrandsAntiparallel Strands •One strand of DNA goes from 5’ to 3’ (sugars) • The other strand is opposite in direction going 3’ to 5’ (sugars) copyright cmassengale
- 14. 14 Nitrogenous BasesNitrogenous Bases •Double ringDouble ring PURINESPURINES Adenine (A)Adenine (A) Guanine (G)Guanine (G) • Single ringSingle ring PYRIMIDINESPYRIMIDINES Thymine (T)Thymine (T) Cytosine (C)Cytosine (C) T or C A or G copyright cmassengale
- 15. 15 Base-PairingsBase-Pairings • Purines onlypair with Pyrimidines • Three hydrogen bonds required to bond Guanine & Cytosine CG 3 H-bonds copyright cmassengale
- 16. 16 T A •Two hydrogenbonds are required to bond Adenine & Thymine copyright cmassengale
- 17. 17 Question:Question: • If thereis 30% AdenineAdenine, how much CytosineCytosine is present? copyright cmassengale
- 18. 18 Answer:Answer: • There wouldbe 20% CytosineCytosine • Adenine (30%) = ThymineAdenine (30%) = Thymine (30%)(30%) • Guanine (20%) = CytosineGuanine (20%) = Cytosine (20%)(20%) • Therefore,Therefore, 60% A-T and60% A-T and 40% C-G40% C-G copyright cmassengale
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- 20. 20 Replication FactsReplication Facts •DNA has to be copiedDNA has to be copied before a cell dividesbefore a cell divides • DNA is copied during theDNA is copied during the SS or synthesis phaseor synthesis phase • New cells will needNew cells will need identicalidentical DNA strandsDNA strands copyright cmassengale
- 21. 21 DNA ReplicationDNA Replication •As the 2 DNA strands open atAs the 2 DNA strands open at the origin,the origin, Replication BubblesReplication Bubbles formform • Prokaryotes (bacteria) have a single bubble • Eukaryotic chromosomes have MANY bubbles Bubbles Bubbles copyright cmassengale
- 22. 22 DNA ReplicationDNA Replication •Begins atBegins at Origins of ReplicationOrigins of Replication • Two strands open formingTwo strands open forming ReplicationReplication Forks (Y-shaped region)Forks (Y-shaped region) • New strands grow at the forksNew strands grow at the forks ReplicationReplication ForkFork Parental DNA MoleculeParental DNA Molecule 3’ 5’ 3’ 5’ copyright cmassengale
- 23. 23 DNA ReplicationDNA Replication •EnzymeEnzyme HelicaseHelicase unwindsunwinds and separates the 2 DNAand separates the 2 DNA strands by breaking thestrands by breaking the weak hydrogen bondsweak hydrogen bonds • Single-Strand BindingSingle-Strand Binding ProteinsProteins attach and keep the 2 DNA strands separated and untwisted copyright cmassengale
- 24. 24 DNA ReplicationDNA Replication •EnzymeEnzyme TopoisomeraseTopoisomerase attaches to the 2 forks of the bubble to relieve stressrelieve stress on the DNADNA moleculemolecule as it separates Enzyme DNA Enzyme copyright cmassengale
- 25. 25 DNA ReplicationDNA Replication •BeforeBefore new DNA strands can form, there must be RNARNA primersprimers present to start the addition of new nucleotides • PrimasePrimase is the enzyme that synthesizes the RNA Primer • DNA polymerase can then add the new nucleotides copyright cmassengale
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- 27. 27 DNA ReplicationDNA Replication •DNA polymeraseDNA polymerase can only addcan only add nucleotides to thenucleotides to the 3’ end3’ end of theof the DNADNA • This causes theThis causes the NEWNEW strand to bestrand to be built in abuilt in a 5’ to 3’ direction5’ to 3’ direction RNARNA PrimerPrimerDNA PolymeraseDNA Polymerase NucleotideNucleotide 5’ 5’ 3’ Direction of ReplicationDirection of Replicationcopyright cmassengale
- 28. 28 Remember HOW theRememberHOW the Carbons Are Numbered!Carbons Are Numbered! O O=P-O O PhosphatePhosphate GroupGroup N Nitrogenous baseNitrogenous base (A, G, C, or T)(A, G, C, or T) CH2 O C1 C4 C3 C2 5 SugarSugar (deoxyribose)(deoxyribose) copyright cmassengale
- 29. 29 Remember the StrandsareRemember the Strands are AntiparallelAntiparallel P P P O O O 1 2 3 4 5 5 3 3 5 P P P O O O 1 2 3 4 5 5 3 5 3 G C T A copyright cmassengale
- 30. 30 Synthesis of theNew DNASynthesis of the New DNA StrandsStrands • TheThe Leading StrandLeading Strand is synthesized as a single strandsingle strand from the point of origin toward the opening replication fork RNARNA PrimerPrimerDNA PolymeraseDNA PolymeraseNucleotidesNucleotides 3’5’ 5’ copyright cmassengale
- 31. 31 Synthesis of theNew DNASynthesis of the New DNA StrandsStrands • TheThe Lagging StrandLagging Strand isis synthesized discontinuouslydiscontinuously against overall direction of replication • This strand is made in MANY short segments It is replicated from the replication fork toward the origin RNA PrimerRNA Primer Leading StrandLeading Strand DNA PolymeraseDNA Polymerase 5 ’ 5’ 3’ 3’ Lagging StrandLagging Strand 5’ 5’ 3’ 3’ copyright cmassengale
- 32. 32 Lagging Strand SegmentsLaggingStrand Segments • Okazaki FragmentsOkazaki Fragments -- series of short segments on the lagginglagging strandstrand • Must be joined together by anMust be joined together by an enzymeenzyme Lagging Strand RNARNA PrimerPrimer DNADNA PolymerasePolymerase 3’ 3’ 5’ 5’ Okazaki FragmentOkazaki Fragment copyright cmassengale
- 33. 33 Joining of OkazakiFragmentsJoining of Okazaki Fragments • The enzymeThe enzyme LigaseLigase joins thejoins the Okazaki fragments together toOkazaki fragments together to make one strandmake one strand Lagging Strand Okazaki Fragment 2Okazaki Fragment 2 DNA ligaseDNA ligase Okazaki Fragment 1Okazaki Fragment 1 5’ 5’ 3’ 3’ copyright cmassengale
- 34. 34 Replication of StrandsReplicationof Strands Replication Fork Point of Origin copyright cmassengale
- 35. 35 Proofreading New DNAProofreadingNew DNA • DNA polymerase initially makesDNA polymerase initially makes aboutabout 1 in 10,0001 in 10,000 base pairingbase pairing errorserrors • EnzymesEnzymes proofread and correctproofread and correct these mistakesthese mistakes • The new error rate for DNA thatThe new error rate for DNA that has been proofread ishas been proofread is 1 in 1 billion1 in 1 billion base pairing errorsbase pairing errors copyright cmassengale
- 36. 36 Semiconservative Model ofSemiconservativeModel of ReplicationReplication • Idea presented byIdea presented by Watson & CrickWatson & Crick • TheThe two strands of the parental molecule separate, and each acts as a template for a new complementary strand • New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA Parental DNA DNA Template New DNA copyright cmassengale
- 37. 37 DNA Damage &RepairDNA Damage & Repair • Chemicals & ultraviolet radiation damage the DNA in our body cells • Cells must continuously repair DAMAGED DNA • Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA • DNA polymerase and DNA ligase replace and bond the new nucleotides together copyright cmassengale
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