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DNA REPLICATION_20260106_095953_0000.pdf
- 1. DNA REPLICATION Name: B.Tharumathi Submittedby: B.Tharumathi Submitted To: dr. Sanmuga priya Molecular Biology II Bsc Biotechnology "B"
- 2. 3 DNA replication: • Complementarybasepairingproducessemiconservative replication –Double helix unwinds –Each strand acts as template –Complementary base pairing ensures that T signals addition of A on new strand, and G signals addition of C –Two daughter helices produced after replication
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- 4. 5 •Dispersive replication: •Conservative replication: Experimentalproof of semiconservative replication – three possible models •Semiconservative replication – – Watson and Crick model –The parental double helixremains intact; –both strands of the daughter double helix are newly synthesized –Atcompletion,bothstrands of both double helices contain both original and newly synthesized material.
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- 6. Meselson-Stahl experiments confirm semiconservativereplication Fig. 6.16 7 • Experimentalloweddifferentiationofparentaland newly formed DNA. •Bacteria were grown in media containing either normal isotope of nitrogen (14N) or the heavy isotope (15N). •DNA banded after equilibrium density gradient centrifugation at a position which matched the density of the DNA: – heavy DNA was at a higher density than normal DNA.
- 7. 8 Meselson-Stahl experiments confirm semiconservativereplication •Whenbacteria grownin 15Nweretransferred to normal 14N containing medium, 14 •They checked the composition of the resulting DNA molecules by density gradient centrifugation, –foundan intermediateband, –indicating a hybrid molecule –containing both 14N and 15N DNA. –thenewly synthesizedDNA strandhadthe N while the parental strand had 15N.
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- 10. 11 The mechanism ofDNA replication • • Two basic steps: Two basic components: • Tightly controlledprocess, • Requires: –aset ofproteinsand enzymes, –and requiresenergy in the form of ATP. – Initiation – Elongation. – template – primer. –occursatspecifictimesduring the cell cycle.
- 11. 12 The mechanism ofDNA replication (prokaryotic) • DNApolymerase – – •additional enzymes/proteins – –i) DNAhelicase •unwindsdouble helix –ii) Single-stranded binding proteins – •keep helix open –iii) Primase – •creates RNA primers to initiate synthesis – iv) Ligase – •welds together Okazaki fragments – the enzyme that extends the primer; – Pol III •producesnewstandsofcomplementary DNA – Pol I •fillsingapsbetweennewlysynthesized Okazaki segments
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- 13. 14 Origins of Replication •Replicationproceeds in both directions (bidirectionally) from a single origin of replication on the prokaryotic circular chromosome •Replication proceeds in both directions (bidirectionally) from hundreds or thousands of origins of replication on each of the linear eukaryotic chromosomes.
- 14. 15 Origins of Replication •Bacteriahave 1 origin of replication per one chromosome •They only have one chromosome = 1 origin! Molecular Biology of the Cell, 4th Edition.
- 15. 16 Eukaryotic Origins ofReplication
- 16. 17 Replication Initiation replication • DNAorigin of •Initiator proteins bind •Recruits DNA helicase •Opening of DNA strands Originof Rep
- 17. 18 •Replication Initiation: •Replication isFinished: •Replication Elongation: •DNA polIII •DNA polI removes primer •Fills gap using 3’ends •DNA ligase connects frags •Uses 5’ ends! •Primase and the RNA Primer •Musthave3’ to add to Originof Rep
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