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DNA Replication: Accuracy, Mechanisms, and Proteins in Genetic Information

This chapter delves into the crucial process of DNA replication, emphasizing the importance of accurate copying during cell division. The semiconservative nature of DNA replication, models proposed for replication, and the different modes of replication are explored. The chapter also discusses the requirements, enzymes, and proteins involved in DNA replication, both in bacterial and eukaryotic cells. Mechanisms such as proofreading and mismatch repair ensuring fidelity in replication are highlighted. Lastly, the replication of telomeres at the ends of chromosomes is also discussed.

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DNA Replication: Accuracy, Mechanisms, and Proteins in Genetic Information

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  1. Chapter 12 Outline • 12.1 Genetic Information Must Be Accurately Copied Every Time a Cell Divides, 316 • 12.2 All DNA Replication Takes Place in a Semiconservative Manner, 316 • 12.3 The Replication of DNA Requires a Large Number of Enzymes and Proteins, 324 • 12.4 Recombination Takes Place Through the Breakage, Alignment, and Repair of DNA Strands, 335

  2. 12.1 Genetic Information Must Be Accurately Copied Every Time a Cell Divides • Replication has to be extremely accurate: • 1 error/million bp leads to 6400 mistakes every time a cell divides, which would be catastrophic. • Replication also takes place at high speed: • E. coli replicates its DNA at a rate of 1000 nucleotides/second.

  3. 12.2 All DNA Replication Takes Place in a Semiconservative Manner

  4. Proposed DNA Replication Models • Conservative replication model • Dispersive replication model • Semiconservative replication

  5. Meselson and Stahl’s Experiment • Two isotopes of nitrogen: • 14N common form; 15N rare heavy form • E. coli were grown in a 15N media first, then transferred to 14N media. • Cultured E. coli were subjected to equilibrium density gradient centrifugation.

  6. Modes of Replication • Replicons: units of replication • Replication origin • Thetareplication: circular DNA, E. coli; single origin of replication forming a replication fork, usually a bidirectional replication • Rolling-circlereplication: virus, F factor of E. coli; single origin of replication

  7. Linear Eukaryotic Replication • Eukaryotic cells; thousands of origins; a typical replicon: 200,000 ~ 300,000 bp in length

  8. Linear Eukaryotic Replication • Requirements of replication: • A template strand • Raw material: nucleotides • Enzymes and other proteins

  9. Linear Eukaryotic Replication • Direction of replication: • DNA polymerase add nucleotides only to the 3′ end of a growing strand. • The replication can only go 5′3′.

  10. Linear Eukaryotic Replication • Direction of replication: • Leading strand: undergoes continuous replication • Lagging strand: undergoes discontinuous replication • Okazaki fragment: the discontinuously synthesized short DNA fragments forming the lagging strand

  11. 12.3 The Replication of DNA Requires a Large Number of Enzymes and Proteins

  12. Bacterial DNA Replication • Initiation: 245 bp in the oriC (single origin replicon); an initiation protein • Unwinding of DNA is performed by Helicase. Gyrase removes supercoiling ahead of the replication fork. Single stranded DNA is prevented from annealing by single stranded binding proteins. • Primers:an existing group of RNA nucleotides with a 3′-OH group to which a new nucleotide can be added; usually 10 ~ 12 nucleotides long Primase: RNA polymerase

  13. Bacterial DNA Replication • Elongation: carried out by DNA polymerase III • Removing RNA primer: DNA polymerase I • DNA ligase: connecting nicks after RNA primers are removed • Termination: when a replication fork meets or by termination protein

  14. Bacterial DNA Replication • The fidelity of DNA replication • Proofreading: DNA polymerase I: 3′5′ exonuclease activity removes the incorrectly paired nucleotide. • Mismatchrepair: correcting errors after replication is complete

  15. Eukaryotic DNA Replication • Eukaryotic DNA polymerase • DNA polymerase a- acts like Primase to initiate • DNA polymerase d- replicates lagging strand • DNA polymerase e- replicates leading strand

  16. Eukaryotic DNA Replication • Replication at the ends of chromosomes: • Telomeres and telomerase

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