Chapter 12 Outline

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

6. 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.

8. 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

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

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

15. 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′.

18. 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

19. 12.3 The Replication of DNA Requires a Large Number of Enzymes and Proteins

20. 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

21. 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

22. 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

26. 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

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