GENETICS ESSENTIALS Concepts and Connections SECOND EDITION GENETICS ESSENTIALS Concepts and Connections SECOND EDITION Benjamin A. Pierce © 2013 W. H. Freeman and Company CHAPTER 9 DNA Replication and Recombination

CHAPTER 9 OUTLINE 9.1 Genetic Information Must be Accurately Copied Every Time a Cell Divides, All DNA Replication Takes Place in a Semiconservative Manner, Bacterial Replication Requires a Large Number of Enzymes and Proteins, Eukaryotic DNA Replication Is Similar to Bacterial Replication but Differs in Several Aspects, Recombination Takes Place Through the Breakage, Alignment, and Repair of DNA Strands, 250

THE CASE OF THE HAPPY TREE Compound found in this plant- cmaptothecin- has anti cancer properties Shown to inhibit one of the crucial enzymes in the process of DNA replication- the topoisomerase

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. 9.1 Genetic Information Must Be Accurately Copied Every Time a Cell Divides

9.2 ALL DNA REPLICATION TAKES PLACE IN A SEMICONSERVATIVE MANNER Proposed DNA Replication Models: Conservative replication model Dispersive replication model Semiconservative replication

Meselson and Stahl’s Experiment: Two isotopes of nitrogen: 14 N common form; 15 N rare heavy form E. coli were grown in a 15 N media first, then transferred to 14 N media Cultured E. coli were subjected to equilibrium density gradient centrifugation 9.2 All DNA Replication Takes Place in a Semiconservative Manner

Modes of Replication Replicons: Units of replication. Replication origin Theta replication: circular DNA, E.coli; single origin of replication forming a replication fork, and it is usually a bidirectional replication. 9.2 All DNA Replication Takes Place in a Semiconservative Manner

Modes of Replication In eukaryotes DNA Replication

Requirements of replication A template strand Raw materials (substrates) Enzymes and other proteins 12.2 All DNA Replication Takes Place in a Semiconservative Manner

Direction of Replication DNA polymerase add nucleotides only to the 3 end of growing strand The replication can only go from 5  All DNA Replication Takes Place in a Semiconservative Manner

- 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. 9.2 All DNA Replication Takes Place in a Semiconservative Manner

9.3 BACTERIAL REPLICATION REQUIRES A LARGE NUMBER OF ENZYMES AND PROTEINS Bacterial DNA Replication Initiation: 245 bp in the oriC. (single origin replicon); an initiation protein. Unwinding Initiator protein DNA helicase Single-strand–binding proteins (SSBs) DNA gyrase (topoisomerase)

Bacterial DNA Replication Primers: an existing group of RNA nucleotides with a 3′-OH group to which a new nucleotide can be added. It is usually 10 ~ 12 nucleotides long. Primase: RNA polymerase 9.3 Bacterial Replication Requires a Large Number of Enzymes and Proteins

Bacterial DNA Replication Elongation: Carried out by DNA polymerase III Removing RNA primer: DNA polymerase I Connecting nicks after RNA primers are removed: DNA ligase Termination: when replication fork meets or by termination protein (Tus) 9.3 Bacterial Replication Requires a Large Number of Enzymes and Proteins

The fidelity of DNA Replication Proofreading: DNA polymerase I: 3′  5′ exonuclease activity removes the incorrectly paired nucleotide. Mismatch repair: correct errors after replication is complete. 9.3 Bacterial Replication Requires a Large Number of Enzymes and Proteins

Eukaryotic DNA Replication Eukaryotic DNA polymerase: 9.4 Eukaryotic DNA Replication Is Similar to Bacterial Replication but Differs in Several Aspects

Eukaryotic DNA Replication Origin licensing (licensing factor) The location of DNA replication within the nucleus: DNA polymerase is fixed in location and template RNA is threaded through it. Replication at the ends of chromosomes: Telomeres and telomerase Fig Eukaryotic DNA Replication Is Similar to Bacterial Replication but Differs in Several Aspects