Lecture Connections 25 | DNA Metabolism © 2009 W H Freeman and Company CHAPTER 25 DNA Metabolism Key topics: – DNA replication – DNA repair – DNA recombination What is DNA Metabolism? • While functioning as a stable storage of genetic information, the structure of DNA is far from static: – A new copy of DNA is synthesized with high fidelity before each cell division – Errors that arise during or after DNA synthesis are constantly checked for, and repairs are made – Segments of DNA are rearranged either within a chromosome or between two DNA molecules giving offspring a novel DNA • DNA metabolism consists of a set of enzyme catalyzed and tightly regulated processes that achieve these tasks The Substrate that Encodes its Own Metabolisms The Meselson-Stahl Experiment • The Meselson-Stahl experiment was about the origin of the two strands in each of the daughter genomes • Cells were grown on a medium containing only 15N isotope until all their DNA became fully 15N labeled • Cells were then switched to 14N medium and allowed to divide once • CsCl density gradient centrifugation was used to determine the mass of genomic DNA before and after each round of replication DNA Replication is Semiconservative • The Meselson-Stahl experiment showed that the nitrogen used for the synthesis of new dsDNA becomes equally divided between the two daughter genomes • This suggests a semiconservative replication mechanism Replication of Circular DNA is Bidirectional • Both strands are replicated simultaneously Methylation of DNA and Repair (2) • The cleaved strand with incorrect nucleotide is degraded by exonucleases and rebuilt by DNA polymerase Base Excision Repair DNA Recombination • Segments of DNA can rearrange their location – within a chromosome – from one chromosome to another • Such recombination is involved in many biological processes – Repair of DNA – Segregation of chromosomes during meiosis – Enhancement of generic diversity • In sexually reproducing organism, recombination and mutations are two driving forces of evolution • Recombination of co-infecting viral genomes may enhance virulence and provide resistance to antivirals Homologous Recombination • Genetic exchange occurs between two molecules that share an extended region of nearly identical sequences Holliday Intermediate Between Two Bacterial Plasmids Chapter 25: Summary In this chapter, we learned that: • DNA replication is catalyzed by DNA polymerases that use one of the strand as a template while adding new nucleotides to the 3’-end of the newly synthesized chain • Several mechanisms exist to correct mismatches and other changes in DNA • During DNA repair, the information encoded in the parent strand can be used to make corrections in the daughter strand • Homologous recombination involves swapping of regions of DNA with similar sequences ...CHAPTER 25 DNA Metabolism Key topics: – DNA replication – DNA repair – DNA recombination What is DNA Metabolism? • While functioning as a stable storage of genetic information, the structure of DNA. .. DNA molecules giving offspring a novel DNA • DNA metabolism consists of a set of enzyme catalyzed and tightly regulated processes that achieve these tasks The Substrate that Encodes its Own Metabolisms... responsible for DNA replication Structure of Bacterial DNA Polymerases is Well Understood • Structure of the Klenow fragment of DNA polymerase I • Structure of DNA Polymerase III bound to DNA Synthesis