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PowerPoint® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter 11 DNA Replication and Recombination Lectures by David Kass with contributions from John C. Osterman. Copyright © 2009©Pearson Education, Inc. Copyright 2009 Pearson Education, Inc. Section 11.1 • DNA Is Reproduced by Semiconservative Replication • The complementarity of DNA strands allows each strand to serve as a template for synthesis of the other. Copyright © 2009 Pearson Education, Inc. Section 11.1 • 3 possible modes of DNA replication are possible: • conservative • semiconservative • dispersive Copyright © 2009 Pearson Education, Inc. Section 11.1 • The Meselson-Stahl experiment demonstrated that: • DNA replication is semiconservative • each new DNA molecule consists of one old strand and one newly synthesized strand Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Figure 11.3 Copyright © 2009 Pearson Education, Inc. Figure 11.4 Section 11.1 • The TaylorWoods-Hughes experiment demonstrated that DNA replication is semiconservative in eukaryotes. Copyright © 2009 Pearson Education, Inc. Section 11.1 • DNA replication begins at the origin of replication and is bidirectional rather than unidirectional. • A replicon is the length of DNA that is replicated following one initiation event at a single origin. Copyright © 2009 Pearson Education, Inc. Section 11.2 • DNA Synthesis in Bacteria Involves Five Polymerases, as well as Other Enzymes • DNA polymerase I catalyzes DNA synthesis and requires a DNA template and all four dNTPs. Copyright © 2009 Pearson Education, Inc. Section 11.2 • Chain elongation occurs in the 5' to 3' direction by addition of one nucleotide at a time to the 3' end. Copyright © 2009 Pearson Education, Inc. Section 11.2 • DNA polymerases I, II, and III can elongate an existing DNA strand (called a primer) but cannot initiate DNA synthesis. • All three possess 3' to 5' exonuclease activity. • But only DNA polymerase I demonstrates 5' to 3' exonuclease activity. Copyright © 2009 Pearson Education, Inc. Section 11.2 • DNA polymerase III is the enzyme responsible for the 5' to 3' polymerization essential in vivo. • Its 3' to 5' exonuclease activity allows proofreading. Copyright © 2009 Pearson Education, Inc. Section 11.2 • Polymerase I is believed to be responsible for: • removing the primer • the synthesis that fills gaps produced during synthesis Copyright © 2009 Pearson Education, Inc. Section 11.2 • DNA polymerases I, II, IV, and V are involved in various aspects of repair of damaged DNA. Copyright © 2009 Pearson Education, Inc. Section 11.2 • DNA polymerase III has 10 subunits whose functions are shown in Table 11.3. Copyright © 2009 Pearson Education, Inc. Section 11.3 Many Complex Tasks Must Be Performed during DNA Replication • 7 key issues that must be resolved during DNA replication: • unwinding of the helix • reducing increased coiling generated during unwinding • synthesis of a primer for initiation • discontinuous synthesis of the second strand • removal of the RNA primers • joining of the gap-filling DNA to the adjacent strand • proofreading Copyright © 2009 Pearson Education, Inc. Section 11.3 – Unwinding DNA Helix • DnaA binds to the origin of replication (oriC) and is responsible for the initial steps in unwinding the helix. Copyright © 2009 Pearson Education, Inc. Section 11.3 - RNA Primer • To elongate a polynucleotide chain, DNA polymerase III requires a primer with a free 3'OH group. • Enzyme primase synthesizes an RNA primer that provides the free 3'-OH required by DNA polymerase III Copyright © 2009 Pearson Education, Inc. Section 11.3 • As replication fork moves, only 1 strand can serve as template for continuous DNA synthesis—the leading strand. • Opposite lagging strand undergoes discontinuous DNA synthesis. Copyright © 2009 Pearson Education, Inc. Section 11.3 • Both DNA strands are synthesized concurrently by looping the lagging strand to invert the physical but not biological direction of synthesis. Copyright © 2009 Pearson Education, Inc. Section 11.3 • Proofreading and error correction are an integral part of DNA replication. • All of the DNA polymerases have 3' to 5' exonuclease activity that allows proofreading. Copyright © 2009 Pearson Education, Inc. Section 11.4 • DNA synthesis at a single replication fork: Copyright © 2009 Pearson Education, Inc. Section 11.6 • Eukaryotic DNA Synthesis Is Similar to Synthesis in Prokaryotes, but More Complex • In eukaryotic cells: • there is more DNA than prokaryotic cells • the chromosomes are linear • the DNA is complexed with proteins Copyright © 2009 Pearson Education, Inc. Section 11.6 • Eukaryotic chromosomes contain multiple origins of replication to allow the genome to be replicated in a few hours. Copyright © 2009 Pearson Education, Inc. Section 11.6 • 3 DNA polymerases are involved in replication of nuclear DNA. • 1 involves mitochondrial DNA replication. • Others are involved in repair processes. Copyright © 2009 Pearson Education, Inc. Section 11.6 • Pol and d • major forms of the enzyme involved in initiation and elongation. • Pol • possesses low processivity. • functions in synthesis of RNA primers during initiation on the leading and lagging strands. • Polymerase switching occurs • Pol is replaced by Pol d, which has high processivity, for elongation. Copyright © 2009 Pearson Education, Inc. Section 11.7 • Telomeres Provide Structural Integrity at Chromosome Ends but Are Problematic to Replicate • Telomeres at the ends of linear chromosomes consist of long stretches of short repeating sequences and preserve the integrity and stability of chromosomes. Copyright © 2009 Pearson Education, Inc. T-Loop in Telomeres http://www.ncbi.nlm.nih.gov/bookshelf/picrender.fcgi?book=eurekah&part=A74250&blobname=ch516f1.jpg Copyright © 2009 Pearson Education, Inc. Section 11.7 • Lagging strand synthesis at end of chromosome is a problem b/c once the RNA primer is removed, there is no free 3'-hydroxyl group from which to elongate. Copyright © 2009 Pearson Education, Inc. Section 11.7 • Telomerase directs synthesis of the telomere repeat sequence to fill gap. • This enzyme is a ribonucleoprotein w/an RNA that serves as the template for the synthesis of its DNA complement. Copyright © 2009 Pearson Education, Inc. Section 11.8 • DNA Recombination, Like DNA Replication, Is Directed by Specific Enzymes • Genetic recombination involves: • • • • • endonuclease nicking strand displacement ligation branch migration duplex separation to generate the characteristic Holliday structure (chi form) Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Figure 11.18 Section 11.9 Gene Conversion Is a Consequence of DNA Recombination • Gene conversion is characterized by nonreciprocal genetic exchange between two closely linked genes. Copyright © 2009 Pearson Education, Inc. The End Copyright © 2009 Pearson Education, Inc.