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DNA Replication and Recombination

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1 DNA Replication and Recombination
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.

2 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.

3 Section 11.1 3 possible modes of DNA replication are possible:
conservative semiconservative dispersive

4 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

5 Figure 11-3 The Meselson–Stahl experiment.

6 Figure 11-4 The expected results of two generations of semiconservative replication in the Meselson–Stahl experiment. Figure 11.4

7 Meselson-Stahl Experiment

8 Section 11.1 The Taylor-Woods-Hughes experiment demonstrated that DNA replication is semiconservative in eukaryotes.

9 Semiconservative Replication - http://youtu.be/yyUNaSQf4zs
Mechanism of DNA Replication (Basic) - Mechanism of DNA Replication (Advanced) - DNA Replication Process -

10 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.

11 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.

12 Polymerase Direction Chain elongation occurs in the 5' to 3' direction by addition of one nucleotide at a time to the 3' end.

13 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.

14 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.

15 Section 11.2 Polymerase I is believed to be responsible for:
removing the primer the synthesis that fills gaps produced during synthesis

16 Section 11.2 DNA polymerases I, II, IV, and V are involved in various aspects of repair of damaged DNA.

17 Section 11.2 DNA polymerase III has 10 subunits whose functions are shown in Table 11.3.

18 Polymerase III Holoenzyme (made of many protein subunits) in E. coli
Shevelev, Igor and Hubschur, Ulrich “The 3’ to 5’ exonucleases.” Nature Reviews Molecular Cell Biology 3, pg Retrieved 11/5/13 from

19 7 key issues that must be resolved during DNA replication:
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

20 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.

21 Section 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

22 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.

23 Section 11.3 Both DNA strands are synthesized concurrently by looping the lagging strand to invert the physical but not biological direction of synthesis.

24 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.

25 Section 11.4 DNA synthesis at a single replication fork:

26 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

27 Section 11.6 Eukaryotic chromosomes contain multiple origins of replication to allow the genome to be replicated in a few hours.

28 Section 11.6 3 DNA polymerases are involved in replication of nuclear DNA. 1 involves mitochondrial DNA replication. Others are involved in repair processes.

29 Section 11.6 Pol  and d and Ɛ Pol  Polymerase switching occurs
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 and Ɛ, which has high processivity, for elongation.

30 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.

31 T-Loop in Telomeres

32 T-Loop - Telomere

33 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.

34 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.

35 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)

36 Figure Model depicting how genetic recombination can occur as a result of the breakage and rejoining of heterologous DNA strands. Each stage is described in the text. The electron micrograph shows DNA in a -form structure similar to the diagram in (g); the DNA is an extended Holliday structure, derived from the ColE1 plasmid of E. coli. David Dressler, Oxford University, England Figure 11.18

37 Section 11.9 Gene Conversion Is a Consequence of DNA Recombination
Gene conversion is characterized by nonreciprocal genetic exchange between two closely linked genes.

38 The End


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