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DNA Replication the big event during S phase

DNA Replication the big event during S phase. The Animation . http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#. 1. Helicase breaks hydrogen bonds between bases, unzips and unwinds the helix. Helicase:

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DNA Replication the big event during S phase

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  1. DNA Replicationthe big event during S phase

  2. The Animation http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

  3. 1. Helicase breaks hydrogen bonds between bases,unzips and unwinds the helix

  4. Helicase: • Is an enzyme (a protein that speeds up chemical reactions) • Is made during G1

  5. Begins to unwind the DNA at the ORIGIN OF REPLICATION (a specific nucleotide sequence)

  6. Helicase enzymes move in both directions from the Point of Origin, forming a REPLICATION BUBBLE. At either end of the replication bubble is a REPLICATION FORK, a Y-shaped region where the new strands of DNA are elongating.

  7. In prokaryotic cells there is one origin In eukaryotic cells there are 100’s to 1000’s of origins

  8. 2. Single stranded binding proteins hold the DNA strands apart • Keeps the separated strands apart and stabilize the unwound DNA

  9. RNAnucleotides bind with complementary base sequences under the direction of RNA primase. These RNA nucleotides act as a primerfor DNA nucleotides. • Primers are short segments of RNA, about 10 nucleotides long • Must have a primer because DNA polymerase can only add nucleotides to another nucleotide

  10. Primase

  11. 4. DNA polymerase III adds DNA nucleosidetriphosphates to the RNA primer sequence in a 5’ to 3’ direction. Two of the phosphates are stripped off in the bonding

  12. New nucleotides can only be added to the 3’ end of a growing DNA chain • So we say DNA grows 5’ to 3’

  13. Leading Strand: • DNA polymerase III can synthesize a complementary strand on one side of the template in the 5’ to 3’ direction with no problem.

  14. What about the other strand?? 5’ 3’ DNA Polymerase III Can only add to this side … AWAY from the replication fork

  15. Lagging Strand • DNA polymerase III must work away from the replication fork. • Makes a short strand of DNA, called an Okazaki fragment. • As the bubble widens, it can make another short strand, and so on.

  16. RNA primers are removed and replaced with DNA nucleotides by DNA Polymerase I.

  17. Along the lagging strand the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.

  18. Proofreading by DNA Polymerase III and I occurs, and replication is complete.

  19. The Animation http://207.207.4.198/pub/flash/24/menu.swf

  20. DNA replication is ... • Accurate • Only 1:1,000,000,000 nucleotides are incorrectly paired

  21. DNA Replication is semi-conservative: • each replicated DNA molecule consists of one “old” and on “new” strand.

  22. DNA replication is ... • Extremely rapid • In prokaryotes, up to 500 nucleotides are added per second • 50 per second in eukaryotes

  23. Other Good Animations • http://www.ncc.gmu.edu/dna/repanim.htm • http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/dna-rna2.swf • http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

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