March%2010%20Replication%20M-S%20Mutation%20Post

March%2010%20Replication%20M-S%20Mutation%20Post - Today...

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Today Complete replication mechanism Meselson–Stahl experiment Mutations
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Summary of the events on lagging strand with the Okazaki fragments 2. DNA polymerase III elongates a new Okazaki fragment 3. The previous Okazaki fragment primer is removed. 4. DNA polymerase I fills in the gap. 5. DNA ligase seals the single nucleotide gap to link fragment to the DNA 5’ 5’ 3’ OH 1. New RNA primer made by primase Direction of helicase
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1. New RNA primer made by primase 2. DNA polymerase elongates a new Okazaki fragment 3. The previous Okazaki fragment primer is removed. 4. DNA polymerase I fills in the gap. 5. DNA ligase seals the single nucleotide gap to link fragment to the DNA 5’ 3’ 3’ 3’ 3’ 3’ 5’ 5’ 5’ 5’ Problem 4. The end of linear DNA (eukaryotic) Telomere Here’s the same picture of the events on the lagging strand with the Okazaki fragments - but we’ll focus on the end.
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In this picture, you can see that when we get to the very 3’ end of the template-there will be no place for primase to make an RNA primer ????? This would happen with every DNA replication, so the chromosome would get shorter every replication Problem 4. The end of linear DNA 3’ 5’ Template strand New strand End of template strand
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Cells (and viruses) use 4 different methods for dealing with how to replicate the 3’ end of the lagging strand. 1. Eliminate the ends 2. Modify the ends by sealing them up 3. Modify the ends by adding a protein 4. Have non coding DNA at the ends and an enzyme to add DNA to the ends (telomerase). Solutions to Problem 4:
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1. Eliminate the ends by having a circular DNA Replication starts at a special sequence: the Origin of replication. This is very common in Bacteria and Archaea
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2. Eliminate the ends by sealing them up Replication starts at a special sequence: the Origin This is found in Bacteria and Archaea Snip! 5’ 3’
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3. Add a “primer protein” to the end of the DNA When replication gets to the end of the lagging strand, DNA polymerase binds to protein to start Replication starts at a special sequence: the Origin. This is found in Bacteria and Archaea
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An enzyme called telomerase adds DNA sequences to the 3’ end of the lagging strand. Then it doesn’t matter if the end is not completed. This happens in Eukaryotes. Replication Telomerase These are non- coding repeats 4. Telomerases add repeating DNA sequences to the 3’ end of the lagging strand
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Semiconservative replication was first proposed by Watson and Crick. Each parental DNA strand serves as a template for a daughter strand. Parental dsDNA unwinds as the new strand forms. How do we know? See the Meselson–Stahl experiment Replication could occur in 3 ways - which is correct?
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Grow E. coli in medium with 15 N as sole nitrogen source for many generations.
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at The University of British Columbia.

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March%2010%20Replication%20M-S%20Mutation%20Post - Today...

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