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DNA REPLICATION

DNA REPLICATION. What would you do?. Imagine being presented with a book that is composed of about 1 billion pages, and you are asked to find a way to make 100 copies, at once, within a 20 minute time period. Your body, every day.

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DNA REPLICATION

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  1. DNA REPLICATION

  2. What would you do? • Imagine being presented with a book that is composed of about 1 billion pages, and you are asked to find a way to make 100 copies, at once, within a 20 minute time period

  3. Your body, every day • If you can wrap your head around the fact that an average cell cycle requires a full 20 minutes to complete – and the fact that each new cell that comes along requires a brand-spanking new copy of your DNA, you can imagine the legwork required to maintain the health of your body • Every time a cell is replaced in order to repair damage, or every time growth has to occur, your DNA has to be copied so a new cell has the complete instruction manual

  4. DNA is annoying to copy • The truth is, never mind having to worry about copying 3 billion base pairs accurately (mistakes are bad – remember this is your body we are talking about) but you also have to deal with an enormous molecule that is twisted and complex in structure • So imagine that your task just got more annoying because the instruction manual you have been asked to copy is housed in complicated binders that you have to open, remove papers from, put in the copier, and copy – all at the same time keeping track of everything and not mixing up pages

  5. On top of that… • Think very carefully about DNA structure – the coded information for your cell is found within the base pair sequence of DNA • But in truth, where are these base pair sequences? They are in fact closed off from the outside world; remember that the structure of DNA is a double helix – it is very difficult to get to the base pairs because DNA is all wound up to begin with

  6. The challenges of DNA replication • Therefore, DNA replication requires some finesse the major steps involved include: • Unwinding DNA to expose the base pairs • Copying the base pairs • Putting the base pairs together in a new DNA molecule • Not too much different from photocopying a big book: • Take the pages from the original • Photocopy the pages from the original • Put the new pages together in the new copy

  7. Use a template • There are a few ways to get around this that DNA replication uses to minimize mistakes

  8. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experimenthttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experiment http://www.tvdsb.on.ca/westmin/science/sbi3a1/genetics/replicat.htm

  9. Matthew Meselson and Franklin Stahl • Provided experimental evidence that DNA replicates semi-conservatively • They grew E.coli bacteria in two different cultures: one containing heavy nitrogen (AMU = 15, therefore 15N) and one containing light nitrogen, 14N • The first generation of bacteria was grown in the heavy nitrogen, and then switched to the lighter nitrogen • The idea behind their experiment was that as each generation of bacteria grew, the density of the DNA would change as they incorporated more and more of the lighter DNA • Therefore, if they were to separate the DNA from the group of bacteria afterwards via centrifuge, they would find that each successive generation would yield a different density in their DNA

  10. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experimenthttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio22.swf::Meselson%20and%20Stahl%20Experiment

  11. Yay! Proteins! • These hosts of proteins go to the DNA strand in order to complete DNA replication

  12. Prep to copy • Before DNA replication, the chromosomes start to unwind in order to reveal the chromatin that makes up DNA • The DNA is unwound until individual strands can be accessed by the proteins responsible for DNA replication

  13. Unwinding • The next step is to unwind DNA – DNA is somewhat like a rope composed of two strands – what happens when you try to pull apart the two strands?

  14. Without topoisomerases, the unwinding DNA strand will bunch up eventually and get knotted up – which would bring DNA replication to an end

  15. Getting started • To begin, a small piece of DNA is laid down by an enzyme known as RNA PRIMASE • This enzymes “primes” or gets DNA replication started by laying down a short piece of RNA on the original strands of DNA

  16. DNA Polymerase III • As its name indicates, DNA polymerase III “polymerizes” nucleotides – it joins them together to form a new DNA strand

  17. Fork it • As DNA is being replicated, it is being split open to form a Y-shaped structure termed a DNA REPLICATION FORK

  18. Leading and lagging • Therefore, there is a difference in the manner in which the two new daughter strands are synthesized • The LEADING STRAND • On the LAGGING STRAND

  19. Okazaki fragments • On the lagging strand, RNA primase makes short RNA primers all along the length of the strand

  20. http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Forkhttp://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork RNA http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio23.swf::How%20Nucleotides%20are%20Added%20in%20DNA%20Replication

  21. Telomeres • In prokaryotes, circular DNA means that DNA polymerase, at the end, always has another nucleotide to attach to • However, in many eukaryotes, DNA is linear • So what happens to those primers that are left at the ends of the daughter strands? • http://web.centre.edu/bmb/movies/Telomeres.html

  22. Save the end bits • Some theories suggest that telomeres may be responsible for aging – a theory proposed after the early death of the cloned sheep Dolly

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