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summ lec 2 (murphy).doc

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Biology 1202B
Shauna Burke

Lecture 2: DNA Replication - Chapter 13 semi conservative: opening of the dna, template = parental strand, one double stranded helix, with a template for both DNA conservative: all or nothing complex, the new strand has no parts from the template dispersive: parental double strand immediately change into soemthign new and then old How was it proven that DNA replicated semi conservatively? 1958 Meselson & Stahl: Bacteria grown for several generations in15 N (heavy) medium. Bacteria transferred to 14N (normal/light) medium & al- lowed to grow & divide for several generations. - took the bacteria, known isotope of nitrogen,took it and grew in heavy nitrogen (15) - nitrogen because its a part of the nucleic acid, when they grew bacteria ( SEVER- AL GENERATIONS), they found that ll the DNA was heavy: i.e they are at the bottom, in N 14 all DNA was light PREDICTED RESULTS: conservative: one --> 14N, one 15 N two ---> 3 of 14 N 1 of 15 N dispersive: each a mixture of 15 N and 14 N two: misture of 14 N and 15 N but more of proven that semiconser-s vative method was used for DNA repli- cation * Look at DNA Syn- thesis Pic polari- ty of the template is v. important, the top = 3’ and bottom is 5’, replication occurs 5’ to 3’ the new nucleotide brings the adenine with the base phosphate inner = beta, external is gamma 3’ to 5’ phsophodiester bond is created, the other 2 phos- phates are removed the gamma and alpha leading strand: when the new nucleotides are added it moves to- wards the right, moving form 5’ to 3’ lagging strands: primers: have 5’ to 3’ polarity, theres a similar orientation but moves towards the extends towards the left ---> okaza- ki fragments are made not continuos Direction of Replication ------------------> Upstream, DNA unwinds more, Leading strand is continuously made, Lagging strand is discontinu- ously made (Okazaki Fragments) helicase: unwinding proteins ( enzyme) unwinds it SSB: single stranded binding proteins, stabilized the molecule, so that it can be replicated appro- priately in replication primase; makes the primers, its an RNA primer, and using this you synthesize molecules * DNA POLYMERASE: adds DNA nucleotides to 3’ end of RNA primers primase: synthesizes and as- semble short complementary rna primers replication fork moves towards the right side, syn- thesis is occurring in that direction, these “nicks” have to sealed Upstream DNA unwinds more DNA Ligase: seals the nick *As DNA continues to unwind, the same steps are repeated *Topoisomerase: Relieve over twisting & strain of DNA ahead of replication fork in circular DNA #note: DNA replication is fast so although process is shown step by step spread out, all enzymes actually operate at the replication fork. So, a short distance behind the replication fork is a new DNA that is fully continuous & wound into a DNA double helix DNA Replication occurs at multiple origins on a eukaryote chromosome Origin of Replication Bidirectional Synthesis of DNA although its a “bubble” theres still a site of origin replication doesn’t go in one direction but in two its bidirectional - bottom part you also have a site of origin, w/in this bubble depends on the direction of the replication and
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