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Lecture 18

Lecture 18: "DNA Replication"

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Western University
Biology 1002B
Tom Haffie

Biology Lecture No. 18: DNA Replication th Wednesday March 14 , 2012 RECALL: -Anti-parallel complementary base pairing allows for accurate replication. -Replication is semi-discontinuous and semiconservative. -Replication “bubble” arises from two “forks” created at one “origin.” -Large, linear, eukaryote chromosomes have several origins. Replication Forks Process Both DNA Strands: -Both strands are replicated in the same replisome at the same time, very close together. Elongation By DNA Polymerase: -DNA synthesis proceeds by adding a new base to the 3’ OH of a properly-paired base. -It is only possible to extend 3’ ends. -RNA primase synthesizes RNA primers, which are extended by DNA polymerase III, and then DNA polymerase I removes the primers and replaces them with DNA. -Both strands have polymerases reading from the 3’ to 5’ end. They are being replicated by the same rules. Replication Forks: -Replisomes replicate both strands at the same time by the same replisome. One strand is replicated continuously; the other strand is replicated discontinuously. The Leading Primer: -The RNA primer on the leading strand is unable to be replaced with DNA by DNA polymerase I because there is no 3’ OH. -Since this primer on the leading strand cannot be replaced, the DNA result is becoming progressively shorter as a result. -It is important to note that polymerases always extend 3’ OH’s and that RNA polymerases can start themselves, whereas DNA polymerases cannot. Telomeres & Telomerase: -As RNA polymerase
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