9 - MCB 102 James Berger Lecture 25 Sharing or distribution of lecture notes or sharing of your subscription is ILLEGAL and will be prosecuted Our

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MCB 102 James Berger 3/19/08 Lecture 25 Sharing or distribution of lecture notes, or sharing of your subscription, is be prosecuted. Our non-profit, student-run program depends on your individual subscription for its continued existence. ILLEGAL and will These notes are copyrighted by the University of California and are for your personal use only. Sharing or copying these notes is illegal and could end note taking for this course LECTURE Last time we talked about replication and one question that has come up is about this distinction between leading and lagging strand synthesis and why you don’t just have a polymerase that moves in the correct direction. The reason you have to do lagging strand synthesis is as follows. If you imagine a replication fork, you have a leading strand polymerase on one strand and the lagging strand polymerase on the other strand. As the fork moves, it’s going to open up single stranded DNA. The leading strand polymerase is in a position to simply move along with that fork. The 5’ to 3’ direction of that polymerase is coincident with the direction of the fork. By contrast, the lagging strand polymerase can’t synthesis in that direction. What has to happen is that eventually once it finishes making the strand that it is working on, it has to disengage from the DNA, hop over, pick up a new piece of single stranded DNA and continue. That’s why you have to give rise to this discontinuous replication. Again, it is because the two strands of DNA are anti-parallel. In addition to leading and lagging strand synthesis we covered some other aspects of replication including initiation and termination, and I left it off last lecture just beginning to discuss repair . As we mentioned there are myriad number of systems that the cell uses to repair DNA. At first glance, all of these look bewilderingly complex, but there is sort of a method to the madness as you’ll see in just a minute. The different kinds of repair system are tuned to predominantly work on particular types of lesions that occur in DNA. It doesn’t mean that they are restricted to those lesions, but it means that they have a preference to work on them in certain instances. The first kind of repair that I want to talk about is the repair that deals with a problem arising right out of replication which is when polymerases do make mistakes. We talked about how polymerases are phenomenally accurate, but if you make a mistake
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ASUC Lecture Notes Online MCB 102 3/19/08 Sharing or copying these notes is illegal and could end note taking for this course 2 every 1 in ten million times, you still are still making mistakes that the genome can’t afford. Cells have evolved a system known as mismatch repair , sometimes abbreviated MMR , to deal with these erroneous bases that make their way into the genome during replication. Mismatch repair boosts the fidelity of DNA replication another 100 to 1000 fold. If you were at 10 7 -10 8 accuracy, you’re now more like 10 11
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This note was uploaded on 05/07/2008 for the course MCB 130 taught by Professor Schekman during the Spring '08 term at University of California, Berkeley.

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9 - MCB 102 James Berger Lecture 25 Sharing or distribution of lecture notes or sharing of your subscription is ILLEGAL and will be prosecuted Our

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