drk1980
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One question from Qbank drove me crazy...
about the functions of DNA topoisomerase I and II.... the negative supercoils go to the right and the positive supercoils to the left and ....for a particular state of the DNA helix(underwound or overwound).. !!!!
will highly appreciate any discussions on this....also any sincere advise that this topic is not high yield
thanks in advance.
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| KEROCHI
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DNA topoisomerases are enzymes that provides mechanism for removing supercoils in the helix.
DNA topoisomerase 1- cut a single strand of the double helix reversibly. It relaxes negative supercoils & both negative & positive supercoils in eukaryotic cells.Negative supercoils contains fewer turns of the helix than relaxed DNA. Positive supercoils contain more more turns of the helix.
DNA topoisomerase II- bind tightly to the DNA double helix & make transient breaks in both strands.
I hope this helps you a little abt. this enzymes. more addt'l info's appreciated.
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| drk1980
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Hi kerochi..thanks for the discussion.
what i could gather from going back to the Qbank explanation was that positive supercoils(which wraps the DNA helix over to the left of itself) are used to stabilize Overwound DNA and Negative supercoils(which wraps the DNA helix over on to its right side) are used to stabilize a partially Underwound helix...the partially underwound DNA is significant bcoz it allows the important processes of replication and transcription to occur, by allowing proteins to bind...
DNA gyrase (topo II in Ecoli) is the only one topoisomerase which introduces Negative supercoils in the DNA helix to stabilize a partially underwound DNA allowing replication and transcription to occur in the Ecoli....and the flouroquinolones inhibit this DNA gyrase...
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| GMOB
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The tighter a DNA molecule is, the less transcriptionally active, therefore inhibition of Topoiso II will affect replication, as the nolecule wont be able to unwind and become more loose (neg. supercoil) in order to be replicated.
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| drk1980
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so if topoisomerase II can do the job whts the use of Topoisomerase I? i mean in what conditions is T-I more useful than T-II?
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| GMOB
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Topo I removes the RNA primer from the 5' end it is an exonuclease, it also synthesizes DNA from the 3' of the Okazaki fragments. Topo II or DNA Gyrase introduces negative supercoils by cutting and resealing BOTH strands of DNA, topo I can only do cuts on ONE dna strand. they are both important for DNA replication. Quinolones block topoisomerases, nalidix acid blocks dna gyrase. eukariotic dna topo II is inhibited by teniposide, etoposide (antineoplastic).
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| drk1980
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thnks
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| Lim
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DNA gyrase in bacteria;
introducing negative supercoils into DNA to stabilize an underwound DNA helix. when it comes to DNA replication,does unreaded DNA get overwounded by helicase going on?
so Gyrase puts in negative supercoils(to the right direction) to relieve tension.
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| Musuq
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