1.25.07 - 1.25.07 So much tension from helicase unwinding...

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1.25.07 So much tension from helicase unwinding that it could cause super coiling, how is the tension resolved? o Topoisomerase is an enzyme that relieves the tension in two ways either Break a phosphodiester bond in one of the two strands to allow the one remaining strand to have free rotation around itself and can unwind (effectively it removes a twist close to the fork) and then it will reconnect the phosphodiester bond that it just broke It does this over and over again really fast taking out twists right at the fork so that helicase can continue to unwind Some topoisomerase break a phosphodiester bond in both of the two strands and does an unwinding of both strands at once, either way the same thing is still accomplished Can consider the fork, helicase moves and topoisomerase moves right in front of it How is DNA made? o By various enzymes called DNA polymerases, they put together DNA nucleotides o The new nucleotide has to be a triphosphate nucleotide (three phosphates on the 5’ end) but only one phosphate makes the phosphodiester bond and two are removed o In phosphate phosphate bonds you have high energy, so when you break this bond energy is released that can be used o It is more stable to have separate nucleotides than connected ones, not spontaneous to form the phosphodiester bond, so the energy from cleaving the P-P bond is necessary for the addition of the nucleotide on the strand o Here the individual nucleotides provide the energy for their own addition onto the strand so only triphosphate nucleotides can be added to DNA (or RNA) o The phosphate on the 5’ carbon of the new nucleotide must attach to the hydroxyl of the 3’ end of the growing strand o DNA polymerase is moving in the 5’ to 3’ direction in the growing strand, the strand being synthesized o No DNA polymerases will add consecutively to the 5’ end and none that use something other than triphosphate nucleotides o DNA polymerases can only add new nucleotides to an EXISTING strand, o RNA polymerases can pick up the first few pairs of nucleotides and base pair them to the template and then make a phosphodiester bond, the RNA polymerase can create a 3’ end while DNA polymerase cannot. All new nucleotides are added still 5’ to 3’ though. o So, you start a DNA strand with RNA polymerases because DNA polymerases cannot do this o Primase is the enzyme that makes the RNA primer, it makes it right along the template, it is not a premade primer, it picks up RNA nucleotides and puts them together on the spot o Primase takes RNA nucleotides and puts them together and moves in a 5’ to 3’ direction so you can say it has a 5’-3’ RNA polymerase activity o How does primase know when to stop? Primer is relatively short so how does it know when to stop? It has an inherent quality of the enzyme itself, it just doesn’t have a strong attachment to the template and it does its job a few times and then it gives up and dissociates from the template (called low processivity)
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1.25.07 - 1.25.07 So much tension from helicase unwinding...

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