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Altius FL1 Bio Q32
Nicole_5521
#1 Posted : Tuesday, June 22, 2021 3:25:05 PM
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Hello,

For Q32 from Altius FL1, I'm wondering why topoisomerase Ib activity is most crucial after helicase. I had thought during DNA replication, after ligase joins together the Okazaki fragments, DNA gyrase or topoisomerase follows to relieve the pressure in the coils.

Thanks!
INSTR_Taylor_140
#2 Posted : Thursday, June 24, 2021 6:53:19 PM
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Hi Nicole!

Topoisomerase is most important after helicase because helicase separates double stranded DNA, introducing superhelical tension in the DNA that needs to be relieved. This is achieved by topoisomerase. DNA ligase seals together free ends of DNA, not separates them, and wouldn't produce the superhelical tension on its own!

Taylor
Nicole_5521
#3 Posted : Saturday, June 26, 2021 12:23:14 AM
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Thanks Taylor!

So, how come DNA gyrase/topoisomerase is the very final step of DNA replication. How come it isn't the step immediately following helicase unwinding?

INSTR_Kennedy_135
#4 Posted : Tuesday, July 20, 2021 9:19:51 PM
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Hello,

Most of DNA replication is all happening simultaneously in reality. After Helicase there are single stranded binding proteins that help stabilize the strands, but also the topoisomerase would occur after to help stabilize.

For the MCAT I would not worry too much about why the steps are in the order, just have a list of the steps and the order they occur.

Goodluck!
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