DNA replication mechanism

[ice2013]

I have a couple of questions regarding the DNA replication mechanism.

1. Kaplan says that helicase unwinds the double helix and gyrase then introduce negative supercoiling. My question is the double helix has supercoiling to begin with, doesn't it? So, gyrase has to undo the supercoil before helicase can unwind, and then once the double helix is unwound, gyrase needs to relieve the tension again. Am I wrong?

2. The leading strand doesn't need ligase, right, because it's continuous?

Thanks for your help in advance!

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[a_borgia]

1. If I understood correctly, helixase doesn't need the supercoils to be unwound. Think about it this way: if you take a loop of yarn and twist it lots of times, you can hold it without tension and have lots of supercoils, or hold it with tension and have a straight helix. If you stick your finger in the middle of the two twisted strands and push, there will be resistance because you'll be compressing the twists in front. But helicase, like your finger, can attach anywhere, even in the yarn without tension. However, you can only push your finger along in one direction so long before there are too many twists per inch to keep going, so gyrase (which you will more often see as topoisomerase) will relieve the tension. Because you are uncoiling behind the helicase, the DNA maintains some negative supercoiling.

2. Correct.

 

[Ad2b]

topoisomerase stops the recoiling of the strands... without it, the strands were super coil

helicase unwinds the helix (using ATP, btw)