DNA Topology || Linking number || Twist || Writhe

#biologyanimation DNA topology refers to the superhelical structures or tangles in a DNA double helix. There are generally two forms of DNA, linear DNA and closed circular DNA. In the case of linear DNA, untangling is possible due to the free rotation of the ends of the DNA. However, generally linear DNA is bound to proteins, so untangling is not possible. It is impossible to untangle two strands of a closed circular DNA. The fundamental topological parameter of a covalently closed circular DNA is called the linking number (Lk). Linking number refers to the number of times one strand would have to be passed through the other strand in order for the two strands to be entirely separated from each other. Linking number is the sum of two geometric components called the twist and the writhe. Twist is the number of times one strand completely wraps around the other strand.
Twist is greater than zero in right-handed helix and less than zero in case of a left-handed helix. Another topological component is writhe.
Writhe refers to the number of times DNA double helix crosses over itself in three-dimensional space. For right-handed helix, writhe will be negative and for a left-handed helix, it will be positive in number.
Writhe can be interwound and spiral. As we know, in a DNA double helix there are 10.5 base pairs in a helical turn. So linking number is equal to base pairs divided by 10.5. DNA in cells is negatively supercoiled.
The extend of supercoiling is measured by the difference between Lk and Lk not, which is called the linking difference. The DNA is called negatively if Delta Lk is less than zero, and it will be called positively supercoiled if delta Lk is greater than zero. The superhelical density is measured by the ratio of delta lk and lk not. It is expressed by the symbol sigma. There are enzymes which can change the topology of DNA, called topoisomerase. Topoisomerase type I increase the linking number by 1, Whereas Topoisomerase type II change the linking number by 2.

[@ShivaniSingh-jp8qw]

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[@wildpredator1616]

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[@PowerhouseCell]

Woah this is really cool! Love seeing the overlap between maths and biology :D It's great to see other biology KZbinrs!

[@RethinkBiology]

Thank you, this means a lot. 😊

[@PowerhouseCell]

@@RethinkBiology You're welcome! I've always wanted to make a DNA Topology video on my channel haha, yours was great!

[@sabitasinku9548]

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[@RethinkBiology]

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[@animeshhb]

Can anyone please explain why writhe is less than zero in case of RH helix??? Because if writhe is less than 0 that means we are applying negative supercoiling to right handed helix which will in turn decrease the number of twist and Linking number. Again, applying writhe greater than 0 will cause the right handed helix to twist more in right handed side causing increase in number of twist and Linking number. I may be wrong. Please explain 🙏🏻

[@Kiwi.an_bean]

I think here the negative and positive sign when calculating only shows the direction of turning with respect to the original clock/aniti clock twisting of DNA. By definition negative supercoiling usually refers to when DNA is un winded whereas positive supercoiling is when DNA is condensed further. But here cause the linking number can't be changed and is kept constant we use -(negative) or +(positive) signs to maintain the ratio between twists(Tw) and writhe(Wr). ( As Lk = Tw+ Wr) So even if a strand of DNA which in RH in nature is being twisted clockwise it's supercoiling would still be positive as the number of twists (in equation) decreases and to maintain constant Lk we're required to add writhe (Wr) hence making it positive. This is what I understand,do provide info if I made a mistake.

[@Kiwi.an_bean]

Just like how DNA in humans is stored via coiling around histone proteins,it's by definition an ex of positive supercoiling and we can say that here the number of twists might be decreasing as it's writhe is increasing (coiling around histone proteins). So as per equation Wr would be + as twist might be decreasing in number but still decreasing of twists doesn't equal DNA not condensing as it is being coiled into smaller size. I.e, just cause number of twists are decreased doesn't mean condensing isn't happening.

[@shibanandasharma9558]

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[@RethinkBiology]

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[@RethinkBiology]

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[@RethinkBiology]

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[@RethinkBiology]

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[@RethinkBiology]

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[@RethinkBiology]

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[@RethinkBiology]

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