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.