Twist Number:
Twist number is equal to the number of times one strand of covalently closed circular DNA (cccDNA) crosses in front of the other strand.
For right handed helices Tw > 0
Writhing Number:
Writhing number is equal to the number of times the double helix crosses over the long axis of the double helix in 3D space.
Types of Writhe
Interwound Writhe
For right handed interwound turn Wr < 0 and or left handed interwound turns Wr > 0
Spiral Writhe
For right handed interwound turn Wr > 0 and for left handed interwound turns Wr < 0
Linking Number = Twisting Number + Writhing Number
Topoisomerases:
Topoisomerases I
Type I Topoisomerases increases the linking number one step at a time.
Topoisomerase II
Type II Topoisomerases increases the linking number two steps at a time.
DNA gyrase
Special type of topoisomerase II found in prokaryotes which decreases linking number by 2.
DNA gyrase requires ATP for activity.
Twist number is equal to the number of times one strand of covalently closed circular DNA (cccDNA) crosses in front of the other strand.
For right handed helices Tw > 0
Writhing Number:
Writhing number is equal to the number of times the double helix crosses over the long axis of the double helix in 3D space.
Types of Writhe
- Interwound Writhe
- Spiral Writhe
Interwound Writhe
For right handed interwound turn Wr < 0 and or left handed interwound turns Wr > 0
Spiral Writhe
For right handed interwound turn Wr > 0 and for left handed interwound turns Wr < 0
Linking Number = Twisting Number + Writhing Number
Topoisomerases:
- Topoisomerases I
- Topoisomerases II
Topoisomerases I
Type I Topoisomerases increases the linking number one step at a time.
Topoisomerase II
Type II Topoisomerases increases the linking number two steps at a time.
DNA gyrase
Special type of topoisomerase II found in prokaryotes which decreases linking number by 2.
DNA gyrase requires ATP for activity.
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