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.
Got something to say about this post? Leave a comment…your comments are valuable for improving the posts.