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Thursday, November 13, 2014

DNA Topology - Enzymes and Modes of action on cccDNA


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
  • 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.