The topology of DNA is defined by how the two complementary single strand are intertwined DNA in a relax state usually assumes the B-Conformation with10.6 bp per turn DNA is subjected to bends or opening of DNA over winding or unwinding, its base pair turn changes and the DNA is subjected to stress and strain DNA Topology also encompasses super coiling

1. SHYLESH IA
FOLLOW FOR MORE BIOLOGICAL SLIDES ( LIFE-SCIENCES)
• TOPOLOGICAL MANIPULATION OF DNA

2. CONTENTS
 Introduction
 Types of Super coiling
 Phenomena of Super coiling
 Energy of Super coiling
 Linking Number
 Topoisomerase
 References

3. INTRODUCTION
 The topology of DNA is defined by how the two complementary single strand are
intertwined
 DNA in a relax state usually assumes the B-Conformation with10.6 bp per turn
 DNA is subjected to bends or opening of DNA over winding or unwinding, its base pair
turn changes and the DNA is subjected to stress and strain
 DNA Topology also encompasses super coiling

4. SUPER-COILING
Picture courtesy to biology200.gsu.edu/houghton/4595%20
Picture courtesy to
www.cbs.dtu.dk/staff/dave/roanoke

5. The two best understood Super coiling occur in DNA are the Positive Supercoiling
and Negative Supercoiling
In both the Supercoiling there is a twisting and untwisting of
Helix

6. POSITIVE SUPER-COILING
 Positive Super coiling
If the twisting of the DNA is in the same direction as that of the Double helix then the super-coiling
formed is Positive
 Over wound is positive super coiling
 Negative super coiling
If the helix is untwisted up before closure then the suercoiling formed is negative super coiling
 Under wound is negative super coiling
 almost all the DNA molecule in the cells are on average negatively super coiled

7. WHEN A CELL IS COPYING A DNA STRAND,
IT WILL UNCOIL A STRAND, COPY IT AND
THEN RECOIL IT

8. LINKING NUMBER(LK)
It is the number of times where the two single strands of DNA molecules linked together corresponding to the number to
double helical turn in a molecules
Twist( Tw)
 Refer to the number of base pair per one helical turn
 Relax DNA, in this case 10.6 bp/turn
Writhe(Wr)
It is the super coil that twist on its own length like a rubber band twist
Lk = Tw + Wr

9. For to regulate the level of super coiling certain topoisomerase enzyme like type I
topoisomerase and type II topoisomerase
These classes of enzyme alter the level of super coiling of double stranded DNA

10. TYPE I TOPOISOMERASE
These enzyme acts by making a transient break in one strand of the
DNA e.g E.coli Topoisomerase
These enzyme does not act on positively super coiled DNA

12. TYPE II TOPOISOMERASE
 Type II Topoisomerase
enzymes acts by introducing a
transient double strand break
e.g.
Bacterial DNA gyrase
 Bacterial DNA gyrase are able
to introduce negative
supercoil into relaxed closed
circular molecules
 Bacterial DNA gyrase can
introduce 100 supercoil per
minutes
Mode of Action of
DNA Gyrase

13. REFERENCES
 Phil turner, Alexander, MC. Lennan and Andy Bates and Mike, Molecular
Biology( Third Edition) DNA Super coiling (page.no.47-50)
 Weaver,Robert F. Molecular biology 1999.(Page No. 650-651)
 Keirc Neuman, The Journal of Biological Chemistry(jbc)
Single-Molecule Measurement of DNA Topology and Topoisomerase ,18
june 2010