GOVT. E. RAGHVENDRA RAO PG. SCIENCE COLLEGE
BILASPUR CG.
AFFLIATED TO ATAL BIHARI BAJPAYEE VISHWAVIDYALAYA BILASPUR CG.
DEPARTMENT OF MICROBIOLOGY
SESSION-2022-23
PAPER II- MICROBIAL GENETICS AND MOLECULAR BIOLOGY
TOPIC- SUPERCOILING OF DNA
SUBMITTED TO SUBMITTED BY
DEPARTMENT OF MICROBILOGY MAHIMA SAHU
M.Sc. –II SEM (MICRO)

CONTENTS
o Introduction
o Positive and Negative supercoiling
o Formation of supercoiling
o Relative terms of supercoiling
o Helpful enzyme
o Importance
o Functions
o Conclusion
o References

SUPERCOILING
 The term supercoiling means literally the coiling of the coil.
 DNA supercoiling is generally a manifestation of structural
strain.
 Supercoiling occurs when the molecule relieves the helical
stress by twisting around itself. Overtwisting leads to positive
supercoiling, while under twisting leads to negative
supercoiling.
 If DNA is in the form of circular molecule, or if the ends are
rigidly held so it form a loop, then over twisting or under
twisting leads to the supercoiled state.

POSITIVE AND NEGATIVE SUPERCOILING
 When the DNA helix has the normal
number of base pairs per helical turn it
is in the relax state.
 If the helix is over twisted so that it
become tighter, the edges of the narrow
grooves more closer together.
 If the helix is under twisted the edges of
the narrow grooves move further apart.
 Changing in the twist from relaxed state
requires adding energy and increases the
stress along the molecules.

 positive supercoiling is the right
handed, double helical form of
DNA .it is twisted tightly in a right
handed direction until the helix
creates knot.
 Positive supercoiling is more
condensed as the supercoil forms at
the direction of DNA helix.

• Negative supercoiling is the left
handed double helical form of
DNA.
• Prokaryotes and eukaryotes usually
have negative supercoiled DNA.
• It is naturally prevalent as it
prepare the molecule for processes
that require separation of the DNA
strands without the need of
additional energy.

FORMATION OF SUPERCOING
 During the process of cellular events like replication
and transcription, the DNA strand needs to be
separated from each other.
 Once the DNA strands are separated from each other
at the location, the twist number gets reduced. This
creates tension in the DNA so the writhe is formed to
compansate for the tension in the strand.
 Some enzymes like topoisomarase can relieve the
stress thus reducing the linking number.

LINKING NUMBER,TWIST AND WRITHE
 Some mathematicians are created three quantities to describe
the DNA loops and their relationship to each other.
 LINKING NUMBER- the number in which the two strands
are intertwined. Also the number of cleaves necessary for
separation two DNA strands. It will be constant.
 TWIST- each of the curves rotates around the central axis of
the double helix.
 WRITHE- the number of times the central axis C makes loops
about itself.
Lk= Tw+Wr

 DNA topoisomerase act to regulate supercoiling by catalysing
the winding and unwinding of DNA strands.
 They make incision that break the DNA backbone, so they can
pass the DNA strands through one another, swiveling and
relaxing/ coiling the DNA before resealing the breaks.
 They can be divided into two groups based on the number of
strand that they break.
 Class I DNA topoisimerase
 Class II DNA topoisomerase
DNA TOPOISIMERASE

CLASS I TOPOISOMERASE
 Breaks one strand of DNA helix.
 ATP independent .
 Mechanism involves rotating the broken strand around the
intact strand to relax (unwined) the strain on the DNA helix,
followed by resealing the ends of the broken strand.
 Play an important in DNA replication and transcription.

CLASS II TOPOISOMERASE
 Breaks two strands of DNA helix.
 ATP dependent.
 Mechanism involves passing an intact DNA helix through the
gap made by the broken DNA helix, then resealing the strands.
 Play an important role in chromosomes condensation and in the
segregation of daughter chromosomes during cell division.

IMPORTANCE OF SUPERCOILING
• For DNA packaging within all cells, because length of DNA
can be thousand of times, packaging this genetic material into
the cell or nucleus is difficult.
• Reduces the space and allow for much more DNA to be
packaged.
• Also required for DNA and RNA synthesis.
• Supercoiling achieved with histones to form a 10 nm fiber, this
fiber is further coiled into 30 nm and again and again itself that
forms chromosome.

FUNCTIONS
1. Allow DNA to fit in a cell:- the stretched DNA of a cell is 2
nm long. Means it should be highly organized to fit in a cell,
this process allow the entire genome to shrink and settle in the
cell.
2. Provides stability to DNA :- Stability is very important for a
cell to perform various metabolic activities.
3. Prevent unnecessary enzymatic reactions.
4. Control and maintain the gene expression profile.
5. Forms chromatin, chromatids and chromosomes.

CONCLUSION
DNA supercoiling describes a higher order DNA structure. The
double helical structure of DNA entails the interwinding of two
complementary strands around one another and around a
common helical axis.

REFERENCES
 A textbook of microbiology “DUBEY AND MAHESHWARI”
page no. 126-161.
 A textbook of microbiology “P.D.SHARMA” page no. 161-
220.
 https://bio.libretexts.org.bookshelves.net
 https://geneticeducation.co.in
 Image source from google.