1. The Fascinating Role of
DNA Topoisomerases
DNA topoisomerases are enzymes that play a
crucial role in DNA replication. These fascinating
proteins help keep DNA from becoming
overwound, untangle strands during replication,
and prevent DNA damage.
by AKASH NIGAM
2. Supercoiling
● "Over and under winding of a DNA strand and strand on that on strand".
● Double helix structure does not only exist but it fold itself to form tertiary structure by
supercoiling. Supercoiling allows for the compact packing of circular DNA.
● Supercoiling DNA moves faster than Relaxed DNA. Important in DNA packaging.
● Two types :
1. Negative Supercoiling(clockwise)
2. Positive Supercoiling(counterclockwise)
3. Topoisomerase
1)Type I
a)IA
i)Topo I i)Topo III
i)Reverse
gyrase
a)IB
i)Topo
I(human)
a)IC
i)Topo V
1)Type II
a)IIA
i)Topo II i)Topo IV i)Gyrase
a)IIB
i)Topo VI
i)Topo VII
, Mini A
Classification Of Topoisomerase
4. The Two Types of DNA
Topoisomerases
Type I Topoisomerases
Type I topoisomerases make a transient
single-strand break in the DNA helix,
enabling the sections of DNA to rotate
freely relative to each other.
Type II Topoisomerases
Type II topoisomerases make a transient
double-strand break in the DNA helix,
enabling one DNA helix to pass through
another.
5. Topoisomerase I Topoisomerase II
The enzymes, which cut one of
the two strands of double-
stranded DNA, relax the strand,
and reanneal the strand
The enzymes, which cut both
strands of the DNA helix
simultaneously in order to
manage DNA tangles and
supercoils
Generates single-strand breaks Generates double-strand breaks
May not require ATP hydrolysis Requires ATP hydrolysis
7. The reversible DNA nicking reaction catalyzed
by a DNA topoisomerase I enzyme
Mechanism
• Cutting a single strand of DNA
• Passing of strand
• Re-ligation
one end of the
DNA double
helix cannot
rotate relative
to the other
end
type I DNA
topoisome
rase with
tyrosine at
the active
site
DNA topoisomerase
covalently attaches
to a DNA phosphate,
thereby breaking a
phosphodiester
linkage in one DNA
strand
the two ends of
the DNA
double helix
can now rotate
relative to each
other, relieving
accumulated
strain
the original
phosphodiester
bond energy is
stored in the
phospho-
tyrosine linkage,
making the
reaction
reversible
spontaneous re-
formation of the
phosphodiester
bond regenerates
both the DNA helix
and the DNA
topoisomerase
9. The DNA-helix-passing reaction catalyzed by
DNA topoisomerase II
Mechanism
• Cleaving of DNA chain
• Crossing of the intact strand through the gap
• Re-ligation
two DNA
double
helices
that are
interlocked
topoisomerase II
topoisomerase
recognizes the
entanglement and
makes a reversible
covalent attachment to
the two opposite strands
of one of the double
helices (orange)
creating a double strand
break and forming a
protein gate
the
topoisomerase
gate opens to
let the second
DNA helix
pass
the gate
shuts
releasing
the red
helix
reversal of the
covalent
attachment of
the
topoisomerase
restores an
intact orange
double helix
two DNA
double helices
that are
separated
13. Topoisomerase Inhibitor
Inhibitor Target enzyme Therapeutic value
Quinolones(e.g.ciprofloxacin)
DNA gyrase and
topoisomerase IV
Effective antibacterial
agents
Coumarins(e.g.novobiocin)
DNA gyrase and
topoisomerase IV
Antibiotics , but
Not widely used
Camptothecin(e.g.topotecan) Human topoisomerase I Anticancer drug
Amsacrine(mAMSA) Human topoisomerase II Anticancer drug
Epipodophyllotoxins(e.g.teniposide) Human topoisomerase II Anticancer drug
14. The Role of Topoisomerase in DNA Replication
Preventing DNA damage
Topoisomerases prevent DNA
damage during replication by
breaking and rejoining DNA strands,
allowing for rotation and release of
tension. This helps to ensure that
the DNA strands remain intact and
the genetic information is
preserved.
Efficient replication
Topoisomerases help to make DNA
replication more efficient by
preventing the formation of knots or
tangles in the DNA strands. This
allows the replication process to
proceed more smoothly and
accurately, without any loss of
genetic information.
Preserving genetic
information
By preventing DNA damage and
ensuring efficient replication,
topoisomerases play a crucial role
in preserving genetic information.
Without topoisomerases, the
replication process would be prone
to errors and mutations, leading to
genetic abnormalities.
End of Replication
Topoisomerase plays a crucial role
in the final stages of DNA replication
by releasing the DNA strands from
the replication machinery and
sealing the ends of the newly
synthesized DNA, preventing the
loss of genetic information.
DNA Tangling Prevention
One important role of DNA topoisomerases during replication is to prevent the tangling of DNA strands. As DNA is replicated, the strands become twisted and can form tangles
that slow down the replication process and potentially cause errors. DNA topoisomerases break and rejoin the strands of DNA to allow them to rotate and release tension, which
prevents tangles from forming. By preventing DNA tangling, topoisomerases ensure that the replication process proceeds smoothly and accurately, without any loss of genetic
information.
15. The Importance of DNA Topoisomerases
1 Preventing DNA Damage
If the tension in the DNA helix
becomes too great, it can result in
DNA damage and possible
mutations. DNA topoisomerases
are crucial in preventing this by
maintaining proper DNA structure.
2
Other Cellular Processes
DNA topoisomerases play a
critical role in diverse
biological processes and
cellular functions, including
transcription and
chromosome segregation.
3 Cancer Drug Targets
Some cancer drugs target DNA
topoisomerases by causing
overwinding of the DNA helix, leading
to cell death.
