DNA exists in different structural forms. The most common form is B-DNA, which has a right-handed double helix structure discovered by Watson and Crick. Other forms include A-DNA, Z-DNA, and C-DNA, which differ in features like diameter, groove size, and handedness. DNA topology refers to the constrained, intertwined nature of the double helix that is influenced by factors like linking number, twist, and writhe. Topoisomerase enzymes play a role in changing DNA topology and are essential for processes like replication and transcription.

1. DNA TOPOLOGY
ALICIA TINY
St. Mary's college ,Thrissur

2. Content
STRUCTURE OF DNA
DIFFERENT FORMS OF DNA
TOPOLOGY

3. Structure of DNA has been studied by many people
for many years
Linus Pauling
Maurice Wilkin and Rosalind Franklin
James Watson and Francis Crick

4. STRUCTURE OF DNA
• The structure of DNA is divided into four different
levels
• Primary
• Secondary
• Tertiary
• Quantanary

5. • Primary structure consists of a linear sequence of
nucleotides that are linked together
by phosphodiester bonds It is this linear sequence of
nucleotides that make up the Primary structure
of DNA or RNA.
• Secondary structure is the set of interactions
between bases, i.e., which parts of strands are
bound to each other. In DNA double helix, the two
strands of DNA are held together by hydrogen bonds.

6. • Secondary structure is the set of interactions
between bases, i.e., which parts of strands are
bound to each other. In DNA double helix, the two
strands of DNA are held together by hydrogen bonds.
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7. • Tertiary structure refers to the locations of the
atoms in three-dimensional space, It is a higher order
than the secondary structure, in which large-scale
folding in a linear polymer occurs and the entire
chain is folded into a specific 3-dimensional shape.
• There are 4 areas in which the structural forms of
DNA can differ.
• Handedness – right or left
• Length of the helix turn
• Number of base pairs per turn
• Difference in size between the major and minor
grooves

8. • The quaternary structure of nucleic acids is similar
to that of protein quaternary structure.
• The quaternary structure refers to a higher-level of
organization of nucleic acids.
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10. Watson and Crick model
DNA is a double stranded helix
The X-ray diffraction helped to identify that DNA
was helical.
Watson and Crick performed no experiment
themselves there tactic was to use other groups data
to build DNA model.
Postulates of Watson and Crick model……

12. Different forms of DNA
DNA the genetic information crrier can have
different structural conformations they are
A-DNA
B-DNA
C-DNA
Z-DNA
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13. A-DNA
• A DNA is of a rare type of structural conformation DNA
can adopt under dehydrated conditions this apparently
protects DNA under conditions such as the extreme
desiccation of bacteria.
• Its double stranded similar to B DNA but shorter and
more compact
• Much wider than B DNA
• The helix diameter of A-DNA is 26
• The conformation of gycosidic bond in A-DNA is anti
form
• Major grove is narrow and deep
• Minor groove is broad and shallow
• A-DNA has a axial hole in the center
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14. B DNA
• Discovered by Watson and Crick
• It is most common and predominant form of DNA
• Its double stranded
• B-DNA has a solid central core
• The major groove is wide and deep
• Minor groove is narrow and deep
• Gycosidic bond is it anti form conformation
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15. Z DNA
• Z-DNA is one of the many possible double
helical structures of DNA. It is a left-handed double
helical structure in which the helix winds to the left in
a zigzag pattern (instead of to the right, like the more
common B-DNA form).
• Left-handed DNA was first discovered by Robert
Wells and colleagues
• It has a flat major groove
• Minor groove is narrow and deep
• Gycocidic bond is anti for pyrimidines and syn-
purines
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16. C DNA
• C-DNA also known as C form DNA. It is one of the
many possible double helical structures of DNA. This
form of DNA can be observed at some conditions
such as relatively low humidity and the presence of
certain ions, such as Li+ or Mg2+.
• Its shape is narrow
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18. Topology
• Topology is only defined that for a continuous strand
• Topological aspect of DNA structure arises primarily
from the fact that the two DNA strands are
repeatedly intertwined
• A DNA segment constrained in such a way that it
ends cannot rotate freely is called a topological
domain.
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20. DNA in its relaxed state usually assumes the B
configuration .
• It has the minimum energy and deviation
• Any deviation can cause a change in the energy level
• In prokaryotes, DNA is always circular and so this
circularity leads to a lot of different properties of
DNA, it leads to the notion of DNA topology.
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21. • DNA is said to be topologically constrained because of
their length, entertainment in chromatin, interaction with
other cellular components.
• The number of times one strand would have to be passed
through the other strand in order for the 2 strands to
entirely separate from each other is called linking
number. It is always an integer. Linking number is the
sum of two geometric components called twist and writh.
• L=W+T AND dL=dw+dT
• Twist(T) is number of helical turns of one DNA about
another
• Wrist (W) a property of super coiled molecules, is the
number of turns of the axis of the double stranded helix is
space .
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23. There are special enzymes called topoisomerases,
which change DNA topology. They are present in
eukaryotes and prokaryotes .
If you shut down or silence the genes of
topoisomerases in eukaryotes, all major processes,
such as replication and transcription, in the cell will
stall.
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24. CONCLUSION
• DNA is the source of genetic information in most
living organisms.
• It has different structures and different
conformational structures
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