Structure & Types
• Deoxyribonucleic Acid
• Usually double stranded.
• double-helix structure.
• found in chromosomes,
mitochondria and chloroplasts.
• It acts as the genetic material
in most of the organisms.
• Carries the genetic information
Friedrich Meischer identified an acidic
substance present in nucleus and
named it as ‘Nuclein’ (1868).
Rosalind Franklin and Maurice
Wilkins studied X-ray diffraction with
DNA and obtained patterns suggested
More than one strand
10 base pairs per complete turn
Few Key Events Led to the Discovery of
the Structure of DNA
✓ These are all derived from their
parent heterocyclic compound
pyrimidine, which contains a six
membered ring with two-nitrogen
atoms and three double bonds.
✓ It has a melting point of 22°C and
a boiling point of 123.5°C.
• These are all derived from
their parent compound
purine, which contains a
ring fused to the five-
• It has a melting point of
Structure of DNA
DNA structure is often divided
into four different levels
O P O CH2
O P O CH2
O P O CH2
O P O CH2
Primary Structure/Single Strand
• Supercoiled, DNA,
chromosomes and chromatin
are the three types of tertiary
structures of DNA
• DNA can be twisted, coiled
into compact structure which
resembles a coiled telephone
• Supercoiling may be right
handed or left handed
• is biologically THE MOST COMMON
It is a -helix meaning that it has a Right handed, or
Complementary base pairing
Ideal B-DNA has 10 base pair per
So each base is twisted 36o relative to adjacent
Base pair are 0.34 nm apart.
So complete rotation of molecule is 3.4 nm.
Axis passes through middle of each basepairs.
✓ A-DNA appears when the DNA fibre (B-DNA) is
dehydrated, i.e., relative humidity is reduced from 92 to
75% and Na+, K+ and Cs+ ions are present in the
medium. In other words, in solution, DNA assumes the
B form and after dehydration it assusmes theAform.
✓ This is because the phosphate groups in the A-DNA
bind fewer water molecules than the phosphates in B-
✓ Z-DNA is the more radical departure from B-DNA and
is characterized by a lefthanded helical rotation
✓ It was discovered by Rich, Nordheim and Wang in
✓ They found that a hexanucleotide, CGCGCG, forms a
duplex of antiparallel strands held together by Watson-
Crick base pairing, as expected.
✓ Surprisingly, they found that this double helix was left-
handed and the phosphates in the DNA backbone were
in a zigzag manner ; hence, they termed this new form
✓ C-DNA is formed at 66% relative humidity in the presence of
✓ This form of DNA is also right-handed, with an axial rise of
3.32 Å per base pair.
✓ There are 9.33 base pair per turn of the helix ; the value of
helix pitch is, therefore, 3.32 × 9.33 Å or 30.97 Å.
✓ The rotation per base pair in C-DNA is 360/9.33 or 38.58°.
✓ The C-helix has a diameter of 19 Å, smaller than that of
both B- andA-helix.
Property B-DNA A-DNA Z-DNA C-DNA
Strand Antiparallel Antiparallel Antiparallel Antiparallel
Type of Helix Right-handed Right-handed Left-handed Right-handed
Overall shape Long and
Long and narrow
Base pair per turn 10 11 12 9.33
0.34 nm 0.23 nm 0.38 nm 0.31 nm
Pitch/turn of helix 3.40 nm 2.82 nm 4.56 nm 3.32 nm
Helical Diameter 2.0 nm 2.3 nm 1.8 nm 1.9 nm
bp to axis
Property B-DNA A-DNA Z-DNA
Major Groove Wide & Deep Narrow &
Minor Groove Narrow,
✓ D-DNA is an extremely rare variant with only 8 base
pairs per helical turn.
✓ This form of DNA is found in some DNA molecules
devoid of guanine.
✓ By contrast, A-, B- and C forms of DNA are found in all
DNA molecules, irrespective of their base sequence.
(300 nm in diameter)
30 nm fiber
(11 nm in diameter)
(2 nm in diameter)
Each chromatid (700
nm in diameter)
Tertiary Structure/Super Coiling
In Eukaryotes DNA associated
with Proteins. (In prokaryotes
DNA is naked)
Nucleosome are the basic unit of
the chromatin organization.
Nucleosomes are basic bead like
units of DNA packing
Made of segment of DNA
wound around a protein core
that is composed of 2 copies
of each 4 types of Histones.
• Nucleosomes have:
• 8 Histones in the core
• DNA wrapped twice around
• the core
• One Histone holding the
• A DNA ‘linker’ continues towards
the next nucleosome.
• The DNA has a negatively charged
backbone(because of PO4 group)
• The Protein(Histones) are positively
• The DNA and Protein are
Electromagnetically attracted to each
other to form chromatin.
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