2. INTRODUCTION
• Nucleic acids are any group of long ,linear
macromolecule that carries genetic information
directing all cellular functions; composed of
linked nucleotides.
• Two types : DNA and RNA
• DNA: Deoxyribonucleic acid: An extremely long,
double-stranded nucleic acid molecule arranged
as double helix that is the main constituent of the
chromosomes and that carries the genes as
segments along its strands.
3.
4.
5. Variations in DNA
• Most of the DNA is in the classic Watson-Crick
model simply called as B-DNA or B-form DNA.
• In certain condition ,different forms of DNAs
are found to be appeared like A-DNA,Z-DNA,C-
DNA,D-DNA,E-DNA.
• This deviation in forms are based on their
structural diversity.
6. DIFFERENT FORMS OF DNA
• X-ray analysis of DNA crystals at atomic resolution have
revealed that DNA exhibits much more structural diversity than
formly envisaged. Such variations are:
B-DNA: Most common ,originally deduced from X-ray diffraction
of sodium salt of DNA fibres at 92% relative humidity.
A-DNA: Originally identified by X-ray diffraction of analysis of
DNA fibres at 75% relative humidity .
Z-DNA: Left handed double helical structure winds to the left in
a zig- zag pattern .
C-DNA: Formed at 66% relative humidity and in presence of Li+
and Mg2+ ions.
7. D-DNA: Rare variant with 8 base pairs per helical
turn ,form in structure devoid of guanine .
E- DNA: Extended or eccentric DNA.
8.
9. B-DNA
• Described by James D. Watson & Francis crick.
• Commonly found in DNA.
• DNA molecule consists of 2 helical polynucleotide
chains coiled around common axis.
• 2 helices are wound in such a way so as to
produce 2 interchain spacing or groove –
• Major/wide groove(width 12A°,depth 8.5A°)
• Minor /narrow groove(width 6A°,depth 7.5A°)
• These grooves provide surface with which
proteins,chemicals,drugs can interact.
10.
11. • 2 helical wind along the molecules.
• 2 chains run in opposite direction ,they are
antiparallel ,the plane of bases are perpendicular
to helix axis.
• Right handed twisting .
• Uniform diameter(20A°)
• Complementary base pairing.
• Base pair per turn is 10.4.
• Rise per base pair is 3.4A°.
12. A-DNA
• A-DNA is one of the possible double helical
structure which DNA can adopt along with
other two biologically active helix structure(B-
DNA,Z-DNA).
• Right handed double helix .
• Short and fat compared to B-DNA.
• Occur only in dehydrated sample of DNA ,Such
those used in crystallographic experiments.
13. • A-DNA was originally identified by X-ray
diffraction analysis of DNA fibres at 75% relative
humidity.
• The grooves are not as deep in B-DNA.
• The bases are more tilted (to 20A°).
• The base pairs per turn is 11.
• Rise per base pair is 2.3A°.
14.
15. Z-DNA
• One of the many possible DNA double helix
structure.
• Left handed double helix structure winds to
left in zig-zag manner.(DNA backbone were in
zig-zag manner)so they are termed as Z-DNA.
• Discovered by Rich, Nordheim &Wang in 1984.
• It has antiparallel strands as B-DNA.
• It is long and thin as compared to B-DNA.
16. • Remarkable characteristic :adjacent sugar have
alternating orientation (against B-DNA which has
same orientation).
• In Z-DNA ,a)Purines: syn confirmation(bases & sugar
are near & on same side)
b) pyramidines: anti (bases & sugar are
distant ,on opposite sides)
• Only one deep helical grooves.
• There are 12 base pairs per turn with axial rise
3.8A°,& angle of twist 60°.
17.
18. C-DNA
• C-DNA formed at 66% relative humidity (low)in
presence of Li+ or Mg2+.
• Right handed ,with axial rise of 3.32A° per base
pair .
• 9.33 base pairs per turn.
• Helical pitch 3.32A°×9.33°A=30.97A°.
• Base pair rotation=38.58°.
• Has diameter of19A°,smaller than that of A-&B-
DNA.
• The tilt of base is 7.8°
19.
20. D-DNA
• Extremely rare variant with only 8base pairs per helical
turn .
• This forms of DNA found in some DNA molecules
devoid of guanine.
• Axial rise of 3.03A°per base pairs .Tilt of 16.7° from axis
of helix.
• Actually 2 different forms of D-DNA
1. D(A):Takes part in D-A-B transition.
2. D(B):Associated with D-B change of confirmation.
• 2 DNA structure have same helical parameters.
22. E-DNA
• Cytosine methylation of or bromination of DNA
sequence d(GGCGCC)2 is to induce a novel
extended &eccentric double helix, which we call
E-DNA.
• E-DNA has a long helical axis rise and base
perpendicular to the helical axis.
• Deep major groove and shallow minor groove.
• E-DNA allowed to crystallize for a period time
longer, the methylated sequence forms standard
A-DNA.
23. • E-DNA is the intermediate in the transition toA-
DNA.
• E-DNA is the intermediate in the
crystallographic pathway from B-DNA to A-DNA.
24.
25. DNA CONFIRMATION OF A-,B-&Z-DNA
A B Z
HELIX RIGHT HANDED RIGHT HANDED LEFT HANDED
WIDTH WIDEST INTERMEDIATE NARROWEST
PLANE OF
BASE
PERPENDICULAR TO
HELIX AXIS
PERPENDICULAR TO
HELIX AXIS
PERPENDICULAR TO
HELIX AXIS
CENTRAL
AXIS
6A° HOLES ALONG
HELIX AXIS
TINY CENTRAL NO INTERNAL
SPACE
MAJOR AXIS NARROW & DEEP WIDE &DEEP NO MAJOR GROOVE
32. CHARACTRISTICS A-DNA B-DNA C-DNA Z-DNA
CONDITIONS 75% RELATIVE
HUMIDITY;NA+,K+,C
s IONS
92% R.H:;LOW ION
STRENGTH
60%R.H;Li/Mg IONS VERY HIGH SALT CONC.
SHAPE BROADEST INTERMEDIATE NARROW NARROWEST
HELIX SENSE RIGHT RIGHT RIGHT LEFT
HELIX DIAMETER 25.5A° 20.7A° 19.0A° 18.4A°
RISE PER BASE
PAIR(H)
2.3A° 3.4A° 3.32A° 3.8A°
BASE PAIR PER
TURN(N)
11 10.4 9.33 12
HELIX PITCH(H×N) 25.5A° 35.36A° 30.97A° 45.60A°
ROTATION PER BASE
PAIR
+32.72° +34.61° +38.58° -60°
BASE PAIR TILT 19° 1° 7.8° 9°
GLYCOSIDIC BOND ANTI ANTI _ ANTI FOR C,T. SYN FOR A,G.
MAJOR GROOVE NARROW &VERY
DEEP
WIDE & QUITE DEEP _ NO
MINOR GROOVE VERY BROAD &
SHALLOW
NARROW & QUITE
DEEP
_ VERY NARROW & DEEP