The document discusses the structure of DNA. It describes DNA as a double-stranded polymer made of deoxyribonucleotides. The structure takes a double helix shape with the strands running in opposite directions. Each strand is composed of a backbone of alternating sugar and phosphate groups joined by ester bonds. The strands are held together by bonds between complementary nitrogenous bases - adenine pairs with thymine and cytosine pairs with guanine. DNA exists in different conformations including A-DNA, B-DNA, and Z-DNA which differ in their helical structures and base orientations.

1. P re p are d B y
ZAHIDA UMAR
D e p t of E nvironm e ntal S cie nce s DNA structure 1
G C , U nive rsity, Lahore

2. D N A s tru ctu re
Wh at D N A is ?
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3. D N A s tru ctu re
Wh at D N A is ?
• A polymer of deoxyribonucleotides i.e. DNA
• Double-stranded
• A “double helical” structure
• common axis for both helices
• antiparallel relationship between 2 DNA strands
• Structural purposes, or are involved in regulating the use of this
genetic information.
• Individual deoxynucleoside triphosphates are coupled by
phosphodiester bonds
• Esterification
• terminal ends : 5’ and 3’
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4. D N A D is cove ry
D is cove ring th e s tru ctu re of D N A
DNA structure was discovered in 1869 by chemist Friedrich Miescher.
but 1st described by James Watson and Francis Crick in 1953.
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5. D N A s tru ctu re
D N A O rganization
This image illustrates how DNA is organized in human body.
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6. D N A Top ology
C h e m ical s tru ctu re of D N A
•DNA forms a double helix in which the nucleotide bases are attached to
deoxyribose units linked through phosphate groups. The bases in the center of
the DNA helix always occur in complementary matched pairs, with cytosine
linking to guanine and thymine linking to adenine through hydrogen bonding.
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7. E s te rification
S u gar P h os p h ate Backb one
•This backbone is constructed from alternating ribose sugar and phosphate molecules which
are highly polar. Because the backbone is polar, it is hydrophilic which means that it likes to
be immersed in water. The asymmetric ends of DNA strands are referred to as the 5′ (five
prime) and 3′ (three prime) ends, with the 5' end being that with a terminal phosphate group
and the 3' end that with a terminal hydroxyl group.
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8. D N A Bas e s
N itroge nou s Bas e s
•The interior portion of a DNA molecule is composed of a series of 4 nitrogenous
bases: Adenine (A),Guanine (G), Thymine (T),Cytosine (C). These bases are classified
into two types; Purines , Pyrimidines
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9. D N A Bas e s
N itroge nou s Bas e s
“Chargoff’s rule”
A=T & C=G
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10. D N A s tru ctu re
D N A G roove s
•There are two grooves twisting around the surface of the double helix:
2.Major groove 2. Minor groove
•The major groove is 22 Å wide and the other, the minor groove, is 12 Å wide.
The narrowness of the minor groove means that the edges of the bases are
more accessible in the major groove. 10

11. N A S TR AN D S
S e ns e & Antis e ns e
D N A s trand s
•If DNA sequence is the same as that of a messenger RNA copy that is
translated into protein is called SENSE & sequence on the opposite strand is
called ANTISENSE
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12. D N A S u p e rcoiling
D N A S u p e rcoiling
Counter helical turn
lmost all DNA molecules in cells can be considered
as circular, and are on average negatively
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13. D N A s tru ctu re
Alte rnative S tru ctu re of D N A
A-D na B-D na Z-D na
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14. D N A s tru ctu re
A – D NA
•Right handed helix •Sugars are in the 3’ endo
conformation.
•planes of bases are
5.9 A
tilted 20 ° relative the O
helix axis. 5' 2' BASE O O
O 3' BASE
•2.3 A rise between base 1' 5'
3' H (OH) O 1'
pairs
O 2'
H (OH)
•11 bases per helix turn 3' endo (A-DNA)
2' endo (3' exo) B-DNA
•Helical pitch = 25.3 A •Bases are the anti conformation
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15. D N A s tru ctu re
B – D NA
•Right handed helix •Sugars are in the 2’
endo conformation.
•planes of bases are
nearly perpendicular
O
to the helix axis. 5' 2' BASE
•3.4 A rise between O 1'
base pairs 7.0A 3' H (OH)
O
•10.4 bases per helix turn
•Bases are2' endo (3' conformation
the anti exo) B-DNA
•Helical pitch = 34 A
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16. D N A s tru ctu re
Z – D NA
•Left handed helix
•Helical pitch = 45.6 A
•planes of bases are
tilted 9° relative the
•Flat major grooves
helix axis.
•3.8A rise between base
pairs •Narrow and deep minor
groove
•12bases per helix turn
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17. D N A s tru ctu re
Z – D NA
5’-GCGCGCGCGCGCG
3’-CGCGCGCGCGCGC
C: sugar is 2’-endo, base is anti
G: sugar is 3’-endo, base is syn
Sugar and base conformations in Z-DNA alternate
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18. Alte rnative D N A
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19. P re p are d B y
ZAH ID A U MAR
B .S c H ons,
D e p t of E nvironm e ntal S cie nce s
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