DNA Structure

1. 1
Presented By
Dr. T,Ugandhar
Asst.Prof of Botany
GDCMAHABUBABAD

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4. Timeline
1800’s F Miescher - nucleic acids
1928 F. Griffith - Transforming principle
1952
Avery, McCleod & McCarty-
Transforming principle is DNA
1944
Hershey-Chase ‘blender’
experiment
http://www.dnai.org/lesson/go/2166/1994
1949 Erwin Chargaff – base
ratios

5. DNA
(Deoxyribonucleic Acid)

6. A HISTORY OF DNA
• Discovery of the DNA double helix
A. Frederick Griffith – Discovers that a
factor in diseased bacteria can transform
harmless bacteria into deadly bacteria
(1928)
B. Rosalind Franklin - X-ray photo of
DNA.
(1952)
C. Watson and Crick - described the
DNA molecule from Franklin’s X-ray.
(1953)
SEE p. 292-293

7. X-ray diffraction patterns produced by
DNA fibers – Rosalind Franklin and
Maurice Wilkins

8. The Watson-Crick Model:
DNA is a double helix
• 1951 – James Watson learns about x-ray
diffraction pattern projected by DNA
• Knowledge of the chemical structure of nucleotides
(deoxyribose sugar, phosphate, and nitrogenous
base)
• Erwin Chargaff’s experiments demonstrate that
ratio of A and T are 1:1, and G and C are 1:1
• 1953 – James Watson and Francis Crick propose
their double helix model of DNA structure

9. Watson & Crick proposed…
•DNA had specific pairing between the
nitrogen bases:
ADENINE – THYMINE
CYTOSINE - GUANINE
•DNA was made of 2 long stands of
nucleotides arranged in a specific
way called the “Complementary Rule”

10. Deoxyribonucleic Acid (DNA):
• Deoxyribonucleic acid, also abbreviated as DNA, is the
principal informational macromolecule of the cell,
which stores, translates and transfers the genetic
information.
• In the prokaryotes, the DNA is found mostly in the
nuclear zone.
• In eukaryotes it is found in the nucleus, mitochondria
and chloroplast.
• The present understanding of the storage and
utilization of the cell’s genetic information is based
upon the discovery of the structure of DNA by Watson
and Crick in 1953.
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11. Structure of DNA:
DNA is made of two helical chains coiled around the
same axis, to form a right-handed double helix.
The two chains in the helix are anti-parallel to each
other, i.e., the 5′-end of one polynucleotide chain and
the 3′-end of the other polynucleotide chain is on the
same side and close together.
The distance between each turn is 3.6 nm (formerly 3.4
nm).
There are 10.5 nucleotides per turn (formerly 10
nucleotides).
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14. Chargaff’s Rule
• Adenine must pair with Thymine
• Guanine must pair with Cytosine
• Their amounts in a given DNA molecule will
be about the same.
G CT A

15.  The spatial relationship between the two strands creates major
and minor grooves between the two strands. In these grooves
some proteins interact.
 The hydrophilic backbones of alternating deoxyribose and
negatively charged phosphate groups are on the outside of the
double helix.
 The hydrophobic pyrimidine and purine bases are inside the
double helix, which stabilizes the double helix of the DNA.
 The double helix is also stabilized by inter-chain hydrogen bond
formed between a purine and pyrimidine base.
 A particular purine base, pairs by hydrogen bonds, only with a
particular pyrimidine base, i.e., Adenine (A) pairs with Thymine
(T) and Guanine (G) pairs with Cytosine (C) only.
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 Two hydrogen bonds pairs Adenine and Thymine (A = T),
whereas three hydrogen bonds pairs Guanine and Cytosine (G
≡ C).
 The base pairs A = T and G ≡ C are known as complementary
base pairs.
 Due to the presence of complementary base pairing, the two
chains of the DNA double helix are complementary to each
other.
 Hence the number of A’ bases are equal to the number of T’
bases (or ‘G’ is equal to ‘C) in a given double stranded DNA.
 One of the strands in the double helix is known as sense strand,
i.e., which codes for RNA/proteins and the other strand is
known as antisense strand.

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18. Genetic material of cells…
• GENES – units of genetic material that
CODES FOR A SPECIFIC TRAIT
• Called NUCLEIC ACIDS
• DNA is made up of repeating molecules
called NUCLEOTIDES

19. DNA Nucleotide
O
O=P-O
O
Phosphate
Group
N
Nitrogenous base
(A, G, C, or T
CH2
O
C1
C4
C3 C2
5
Sugar
(deoxyribose)

20. DNA Double Helix
Nitrogenous
Base (A,T,G or C)
“Rungs of ladder”
“Legs of ladder”
Phosphate &
Sugar Backbone

21. DNA Double Helix
P
P
P
O
O
O
1
2
3
4
5
5
3
3
5
P
P
P
O
O
O
1
2 3
4
5
5
3
5
3
G C
T A

22. Nitrogenous Bases
• PURINES
1. Adenine (A)
2. Guanine (G)
• PYRIMIDINES
3. Thymine (T)
4. Cytosine (C)
T or C
A or G

23. BASE-PAIRINGS
CG
H-bonds
T A

24. Genetic Diversity…
• Different arrangements
of NUCLEOTIDES in a
nucleic acid (DNA)
provides the key to
DIVERSITY among
living organisms.

25. The Code of Life…
• The “code” of the chromosome is the
SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…

26. DNA is wrapped tightly around
histones and coiled tightly to
form chromosomes
See p. 297

27. DNA Replication
• DNA must be copied
• The DNA molecule produces 2
IDENTICAL new complementary
strands following the rules of
base pairing:
A-T, G-C
•Each strand of the
original DNA serves
as a template for the
new strand See p.
298

28. DNA Replication
• Semiconservative
Model:
1. Watson and Crick
showed: the two
strands of the parental
molecule separate, and
each functions as a
template for synthesis
of a new
complementary strand.
.
Parental DNA
DNA Template
New DNA

29. Essential features of B-DNA
• Right twisting
• Double stranded
helix
• Anti-parallel
• Bases on the inside
(Perpendicular to
axis)
• Uniform diameter
(~20A)
• Major and minor
groove
• Complementary
base pairing

30. • Structurally, purines (A and G pair best with
pyrimidines (T and C)
• Thus, A pairs with T and G pairs with C, also
explaining Chargaff’s ratios

31. Maybe because
RNA but not DNA
is prone to base-
catalysed
hydrolysis
Why DNA
evolved as the
genetic material
but not RNA?

32. B-DNA
Biologically dominant
Right-handed double
helix
planes of base pairs
are nearly
perpendicular to the
helix axis.
helix axis passes
through the base pairs
and hence B-DNA has
no internal spaces
B-DNA has a wide and
deep major groove
and a narrow and deep
minor groove

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34. 34

35. DNA conformations
B-DNA:
– right-handed double helix with a wide and narrow
groove.
A-DNA
– major groove is very deep and the minor groove is quite
shallow
Z-DNA
– consists of dinucleotides, each with different
conformations
4 stranded DNA
– Telomeric DNA

36. DNA conformations
both form right-handed double
helices
B-DNA helix has a larger pitch and
hence a smaller width than that of
A
In B-DNA, the helix axis passes
through the base pairs and hence
B-DNA has no internal spaces,
whereas that of A-DNA has a 6
Angstrom diameter hole along its
helical axis.
The planes of the base pairs in B-
DNA are nearly perpendicular to
the helix axis, whereas in A-DNA,
they are inclined from this.
Therefore, B-DNA has a wide and
deep major groove and a narrow
and deep minor groove, whereas A-
A DNA B DNA

37. DNA conformations
B-DNA forms a right-handed
double helix in which the
repeating unit is a
nucleotide,
whereas Z-DNA forms a left-
handed double helix in
which the repeating unit is a
dinucleotide.
The Z-DNA helix has a
larger pitch and is therefore
narrower than that of B-
DNA.
B-DNA has a wide and deep
major groove and a narrow
and deep minor groove,
whereas Z-DNA has a
narrow and deep minor
B DNAZ DNA

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44. linear
human chromosomes
Double stranded DNA
Genetic material may be DNA
Single stranded DNA
circula
r
linear
circula
r
Prokaryotes
Mitochondria
Chloroplasts
Some viruses
(pox viruses)
Parvovirus
adeno-associated viruses

45. reoviruses
Double stranded RNA
Genetic material may be RNA
Single stranded RNA
Retroviruses like HIV

46. RNA / DNA hybrids
e.g. during retroviral
replication

47. AMAZING DNA FACTS…
• DNA from a single human cell
extends in a single thread for
almost 2 meters long!!!
• It contains information equal
to some 600,000 printed
pages of 500 words each!!!
(a library of about 1,000 books)

48. Functions of DNA:
• The base sequence of the DNA constitutes the
informational signal called the genetic material.
This nucleotide base sequence enables the DNA to
function, store, express and transfer the genetic
information.
• Hence it programs and controls all the activities
of an organism directly or indirectly throughout
its life cycle.
• (a) DNA stores the complete genetic information
required to specify (form) the structure of all the
proteins and RNA’s of each organism.
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49.  DNA is the source of information for the synthesis of
all cellular body proteins. Some of the proteins are
structural proteins and some are enzymes. These
enzymes arrange micro-molecules to form
macromolecules. It determines the activities of an
organism throughout its life cycle, i.e., the period of
gestation, birth, maturity, senescence and death.
It determines the individuality and identity of a given
organism.
It duplicates (replicates to form two daughter DNA)
itself and transfers one of the copy to the daughter cell
during cell division, thus maintaining the genetic
material from generation to generation.
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50. THANK YOU