The document provides a history of discoveries related to DNA structure and function. Some key points summarized: 1) In the 1800s, nucleic acids were discovered but their role was unknown. In the 1950s, experiments showed that DNA was the transforming principle that carried genetic information. 2) In 1953, Watson and Crick proposed the double helix structure of DNA, with base pairing of A-T and G-C. This explained prior findings on base ratios by Chargaff. 3) DNA contains the genetic code as a sequence of nucleotide bases that provides instructions for protein synthesis and cell function. Its double helix structure allows for replication and transmission of genetic material from parent to daughter cells.

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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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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