The major scientific achievements of the 20th century was discovering that genetic information is coded in DNA, a polymeric molecule composed of four nucleotide units. DNA is organized into genes, which are the basic units of heredity. Knowledge of DNA and RNA structure and function is essential for understanding genetics, disease pathogenesis, and the genetic basis of disease. In 1953, Watson and Crick described the double helix structure of DNA based on Rosalind Franklin's X-ray crystallography photo, revealing how nucleotides on two intertwined DNA strands are paired through hydrogen bonds between complementary bases.

2. The major scientific achievements of the 20th century
is the discovery that genetic information is coded
along the length of a polymeric molecule composed
of only four types of monomeric units.
This polymeric molecule is DNA, the chemical basis of
heredity
DNA is organized into genes, the fundamental units
of genetic information.
The basic information pathway. ie,
Knowledge of the structure and function of nucleic
acids is essential in understanding genetics and many
aspects of pathophysiology as well as the genetic basis
of disease.
DNA RNA Protein

3. A HISTORY OF DNA
Discovery of the DNA double helix
 Frederick Griffith –(1928) Discovers that a factor
in diseased bacteria can transform harmless
bacteria into deadly bacteria.
 Maurice Wilkines – worked on the structure of
DNA.
 Rosalind Franklin -(1952) - Worked out helical
structure of DNA by X-ray photo of DNA.
 Watson and Crick - (1953)
described the DNA molecule from Franklin’s X-ray.

4. DNA is very long, thread like macromolecule
Mead up of a polymer of deoxyribonucleotide
Composed monomeric deoxynucleotide……
 deoxyadenylate,
deoxycytidylate,
 These monomeric units of DNA are held in
polymeric form by 3′,5′-phosphodiester bridges
constituting a single strand.
The informational content of DNA resides in
the sequence in which these monomers are
ordered.
Primary Structure of Nucleic Acids
(DNA)
 thymidylate
 deoxyguanylate

5. Formation of phosphodiester bond
Bases in DNA carry genetic information.
Sugar and phosphate perform structural role

6. Nucleotides Linked by Phosphodiester Bond
OH
PO4
2-
Phosphodiester bond
O-P=O-
O
O
5’
3’
PO4
2-
OH3’
5’
P
R
P
R
P
R
P
R
P
R
P
R
1
2
3’
5’
1
2
3
4
5
6
Akindofphospho-polysaccharides

7. The Notation for Nucleic Acids
P
R
B
3’
2’
5’
1’
A T C G A T C G
P OH
5’ 3’
5’ pApTpCpGpApTpCpG-OH 3’
5’ p-ATCGATCG-OH 3’
ATCGATCG

8. Erwin Chargaff(1940) quantitatively analyzed
DNA. He found that,
DNA had equal no of A & T and equal no of G &
C. this is known as Chargaff’s rule of molar
equivalence between the purines & pyrimidines in
DNA. ie.
A=T and G=C
Single stranded DNA & RNAs do not obey rule
Double stranded DNA & RNA(in some viruses)
satisfies chargaff’s rule.
Chargaff’s rule

9. The secondary structure of
DNA is the double helix

10. Model they built of the structure of DNA
James
Watson
Francis
Crick

11. The Watson-Crick
DNA Double Helical Structure.
DNA consists two strands of nucleotides
that wind together in a double helix
(comparable to twisted ladder).
DNA is a right handed double helix.
These two DNA stands are antiparallel. The
two strands run in opposite directions.
The width of double helix is 20A°(2nm).
Each turn of the helix is 34A°(3.4nm) with
10 pairs of nucleotide, each pairs placed at
a distance of about 3.4 A°

12. The bases on the inside of the helix and
the sugar-phosphate backbones on the
outer side of the helix.
The two chains are held together by
hydrogen bonds between complementary
pairs of bases. ie.
 A pairs with T by two hydrogen bonds.
 G pairs with C by three hydrogen bonds.
DNA contains grooves of alternate size
called major grooves and minor grooves.
The sequence of the bases carries the
Genetic Information.

13. BASE-PAIRINGS
H-bonds
G
N
N
N
N
O
=
O
=
N N
N
H
H
H
H
N
H
C
A
N
N
N
N
O
=
N N
H
H
T
N
H
O
CH3 H-bonds

14. sugar-phosphate
chain
bases
THE DOUBLE HELIX
Width
20A°
Each turn
34A°
Each base pair
3.4A°
5’ 3’
3’5’
Major grooveMinor groove

15. TYPES OF DNA
(different structural form of DNA)
DNA can exist in three structural forms
A-DNA
B-DNA (Watson-Crick model)
Z-DNA
Under physiological condition, DNA is
present in B form.
The characteristics of A,B and Z-DNA
are……..

16. CHARACTERISTICS OF A,B AND Z-DNA
Characteristics
property
A-DNA B-DNA Z-DNA
Shape Broadest Intermediate narrowest
Type of helix Right handed Right handed Left handed
Base pairs/turn 11 10 12
Rise / base pair 2.3 A° 3.4 A° 3.8 A°
Helix diameter 25.5 A° 23.7 A° 18.4 A°
Pitch/turn of
helix
25.3 A° 34 A° 45.6 A°
Major groove Narrow Wide Flat
Minor groove Very broad Narrow Very Narrow

18. Organization of DNA

19. Prokaryotic DNA
P.cell generally contains single chromosome.
Composed of double stranded circular DNA
Which contains over 4x106 base pairs.
The circular DNA is supercoiled and
attached to an RNA-protein core.eg E.coli.

20. Eukaryotic DNA
Eukaryotes contain over 1000 times the amount
of DNA found in prokaryotes.
Human haploid DNA contains 2.8x109 base
pairs.
This is distributed between 23 chromosomes.
A typical human cell contains 46 chromosomes
Whose total DNA is approx. 2 meter in length
The packing of DNA in a chromosome
represents 10,000 fold shortening of its length

21. Condensed loops
Chromosome
Non-condensed loops
30-nm chromatin fiber
(nucleosomes)
10-nm chromatin fiber
DNA double helix

24. Thank you