Watson and Crick proposed a model of DNA structure in 1953 as a double helix with two antiparallel strands coiled around the same axis. Each strand consists of alternating deoxyribose and phosphate groups with purine or pyrimidine bases stacked inside. Adenine always pairs with thymine via two hydrogen bonds and guanine pairs with cytosine via three hydrogen bonds. Their model explained experimental X-ray diffraction data and Chargaff's rules of base equivalence. The discovery revolutionized our understanding of genetics and heredity.

1. Watson and Crick model of DNA

2. Watson and Crick model of DNA
Some Basic
 Nucleoside- is a compound formed by the
combination of a pentose sugar and nitrogen base.
 Nucleotide-is a compound formed by the
combination of nucleoside and phosphate group.
 Nucleotides building blocks of nucleic acids.
 Nucleotide have three characteristic components.
1. A nitrogenous base
2. A pentose sugar
3. A phosphate group

3. Nitrogen bases present in DNA
1. Adenine 3.Cytosine
2. Guanine 4.Thyrosine
The different types of nucleosides of DNA
1. Deoxy adenosine 3.Deoxy cytidine
2. Deoxy guanosine 4.Deoxy tymidine
The DNA contain 2 major purine bases
A. Adenine
B. Guanine
The DNA contain 2 pyrimidine bases
A. Cytosine
B. Thymine

7. Phosphodiester linkages in the covalent backbone of DNA

11. X ray diffraction
 x-ray crystallography was originaly used
to look at the structures of simple
organic minerals,but also progressively
applied to more and complex molecules.
 It aided in determining the structures of
the alpha helix,the beta sheets,
hemoglobin and DNA.

12. X-ray diffraction of DNA

13. DNA is double helix
 To shed more light on the structure of DNA. Rosalind, Franklin
and wilkins used the powerful method of X-ray diffraction to
analyses DNA fibres.
 They showed in the early 1950s that DNA produces a
charectestic X-ray diffraction pattern
 From this pattern it was deduced that DNA molecules are
helical with two periodicities along their along axis
 A primary one is 3.4A and a secondary one is 34A
 The problem then was to formulate a three dimensional model
of DNA molecules that could account not only for X-ray
diffraction data but also for the specific A=T and G=C
base.equivalence discovered by chargaff and for the other
chemical properties of DNA

14. X-ray diffraction and the structure of DNA
Watson was shown this picture by wilkins in early 1953
from this picture it was possible to calculate
1) The distance between base (3.4A)
2) The length of the period (34A)
3) The rise of the helix (36degree)

16. Watson and Crick model of DNA
 In 1953 watson & crick proposed a three dimensional
model of DNA Structure
 It consists of two helical DNA chains wound around
the same axis to form a right handed double helix.
 The hydrophilic back bones of alternating deoxy-
ribose and phosphate groups are on the outside of
the double helix .
 The purine and pyrimidine bases of both strands are
stacked inside the double helix.
 The hydrophobic planar ring structure very close
together and perpendicular to the long axis

18.  The offset pairing of two strands creates a major
groove and minor groove on the surface of the duplex.
 Each neucleotide base of one strand is paired in the
same plane with a base of the other strand.
 According to this model DNA is antiparallel it is
ultimately confirmed by x-ray analysis.
 The distance between successive base pairs is 3.4A.
 The every turn of the helix measures 34A and contain
10 base pairs.

19. The steps are formed by paired nitrogen base
 A always pairs with T with 2 hydrogen bonds and G
pairs with C with 3 hydrogen bonds
 Hence two strands are complementary to each other
 Because of the specificity in base pairing the amount
of purines is equal to the amount of pyrimidines A=T &
G=C
 This is called chargaff`s rule of base equivalence.
 The DNA double helix is held together by two
forces,hydrogen bonding between complementary
base pairs and base stacking interactions.
 These two force gives the stability of the double helix.

20.  For their outstanding work in discovering the double
helical structure of DNA
 Watson and crick shared the 1962 Nobel price for
physiology and medicine with wilkins,sadly and
franklin whose working greatly contributed to this
key discovery.

21.  The discovery of the structure of DNA by
watson and crick in 1953
 It was a momentous event in science
 An event gave rise to entirely new disciplines
and influenced the course of many established
ones
 Our precise understanding of the storage and
utilization of cells genetic information is based
on work made possible by this discovery