This document summarizes the DNA double helix structure proposed by James Watson and Francis Crick in 1953. They discovered that DNA exists as two intertwined strands that coil around each other to form a double helix. Each strand is made up of alternating sugar and phosphate groups with nitrogenous bases protruding inward. The bases on one strand form hydrogen bonds with complementary bases on the other strand according to Chargaff's rules - adenine pairs with thymine and guanine pairs with cytosine. Watson and Crick received the Nobel Prize in 1962 for their discovery of the DNA double helix structure.

1. DNA DOUBLE
HELICAL
STRUCTURE
PRESENTED BY,
JAMUNA S
M.Sc BIOCHEMISTRY

2. INTRODUCTION :
 Chromosomes are the carrier of genetic material.
Chromosome contain DNA, RNA, proteins.
 DNA is the genetic material in most of the organism
and higher organism.
 In 1953 , Jameson Watson and Francis Crick proposed
double helix DNA model on the basis of X-ray
diffraction studies with photographs of DNA taken by
Wilkins and Franklin.
 It is also known as Watson and Crick model DNA
structure.
 Watson and Crick shared a noble prize in 1962.

3. DNA is a double stranded structure in which the two
strands are coiled around each other forming a double
helix.
The DNA duplex is “coil of life”.
There are two grooves in DNA namely, 1. Major groove,
2. Minor groove.
The backbone of the helix is formed of sugar and
phosphate molecule. The nitrogenous bases are attached
to sugar molecule.
The two nucleotide strands are held together by unstable
hydrogen bonds.

4. DNA DOUBLE HELIX DIMENSION OF DNA

5. The two strand run antiparallely in opposite direction i.e.,
they run in opposite direction 5’ to 3’ end and 3’ to 5’ end.
The strand are interwined in clockwise direction.
The width of the DNA molecule is 20 Å. The strand
completes a turn every 34 Å along its length.
There are ten nucleotide per turn. The internucleotide
distance is 3.4 Å.
Watson and Crick model of DNA is called B-form DNA.
The chain in B-form DNA are in right handed orientation.

6. STABILITY OF DOUBLE HELIX STRUCTURE :
 Internal and external hydrogen bonds
 Negative charge of phosphate groups
 Base pair stacking.
MAJOR AND MINOR GROOVE :

7.  Each nucleotide is made up of pentose sugar, a phosphate
group and a nitrogenous base. Deoxyribose is the
constituent sugar in DNA.
The nitrogenous bases are of two kinds,
 Purines
 Pyrimidines
 Adenine and guanine are the purines and thymine and
cytosine are pyrimidines.
The nitrogenous bases found in DNA are adenine,
guanine, cytosine and thymine.

8. There are two hydrogen bonds
between adenine and thymine
(A=T) and there are three
hydrogen bonds between guanine
and cytosine (G  C) pairing.
 The sub-unit containing only
sugar and nitrogenous base is
known as nucleoside. A nucleoside
combines with phosphate to form
a nucleotide.
 Nucleotides are building blocks of
DNA.

9.  Erwin Chargaff in 1949 showed that
 (i) The bases pair in specific manner. Adenine always pairs with
thymine and guanine pairs with cytosine.
 (ii) Total amount of purine nucleotides is always equal to the total
amount of pyrimidine nucleotides i.e.[A] + [G] = [T] + [C].
 (iii) The proportion of adenine is equal to thymine and so also of
guanine is equal to cytosine. But the [A] + [T] need not necessarily
be equal to [G] + [C].
 These empirical rules regarding the composition of bases in DNA is
collectively known as Chargaff’s law or Base pair rules.