The document summarizes the key discoveries around DNA structure. It describes how in 1953, Francis Crick and James Watson described DNA as having a double helix structure, with two strands coiling around each other. Each strand is made of nucleotides consisting of a phosphate, sugar, and nitrogenous base. The bases bond together between the strands in specific patterns - adenine pairs with thymine, and cytosine pairs with guanine. This discovery helped explain how genetic information is stored and replicated in living organisms.

1. Double standard helix
DNA structure
Presented To:
Dr.Chacko Vijai Sharma,
Asst.Prof of Microbiology,
Sacred Heart College,
Tirupattur.
Presented By:
V.Salomi (BP211509) ,
I M.Sc.Microbiology,
Sacred Heart College,
Tirupattur.

2. DNA model

3. DNA Structure
 The double helix is a description of the molecular shape of a double-stranded
DNA molecule.
 In 1953, Francis Crick and James Watson first described the molecular
structure of DNA, which they called a "double helix," in the journal Nature.
 For this breakthrough discovery, Watson, Crick, and their colleague Maurice
Wilkins won a Nobel Prize in Physiology, or Medicine, in 1962.
 However, a crucial contribution that enabled this discovery was made by
Rosalind Franklin, who was not acknowledged at that time.
 After her death, Crick said that her contribution had been critical.

5. Structure of DNA

6.  The double helix describes the appearance of double-stranded DNA, which is
composed of two linear strands that run opposite to each other, or anti-parallel,
and twist together.
 Each DNA strand within the double helix is a long, linear molecule made of
smaller units called nucleotides that form a chain.
 The chemical backbones of the double helix are made up of sugar and
phosphate molecules that are connected by chemical bonds, known as sugar-
phosphate backbones.
 The two helical strands are connected through interactions between pairs of
nucleotides, also called base pairs.
 Two types of base pairing occur:
 nucleotide A pairs with T, and nucleotide C pairs with G.

7. Nucleotide

9. Composition of DNA
 we now know that DNA is in fact composed of a series of nucleotides and that
each nucleotide has three components:
 a phosphate group; either a ribose (in the case of RNA) or a deoxyribose (in the
case of DNA) sugar; and a single nitrogen-containing base.
 We also know that there are two basic categories of nitrogenous bases: the
purines (adenine [A] and guanine [G]), each with two fused rings, and the
pyrimidines (cytosine [C], thymine [T], and uracil [U]), each with a single ring.
 Furthermore, it is now widely accepted that RNA contains only A, G, C, and U
(no T), whereas DNA contains only A, G, C, and T (no U)

10. Composition

11. Covalent bonds
 Hydrogen bonds ( Nitrogenous base A= T , G -= C)
 N- Glycosidic bonds ( sugar + Nitrogenous base)
 Phosphodiester bond ( phosphate + sugar + phosphate + nucleotide)
 Composition:
 Sugar – pentose ( deoxyribose)
 Nitrogenuos base – purine & pyrimidine
 Phospahte

13. Thank you