2. DNA MODEL
• The three-dimensional structure of DNA, first proposed by
James D. Watson and Francis H. C. Crick in 1953, consists of
two long helical strands that are coiled around a common axis
to form a double helix.
• Each DNA molecule is comprised of two biopolymer strands
coiling around each other.
• Each strand has a 5′end (with a phosphate group) and a 3′end
(with a hydroxyl group).
• The strands are antiparallel, meaning that one strand runs in a
5′to 3′direction, while the other strand runs in a 3′to
5′direction.
• The diameter of the double helix is 2nm and the double helical
structure repeats at an interval of 3.4nm(34A) which
corresponds to ten base pairs which is none other than helix
measure
3. • The distance between two succesive base pairs is 0.34nm
(3.4A)
• The two strands are held together by hydrogen bonds and
are complementary to each other.
• The two DNA strands are called polynucleotides, as they
are made of simpler monomer units called nucleotides.
Basically, the DNA is composed of deoxyribonucleotides.
• The deoxyribonucleotides are linked together by 3′-
5′phosphodiester bonds.
• The nitrogenous bases that compose the
deoxyribonucleotides include adenine, cytosine, thymine,
and guanine.
• The structure of DNA -DNA is a double helix structure
because it looks like a twisted ladder.
4. • The sides of the ladder are made of alternating sugar
(deoxyribose) and phosphate molecules while the steps of the
ladder are made up of a pair of nitrogen bases.
• As a result of the double helical nature of DNA, the molecule
has two asymmetric grooves. One groove is smaller than the
other.
• The larger groove is called the major groove, occurs when the
backbones are far apart; while the smaller one is called
the minor groove, and occurs when they are close together.
• A always pairs with T with 2 hydrogen bonds and G pairs with
C with 3 hydrogen bonds hence two strands are
complimentary to each other
• Because of specificity in base pairing the amount of purines is
equal to the amount of pyramidines this is called as chargaffs
rule of base equivalence.
6. COMPONENTS OF DNA
• Nucleoside is a compound formed by the combination
of a pentose sugar and nitrogen base
• Nucleotide is compound formed by the combination of
nucleoside and phosphate group
• Nucleotides are the building blocks of nucleic acids
• Nucleotide have three charecteristic components
• Nitrogen base
• Deoxyribose sugar
• Phosphate
9. DEOXYRIBOSE SUGAR
• Deoxyribose, also known as D-Deoxyribose and 2-
deoxyribose, is a pentose sugar (monosaccharide
containing five carbon atoms) that is a key
component of the nucleic acid deoxyribonucleic acid
(DNA).
• It is derived from the pentose sugar ribose.
Deoxyribose has the chemical formula C5H10O4.
• Deoxyribose is the sugar component of DNA
• Alternating with phosphate bases, deoxyribose forms
the backbone of the DNA, binding to the nitrogenous
bases adenine, thymine, guanine, and cytosine.
10. • The sugar-phosphate backbone forms the structural
framework of nucleic acids, including DNA.
• This backbone is composed of alternating sugar and
phosphate groups and defines directionality of the
molecule.
PHOSPHATE GROUP
11. • DNA are composed of nucleotides that are linked to one
another in a chain by chemical bonds, called ester bonds
• These bonds are called phosphodiester bonds
• In double-stranded DNA, the molecular double-helix
shape is formed by two linear sugar-phosphate backbones
that run opposite each other and twist together in a helical
shape.
• The sugar-phosphate backbone is negatively charged and
hydrophilic, which allows the DNA backbone to form
bonds with water.
12.
13. Secret behind the discovery
• At King's College in London, Rosalind Franklin and
Maurice Wilkins were studying DNA. Wilkins and
Franklin used X-ray diffraction as their main tool --
beaming X-rays through the molecule yielded a shadow
picture of the molecule's structure, by how the X-rays
bounced off its component parts.
Franklin, a shy and inward young woman, suffered from
patronizing attitudes and sexism that forced her to do
much of her work alone. And her senior partner, Wilkins,
showed some of Franklin's findings to Watson in January
1953 without her knowledge.
14. • Shortly after, Watson and Crick made a crucial advance when
they proposed that the DNA molecule was made up of two
chains of nucleotides paired in such a way to form a double
helix, like a spiral staircase. This structure, announced in their
famous paper in the April 1953 issue of Nature, explained how
the DNA molecule could replicate itself during cell division,
enabling organisms to reproduce themselves with amazing
accuracy except for occasional mutations.
For their work, Watson, Crick, and Wilkins received the Nobel
Prize in 1962. Despite her contribution to the discovery of
DNA's helical structure, Rosalind Franklin was not named a
prize winner: She had died of cancer four years earlier, at the
age of 37.
