Prepared by – Santosh Kumar Das (6c/15)
2nd Year B.Sc (Ag)
Submitted To- Dr. Kaushik Kumar Panigrahi
Asst. Professor
Plant Breeding And Genetics
DNA STRUCTURE AND CHEMICAL
COMPOSITION

INTRODUCTION:
• DNA called as the genetic blueprint as it carries the information from one
generation to another.
• It is the main constituent of chromosome.
• It is the genetic material of all organism.
• In bacterial cells, in mitochondria and chloroplast single circular double
stranded DNA is found.
• In Eukaryotic cell linear helically coiled double stranded DNA present inside
the nucleus & its number equivalent to number of chromosome per cell.

CHEMICAL COMPOSITION OF DNA
• DAN is called as poly deoxy ribonucleotide molecule due to repeating of
deoxy ribonucleotides.
• These nucleotides are formed by nucleosides and phosphate and nucleosides
are formed by nitrogenous bases and deoxyribose sugar.
DNA = Nitrogenous base + Deoxyribose Sugar + Phosphate
Nitrogenous base + Deoxyribose Sugar= nucleosides
Nucleosides + Phosphate = Nucleotides

NITROGENOUS BASES :
PURINE: PYRIMIDINE:
• Double ring structure having a
pyrimidine ring and an imidazole ring.
• It consists of 9 members
• Nitrogen present at 1,3,7 and 9 position.
• N-9 always binds to sugar.
• Single ring structure.
• It consists of 6 members.
• Nitrogen present at 1 and 3 position.
• N-1 always binds to sugar.

DEOXY RIBOSE SUGAR (C₅H₁₀O₄):
• It is a pentose sugar.
• 5 membered ring with one Oxygen and 4 carbon. C-2
has H instead of –OH so it is called as deoxyribose.
• It binds to base by β-N glycosidic linkage.
• Derivative of phosphoric acid and gives acidity to
DNA molecule.
• It binds to sugar by phosphodiester bond.
PHOSPHATE:

NUCLEOSIDES:
• Bases binds to sugar to form Nucleosides.
• N-9 of Purine and N-3 of Pyrimidine binds to C-1
of sugar.
• it is of 4 types- dA(Deoxyadenosine)
dG(Deoxy guanosine)
dC(Deoxy cytidine)
dT(Deoxy thymidine)
NUCLEOTIDE:
• Nucleoside binds with phosphate to form
nucleotide.
• It is of 4 types – dAMP , dTMP, dGMP, dCMP

BONDS IN DNA :
• A pairs with T by 2 hydrogen bond
and C with G by 3 hydrogen bond .
T A
• Sugar links to phosphate by
phosphoester bond and to bases by
glycosidic bond.
G C
Phosphoester bond
Glycosidic bond
Hydrogen bond

DISCOVERY OF DNA DOUBLE HELIX :
Double helical structure of DNA was proposed by J D Watson and F S C Crick(1953).
This structure of DNA is based on X ray
crystallographic studies on DNA done by
Wilkins and Rosalind Franklin .

Watson & Crick proposed…
• A DNA molecule consist of 2 polynucleotide chains called as
strands , coiled around a common axis to form right handed
helix .
• This coiling forms interchain spacing called major and minor
grooves to which histone and non-histone protein interact .
• ANTI-PARALLEL STRAND : One strand runs from 3′5′ and
other from 5′3′ .
• Sugar and phosphate form the periphery and the core is
formed by nitrogenous bases.
• BASE COMPLIMENTARITY : 6 aminopurine always link to 6
kitopyrimidine and 6 kitopurine link to 6 aminopyrimidin. It
follows Chargaff's Rule i.e A+T=C+G
• Adenine must pair with Thymine & Guanine must pair with
Cytosine. Their amounts in a given DNA molecule will be
about the same.

MEASUREMENTS OF DNA :
• A DNA completes one turn at every 34 Aᵒ
(3.4nm) which is called Helix.
• One helix contains 10 basepairs called as
steps.
• Space between each step 3.4 Aᵒ .
• Diameter of B-DNA is 20 Aᵒ .
• A major groove is about 22 Aᵒ
• A minor groove is about 12Aᵒ

Function of DNA :
• It is the genetic material of all organism except some viruses .
• It can form the exact copy of its own kind . This is called as replication.
• It helps in synthesis of mRNA . This process is called as transcription.
Conclusion:
The structure of DNA given by Watson and Crick resembles with
winding staircase where sugar and phosphate form the railings and
linked bases formed the steps . This model have similarities with X-ray
crystallography study of DAN . Therefore this structure is universally
accepted.