Deoxyribonucleic acid is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. DNA and ribonucleic acid are nucleic acids.

1. Name: Shishir Mahatara Chhetri
Roll No. 48 (Structure and function of DNA)
Email id: mcshishirsm58@gmail.com
Home assignment: Cytology, Genetic and Plant Breeding (first semester)
Deoxyribonucleic Acid (DNA)
Discovered by F.Meischer
Franklin, Wilkins and Astuburry 1st studied double helical structure of
DNA and the technique used was X-Ray Diffraction.
A.Kornberg 1st invitro synthesis of DNA = Non-functional .
H.G Khorana 1st invitro synthesis of functional segement of DNA i.e.gene.
DNA is the basic genetic material was proved by Hershey and chase by
performing an experiment in T2 bacteriophase.
Watson and Crick (1953) purpose double helical mode of DNA .
helically twisted double chain polydeoxyribonucleotide macromolecule.

2. Deoxyribonucleic Acid (DNA)
Nucleotides are the building blocks of DNA.
It constitutes the genetic material of all organism except RNA viruses.
It is double stranded structure found in cytoplasm and prokaryotic cell.
In eukaryotic cells, mainly located in nucleus as chief constituent of
chromosome in combination with histone protein.
Little amount of DNA is also found in organelles like mitochondria and
chloroplast of eularyotes.
DNA is a formed by the end-to-end polymerization of a large number of
repeated units called deoxyribonucleotides.
Deoxyribonucleotide is formed of a Deoxyribose sugar, nitrogenous base and
phosphoric acid.
Nitrogenous base are two types:
• Purines: Adenine(A), Guanine(G)
• Pyrimidines: Cytosine(C), Thymine(T) and Uracil(U)

3. Structure of DNA
According to Watson and Crick (1953) mode of DNA (B-DNA)
• 2 Strands of DNA are coiled around each other By plectonemic coiling
• DNA is coiled around a hypothetical axis in right
• handed manner.
• DNA is antiparallel (responsible bond is phospo-
diester bond).
• The two strands of DNA are complementary i.e.
Sequence of one chain is completely determined by that
of another.
• The sugar phosphate backbone face outward.
• In DNA, adenine (A) always paired with thymine (T) and guanine (G) with cytosine (C).
• Adenine and thymine shows the presence of two hydrogen bond A=T and Guanine,
cytosine are linked together by three hydrogen bond G C .

4. Structure of DNA
• The bond between two adjacent nucleotide is phospodiester bond (strongest bond in DNA).
This bond is between phosphate of one nucleotide and 3rd carbon of sugar of another
nucleotide.
• The bond between sugar and nitrogen base is Glycosidic
bond.
• One compound turn (helix) of DNA consist of 10 base
Pairs. i.e. 20 nucleotides. So length of a complete turn is
about 34A° or 3.4nm.
• The distance between two successive nucleotides is
3.4A° or 0.34nm.
• Each nucleotide turns 36° from successive nucleotide.
• The diameter of double helix is about 20A°.
• The two strands of DNA molecules run in opposite
or anti parallel directions. It means one strand runs in
3’ 5’direction and other strand run in 5’ 3’ direction.

5. Function of DNA
DNA has two important functions autocatalytic and hetero-catalytic represented
by
Autocatalytic function Heterocatalytic function
DNA RNA Protein
• It is genetic materials and transmits characters from parents to young ones.
• It controls the metabolic activities of the cell directly or indirectly.
• By undergoing mutation, DNA brings about the evolutionary changes.
• It has unique properties of formation of carbon copies(autocatalytic). This is
essential for transfer of genetic information.
• The DNA synthesizes the RNA
• The DNA guides the process of protein synthesis.
• Cause heredity by replication.
• It regulates development, growth and differentiation by transcription.