Structure of DNA

Deoxyribonucleic acid (DNA) is the single most important molecule in living cells and contains all of the information that specifies cellular properties, every aspect of cellular function is under the control of DNA. It is polymer of deoxyribonucleotides. 

   Nucleic acids are serve as repositories and transmitters of genetic information. Nucleic acids are two types :

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

Nucleic acids are the polymers of  nucleotides. Nucleotides are composed of a nitrogenous base, a pentose sugar and a phosphate. The term nucleoside refers to base + sugar thus nucleotide is nucleoside + phosphate.

Nitrogenous base :

    The nitrogenous bases found in nucleotides are aromatic heterocyclic compounds. The bases are of two types : Major bases and Minor bases.
Major Bases:

1. Purines
2. Pyrimidines

Purines :

Pyrimidines :

Minor bases :
  Several minor and unusual bases are often found in DNA and RNA. These include:

• 5-methylcytosine
• N⁴-acetylcytosine
• N⁶-methyladenine
• N⁶-dimethyladenine etc.,

The unusual bases in nucleic acids will help in the recognition of specific enzymes.
    The information was obtained about the arrangement and dimensions of various parts of the molecule by X-ray diffraction analysis. The most important significant observations were that the molecule is helical and that the base of the nucleotides are stacked with their planed separated by a spacing of 3.4 A⁰.
 James Watson and Francis Crick combined chemical and physical data for DNA with a feature of the X-ray diffraction diagram that suggested that two helical strands are present in DNA and showed that two strands are coiled about one another to form a double-stranded helix.

The sugar-phosphate backbones follow a helical path at the outer edge of the central core. The helix has two external helical grooves, a deep wide one (the major groove) and a shallow narrow one (the minor groove), both of these grooves are large enough to allow protein molecule to come in contact with bases.
The monomeric deoxyribonucleotides in DNA are held together by 3', 5'-phosphodiester bridges. 

Silent features of Watson-Crick model of DNA :

1. The DNA is a right handed double helix. It consists of two polydeoxyribonucleotide chains twisted around each other on a common axis.
2. The two strands are antiparallel, i.e., one strand runs in the 5' to 3' direction while the other 3' to 5' direction.
3. The Width of a double helix is 20 A⁰ (2 nm).
4. Each turn (pitch) of the helix is 34 A⁰ (3.4 nm) with 10 pairs of nucleotides, each pair placed at a distance of about 3.4 A⁰.
5. Each strand of DNA has a hydrophilic deoxyribose phosphate backbone on the outside of the molecule while the hydrophlic bases are stacked inside.
6. The two polynucleotide chains are not identical but complementary to each other due to base pairing.
7. The two strands are held together by hydrogen bonds formed by complementary base pairs. The A-T pair has two hydrogen bonds while G-C pair has three hydrogen bonds. The G≡C is stronger by about 50% than A=T.

8. The Complementary base pairing in DNA helix proves Chargaff's rule.

Erwin Chargaff in late 1940s quantitatively analysed the DNA hydrolysates from different species. He observed that in all the species he studied DNA had equal number of adenine and thymine residues (A=T) and equal number of guanine and cytosine residues (G≡C). This is known as Chargaff's rule of molar equivalence between the purines and pyrimidines in DNA structure.